This has been one of the most difficult storms to forecast that I can remember. As meteorologists, I think this serves as a reminder that even in this age of advanced technology: powerful computers, advanced radar - the atmospheric system is incredibly complex. Sometimes these kinds of weather patterns are going to occur and there isn't a hell of a lot we can do but make a good guess.
Forecasts are still all over the place but just as I was typing this, the National Weather Service dropped the Winter Storm Warning for the immediate DC area and reduced our snow forecast to around an inch. Further east there's still a chance for more significant snow but the real show will be from Philadelphia northward along I-95 to Boston and beyond. Up in New England they will have the chance for more than a foot.
Here - some light snow I think. Short-term models haven't given up on the idea of some snow falling but it doesn't look like it will amount to much.
What a crazy ride it's been with this storm.
Sunday, December 26, 2010
Saturday, December 25, 2010
Keep an Eye on This One
We're under a Winter Storm Watch and it's likely to be upgraded to a warning once the 12Z guidance comes in. Models still in remarkable disagreement given how close we are to the event. 5 inches seem possible here in DC - maybe more if the storm continues to trend westward - maybe much more if a serious track change happens.
I won't be able to check the modes as regularly today but I'd say it's likely that it will snow tomorrow - at least 4 inches maybe more. Have to keep an eye on that track.
Merry Christmas!
I won't be able to check the modes as regularly today but I'd say it's likely that it will snow tomorrow - at least 4 inches maybe more. Have to keep an eye on that track.
Merry Christmas!
Friday, December 24, 2010
GFS Won't Give Up
I've been reduced to a spectator. I can't remember a case where the models had so much flip-flopping and disagreement this close to an event. There have been plenty of times when the models have all agreed and turned out to have been smoking the same hallucinogenic drug. But this is insane.
Several afternoon models (including some of the near-term models like the RUC - usually disregarded except inside of 40 hours to the event) are leaning towards the GFS solution.
Which looks like this:
That's many things (crazy is one of them) but what it's not is "no snow".
The 0Z runs tonight (which don't come out until like 1:30 AM) are going to be crucial. In any case, I still think this is a Sunday-Monday issue so it shouldn't create any serious disruptions on Christmas Day except for in the South.
Will try and check the models later tonight. Merry Christmas everyone.
Several afternoon models (including some of the near-term models like the RUC - usually disregarded except inside of 40 hours to the event) are leaning towards the GFS solution.
Which looks like this:
That's many things (crazy is one of them) but what it's not is "no snow".
The 0Z runs tonight (which don't come out until like 1:30 AM) are going to be crucial. In any case, I still think this is a Sunday-Monday issue so it shouldn't create any serious disruptions on Christmas Day except for in the South.
Will try and check the models later tonight. Merry Christmas everyone.
12Z Euro Still Out to Sea
So the Euro didn't come around to the GFS solution. With so many models agreeing on an out-to-sea scenario and the GFS now the only true outlier, have to go with consensus right now and stick to the no-major-storm scenario for the mid-Atlantic.
Still watching other models and available information.
Still watching other models and available information.
...right up Until the 12Z GFS. I need an Excedrin.
Yeah, remember that out-to-sea thing I posted about like, what, an hour and a half ago?
The GFS comes out west of its last track making the miss out to sea more of a "brush" and keeping the game alive. This is akin to watching your favorite team down 21 with 7 minutes left in the fourth quarter, turning off the TV because you figure it's over and then having them tie it up with seconds left in the game.
Will the GFS win (i.e. - will the other models also shift towards us?)...I'll see what the afternoon models show.
Here's the 12Z GFS run:
The GFS comes out west of its last track making the miss out to sea more of a "brush" and keeping the game alive. This is akin to watching your favorite team down 21 with 7 minutes left in the fourth quarter, turning off the TV because you figure it's over and then having them tie it up with seconds left in the game.
Will the GFS win (i.e. - will the other models also shift towards us?)...I'll see what the afternoon models show.
Here's the 12Z GFS run:
Models Agree: Out to Sea
Will be watching this closely. The miss is about 100 miles - not more than that. But for now, the threat of a major winter storm for the mid-Atlantic has subsided. There will be snow in the south - maybe from Atlanta through eastern North Carolina which is fun for Christmas. And there's the chance for a major storm for eastern New England. But, it looks like this will miss us in the mid-Atlantic out to sea.
The morning model runs have the suggestion of a change again...but I'm not sure it will be close enough to matter a lot for us.
As the first wave passes, the phase happens too late. Even so, the first wave could give us a bit of light snow Christmas Day afternoon and evening.
I'll be watching the afternoon runs to make sure there are no surprises.
Thursday, December 23, 2010
Models Flip Flopping Again
Quick update this morning: if you like snow, the morning models are not for you. It's still basically the Euro vs. the GFS but the GFS has gone back to its WAY out-to-sea forecast in which only eastern Maine is really affected. The Euro still has the storm but even it is a little further east than in previous runs.
It's not a good idea to latch onto any one of the model runs - good or bad. I'll be looking for some run-to-run consistency before making a call. Hopefully that comes before Saturday.
Right now - some snow is still probable but the megastorm that was in the Euro yesterday seems less likely.
Waiting for the evening runs of the models to see if this eastward shift continues.
It's not a good idea to latch onto any one of the model runs - good or bad. I'll be looking for some run-to-run consistency before making a call. Hopefully that comes before Saturday.
Right now - some snow is still probable but the megastorm that was in the Euro yesterday seems less likely.
Waiting for the evening runs of the models to see if this eastward shift continues.
Wednesday, December 22, 2010
DC, Philly, NYC Would Disappear Beneath the Snow
The 12Z ECMWF forecast model for Monday morning. If this verifies - and that's still a big IF - we're all going to be hoping we received tunneling equipment for Christmas. QPF totals around 1.6 inches with ratios of about 15:1. Ladies and gentlemen, that would be 2 feet of snow - more for Philly and NYC. Hold off the excitement because there's still a LOT of time (4-5 days) for the models to change but if this continues for another 3-4 model runs, we're going to be talking about something fairly serious.
Christmas Storm Update and Teleconnections
Quick update on the Christmas storm (which is really now a Christmas night through Sunday storm) possibility: it's basically the GFS vs the Euro in terms of models. The GFS says "no phase, out to sea, little to no impact". The Euro model says "early phase, storm rides up the coast, stalls and buries the East Coast from northern Georgia to the Northeast in heavy snow totals". Remember that consensus we had earlier this week? Yeah, it's gone.
This is a good time to talk about the other things meteorologists look at to determine the likelihood of major winter storms. First, we'll look at something meteorologists and climatologists call "teleconnections".
The concept of teleconnections is based on the idea that the atmosphere is one large system. Air is essentially a fluid and it's modeled that way in terms of equations and concepts. Since it's a fluid, atmospheric anomalies like low pressure areas and high pressure areas, fronts, storms, etc can be thought of as blocks of styrofoam floating in a pool. If we throw a rock into the pool to create waves, these blocks will move. The movement of the blocks can be measured whether we threw the rock in the middle of all the blocks or somewhere at the other end of the pool. Further, the blocks will interact with each other - some of them will move in opposite directions essentially blocking the movement of others.
The atmosphere is a similar system. Simply put, the study of teleconnections is the study of linkages between weather patterns that occur in widely separated regions of the globe. Let's take a look at two of the teleconnections that most profoundly affect the type of weather we have here in the Mid-Atlantic during the winter.
NAO
NAO stands for "North Atlantic Oscillation" and it refers to the position and strength of two pressure centers in the North Atlantic. One is a low pressure area typically located near Iceland (the "Icelandic Low"). The other is an area of high pressure often located over the Azores in the eastern Atlantic (the "Azores High").
The strength and positioning of these pressure centers affects the alignment of the jet stream - a fast moving current of air - two branches of which control the nature of our weather. It's these branches, by the way, that we look to have phase when we're talking about strong winter storms.
Over time, climatologists and meteorologists have found that specific conditions of NAO cause specific patterns of weather over the globe. We'll concentrate on the eastern U.S. for obvious reasons.
POSITIVE NAO (+NAO)
The positive condition of NAO (shorthand: +NAO) occurs when the Icelandic Low and the Azores High are both stronger-than-normal. The stronger pressure centers cause a stronger pressure gradient (pressure changes more rapidly with distance from either center towards the other center). Pressure gradients cause stronger winds - in this case because the flow around the Icelandic Low (and all low pressure centers in the Northern Hemisphere for that matter) is counterclockwise and the flow around the Azores High (and all high pressure centers in the Northern Hemisphere) is clockwise, the increase in gradient causes an increased westerly wind.
This increase in wind essentially "drains" the cold air from the eastern U.S. into the Atlantic. Therefore, generally, the following is true during +NAO:
NEGATIVE NAO (-NAO)
The negative condition of NAO (shorthand: -NAO) is, sensibly, the opposite of +NAO. A weaker-than-normal Icelandic Low and Azores High results in a weaker pressure gradient which, in turn, induces weaker winds that then allows cold air to build in the eastern U.S. and expand southward. The general trends during -NAO:
We are currently in a rather strong -NAO condition. This is believable given the temperatures around here the last couple of weeks and the increase in snow chances recently. This coming storm will be the third chance for snow in that time period. So we have NAO going for us in this.
Next!
PNA
The Pacific/North American teleconnection pattern is a complicated series of pressure centers over the Pacific Ocean and North America. The positive condition of this teleconnection (+PNA) is characterized by high pressure over Hawaii and the intermountain region of North America and low pressure over the Aleutian Islands and the southeastern United States. One effect of +PNA is to cause low temperatures in the southeastern U.S. Interestingly, +PNA is mostly associated with El Niño and -PNA with La Niña. We're in a pretty significant La Niña right now but the PNA is currently positive. This also works in our favor.
So. These two teleconnections are working in our favor to suggest that the cold air will stick around and cause snow to be most of what falls.
Right now, the big questions are:
The only thing to do right now is to keep watching. Too many parameters have yet to come together for a high-confidence forecast. It's worth noting, though, that the Hydrometeorological Prediction Center - a group whose job is to monitor and forecast these types of storms - totally believes in the European model forecast. I leave you with an excerpt from this morning's discussion (Euro = ECMWF):
...EUROPEAN CENTRE GUIDANCE STAYS THE COURSE ON POWERFUL EAST
COAST STORM DECEMBER 26-27...
WILL STICK WITH THE CONSISTENT ECMWF FOR THE PRELIMINARY FRONTS
AND PRESSURES FOR DAYS 3-7. FOR THREE CONSECUTIVE MODEL
CYCLES...THE DETERMINISTIC ECMWF HAS INDICATED A
MASSIVE...DEEP...SLOW-MOVING CYCLONE HUGGING THE CAROLINA AND MID
ATLANTIC COASTS THE TWO DAYS FOLLOWING CHRISTMAS...
...THE EUROPEAN CENTRE GUIDANCE HAS THE ADVANTAGE IN DEVELOPMENTAL
SYSTEMS WITH ITS HIGH RESOLUTION...AND WITH DYNAMIC...DEVELOPMENTAL
SYSTEMS...THE EUROPEAN GUIDANCE SHOULD HAVE THE EDGE.
THE MOST UNUSUAL...AND THREATENING...ASPECT OF THE ECMWF FORECAST
IS THE RELATIVELY LOW LATITUDE AT WHICH THE STORM MATURES AND
STALLS/LOOPS. MORE TYPICALLY...SYSTEMS WILL CLOSE OFF AND HOVER
OVER THE WATERS SOUTH OF NOVA SCOTIA...AS WITH THE MOST RECENT
CYCLONE STILL AFFECTING DOWNEAST MAINE. THE ECMWF SHOWS THE
UPCOMING CIRCULATION COMING TO A HALT JUST EAST OF THE DELMARVA
PENINSULA...WHICH PUTS THE VERY POPULATED AREAS OF THE EASTERN MID
ATLANTIC...LONG ISLAND...AND SOUTHERN NEW ENGLAND IN A PROLONGED
PERIOD OF HIGH WINDS AND HEAVY PRECIPITATION...MOSTLY SNOW. THE
EXTRAORDINARY NEGATIVE NAO THIS MONTH COULD CERTAINLY SUPPORT SUCH
AN ANOMALY. EVEN BEFORE THE RAPID DEEPENING PHASE OF THE
SYSTEM...AREAS OF THE DEEP SOUTH FROM THE INTERIOR GULF COAST
STATES TO GEORGIA AND THE MOUNTAINS AND PIEDMONT OF THE CAROLINAS
MAY SEE A VERY RARE WHITE CHRISTMAS...AGAIN...CONSISTENT WITH THE
REMARKABLE DISRUPTION AT HIGH LATITUDES.
This is a good time to talk about the other things meteorologists look at to determine the likelihood of major winter storms. First, we'll look at something meteorologists and climatologists call "teleconnections".
The concept of teleconnections is based on the idea that the atmosphere is one large system. Air is essentially a fluid and it's modeled that way in terms of equations and concepts. Since it's a fluid, atmospheric anomalies like low pressure areas and high pressure areas, fronts, storms, etc can be thought of as blocks of styrofoam floating in a pool. If we throw a rock into the pool to create waves, these blocks will move. The movement of the blocks can be measured whether we threw the rock in the middle of all the blocks or somewhere at the other end of the pool. Further, the blocks will interact with each other - some of them will move in opposite directions essentially blocking the movement of others.
The atmosphere is a similar system. Simply put, the study of teleconnections is the study of linkages between weather patterns that occur in widely separated regions of the globe. Let's take a look at two of the teleconnections that most profoundly affect the type of weather we have here in the Mid-Atlantic during the winter.
NAO
NAO stands for "North Atlantic Oscillation" and it refers to the position and strength of two pressure centers in the North Atlantic. One is a low pressure area typically located near Iceland (the "Icelandic Low"). The other is an area of high pressure often located over the Azores in the eastern Atlantic (the "Azores High").
The strength and positioning of these pressure centers affects the alignment of the jet stream - a fast moving current of air - two branches of which control the nature of our weather. It's these branches, by the way, that we look to have phase when we're talking about strong winter storms.
Over time, climatologists and meteorologists have found that specific conditions of NAO cause specific patterns of weather over the globe. We'll concentrate on the eastern U.S. for obvious reasons.
POSITIVE NAO (+NAO)
The positive condition of NAO (shorthand: +NAO) occurs when the Icelandic Low and the Azores High are both stronger-than-normal. The stronger pressure centers cause a stronger pressure gradient (pressure changes more rapidly with distance from either center towards the other center). Pressure gradients cause stronger winds - in this case because the flow around the Icelandic Low (and all low pressure centers in the Northern Hemisphere for that matter) is counterclockwise and the flow around the Azores High (and all high pressure centers in the Northern Hemisphere) is clockwise, the increase in gradient causes an increased westerly wind.
This increase in wind essentially "drains" the cold air from the eastern U.S. into the Atlantic. Therefore, generally, the following is true during +NAO:
- Above-average temperatures in the eastern U.S.
- Higher precipitation (more likely to be rain because of the temperatures)
- Lower likelihood of snowstorms
NEGATIVE NAO (-NAO)
The negative condition of NAO (shorthand: -NAO) is, sensibly, the opposite of +NAO. A weaker-than-normal Icelandic Low and Azores High results in a weaker pressure gradient which, in turn, induces weaker winds that then allows cold air to build in the eastern U.S. and expand southward. The general trends during -NAO:
- Below-average temperatures in the eastern U.S.
- Lower precipitation
- Increased likelihood of snowstorms (essentially: lower precipitation but when it happens, significant winter weather).
We are currently in a rather strong -NAO condition. This is believable given the temperatures around here the last couple of weeks and the increase in snow chances recently. This coming storm will be the third chance for snow in that time period. So we have NAO going for us in this.
Next!
PNA
The Pacific/North American teleconnection pattern is a complicated series of pressure centers over the Pacific Ocean and North America. The positive condition of this teleconnection (+PNA) is characterized by high pressure over Hawaii and the intermountain region of North America and low pressure over the Aleutian Islands and the southeastern United States. One effect of +PNA is to cause low temperatures in the southeastern U.S. Interestingly, +PNA is mostly associated with El Niño and -PNA with La Niña. We're in a pretty significant La Niña right now but the PNA is currently positive. This also works in our favor.
So. These two teleconnections are working in our favor to suggest that the cold air will stick around and cause snow to be most of what falls.
Right now, the big questions are:
- When (or if) phasing will occur. Too early and the storm rides inland up the coast which would pull warmer air off the Atlantic and give us serious mixing issues thereby cutting down snow totals. No model that I've seen is forecasting an early phase. Too late and the storm doesn't intensify and "hook" up the coast until it's either past us to the north or out-to-sea. Some models do have this solution and predict a big snowstorm for the Northeast with lesser impact in the mid-Atlantic. A near-perfect phase happens in the Euro model forecast which shows a storm coming from the northern Gulf of Mexico, across the Southeast and hooking up the coast, quite slowly, dumping serious amounts of snow here and even south of here. The GFS model doesn't think phasing happens at all - the storm remains weak and goes out to sea.
- Once the phasing happens, what is the storm track and speed? Again, the Euro model has predicted a dream storm track and speed for us. Other models have variations on the theme with the GFS being the worst of them: an out-to-sea solution that takes most, if not all, of the snow with it.
The only thing to do right now is to keep watching. Too many parameters have yet to come together for a high-confidence forecast. It's worth noting, though, that the Hydrometeorological Prediction Center - a group whose job is to monitor and forecast these types of storms - totally believes in the European model forecast. I leave you with an excerpt from this morning's discussion (Euro = ECMWF):
...EUROPEAN CENTRE GUIDANCE STAYS THE COURSE ON POWERFUL EAST
COAST STORM DECEMBER 26-27...
WILL STICK WITH THE CONSISTENT ECMWF FOR THE PRELIMINARY FRONTS
AND PRESSURES FOR DAYS 3-7. FOR THREE CONSECUTIVE MODEL
CYCLES...THE DETERMINISTIC ECMWF HAS INDICATED A
MASSIVE...DEEP...SLOW-MOVING CYCLONE HUGGING THE CAROLINA AND MID
ATLANTIC COASTS THE TWO DAYS FOLLOWING CHRISTMAS...
...THE EUROPEAN CENTRE GUIDANCE HAS THE ADVANTAGE IN DEVELOPMENTAL
SYSTEMS WITH ITS HIGH RESOLUTION...AND WITH DYNAMIC...DEVELOPMENTAL
SYSTEMS...THE EUROPEAN GUIDANCE SHOULD HAVE THE EDGE.
THE MOST UNUSUAL...AND THREATENING...ASPECT OF THE ECMWF FORECAST
IS THE RELATIVELY LOW LATITUDE AT WHICH THE STORM MATURES AND
STALLS/LOOPS. MORE TYPICALLY...SYSTEMS WILL CLOSE OFF AND HOVER
OVER THE WATERS SOUTH OF NOVA SCOTIA...AS WITH THE MOST RECENT
CYCLONE STILL AFFECTING DOWNEAST MAINE. THE ECMWF SHOWS THE
UPCOMING CIRCULATION COMING TO A HALT JUST EAST OF THE DELMARVA
PENINSULA...WHICH PUTS THE VERY POPULATED AREAS OF THE EASTERN MID
ATLANTIC...LONG ISLAND...AND SOUTHERN NEW ENGLAND IN A PROLONGED
PERIOD OF HIGH WINDS AND HEAVY PRECIPITATION...MOSTLY SNOW. THE
EXTRAORDINARY NEGATIVE NAO THIS MONTH COULD CERTAINLY SUPPORT SUCH
AN ANOMALY. EVEN BEFORE THE RAPID DEEPENING PHASE OF THE
SYSTEM...AREAS OF THE DEEP SOUTH FROM THE INTERIOR GULF COAST
STATES TO GEORGIA AND THE MOUNTAINS AND PIEDMONT OF THE CAROLINAS
MAY SEE A VERY RARE WHITE CHRISTMAS...AGAIN...CONSISTENT WITH THE
REMARKABLE DISRUPTION AT HIGH LATITUDES.
Tuesday, December 21, 2010
Christmas Storm Still on Track
All of that lovely model agreement that was present a few days ago is gone. The models are back to their bickering with one camp (the GFS and its ensembles) preferring a no-phase, out-to-sea solution for the storm this weekend and the other camp (GGEM, Euro) pointing to a big coastal storm. In fact, the QPF (forecast precipitation amounts) in the Euro model have increased significantly for our area.
Overall, the models are slowing the storm both in onset and duration. This is significant for two reasons: the first is that it's likely any snow wouldn't begin until Christmas evening/night and the second is that a longer duration means greater moisture conveyance from the Atlantic Ocean which means more snow.
With the models divided between two storm track possibilities and even within the factions some in-fighting in regards to intensity and timing - it's far too early to pinpoint things precisely. I don't think I'll feel comfortable making a first call until Thursday.
The National Weather Service has no such luxury and they're sticking to their relatively-high probabilities of snow but they're also taking heed of the timing and have moved everything up about 12-16 hours.
So "stay tuned" is still the rule of this game.
I hope to make a longer post later tonight explaining the things I'm looking at as I try and interpret all of this model data and the potential for a major winter storm on Christmas Day into Sunday.
Overall, the models are slowing the storm both in onset and duration. This is significant for two reasons: the first is that it's likely any snow wouldn't begin until Christmas evening/night and the second is that a longer duration means greater moisture conveyance from the Atlantic Ocean which means more snow.
With the models divided between two storm track possibilities and even within the factions some in-fighting in regards to intensity and timing - it's far too early to pinpoint things precisely. I don't think I'll feel comfortable making a first call until Thursday.
The National Weather Service has no such luxury and they're sticking to their relatively-high probabilities of snow but they're also taking heed of the timing and have moved everything up about 12-16 hours.
So "stay tuned" is still the rule of this game.
I hope to make a longer post later tonight explaining the things I'm looking at as I try and interpret all of this model data and the potential for a major winter storm on Christmas Day into Sunday.
Sunday, December 19, 2010
Some Snow Showers Tuesday, All Eyes on Christmas Eve/Christmas Day
So...the weekend storm formed as forecast but moved out to sea before we could see much more than a few clouds. Not entirely surprising - a few models did call the storm properly back on Thursday but remember that we look for consensus from the models before making a forecast. The consensus just wasn't there until Friday.
Right now, the meteorology world is buzzing about Christmas Eve and Christmas Day. This is the best chance for a true White Christmas (which I'd define as "significant snow" actually falling on Christmas Day) in quite some time. The amazing thing is, in fact, the model consensus this far out. We're some 120 hours away from the forecast start of the storm and this time many of the models are fairly close in their forecast of storm track. It's actually a little unsettling to have this much agreement this far out - each successive run of the models simultaneously increases confidence and leaves you wondering if it can possibly last until the 24th.
But, we'll take it after the craziness of this past week.
For now, here's the GFS forecast total precipitation for Sunday, December 26. This means all precipitation that falls from storms up to and including 12/25.
This shows our area pretty well within the 0.50-1.00 inch zone of liquid precipitation which, when you use the 10-to-1 rule of thumb leads to a forecast of 5-10 inches. Based on the storm track and strength forecasts I've seen, this may be a bit conservative.
BUT.
It's 120 hours before any snow is going to fall. A whole LOT of stuff could happen between now and then. So I'll try to post a daily update as we go through the week. For many of us, it's an exciting event on a major Holiday. For some of us, it's a possible impediment to travel.
Stay tuned. Right now, for reference, the National Weather Service has a 40% chance of snow Friday night and a 50% chance on Christmas Day. Those are pretty high percentages for an event this far away.
Right now, the meteorology world is buzzing about Christmas Eve and Christmas Day. This is the best chance for a true White Christmas (which I'd define as "significant snow" actually falling on Christmas Day) in quite some time. The amazing thing is, in fact, the model consensus this far out. We're some 120 hours away from the forecast start of the storm and this time many of the models are fairly close in their forecast of storm track. It's actually a little unsettling to have this much agreement this far out - each successive run of the models simultaneously increases confidence and leaves you wondering if it can possibly last until the 24th.
But, we'll take it after the craziness of this past week.
For now, here's the GFS forecast total precipitation for Sunday, December 26. This means all precipitation that falls from storms up to and including 12/25.
This shows our area pretty well within the 0.50-1.00 inch zone of liquid precipitation which, when you use the 10-to-1 rule of thumb leads to a forecast of 5-10 inches. Based on the storm track and strength forecasts I've seen, this may be a bit conservative.
BUT.
It's 120 hours before any snow is going to fall. A whole LOT of stuff could happen between now and then. So I'll try to post a daily update as we go through the week. For many of us, it's an exciting event on a major Holiday. For some of us, it's a possible impediment to travel.
Stay tuned. Right now, for reference, the National Weather Service has a 40% chance of snow Friday night and a 50% chance on Christmas Day. Those are pretty high percentages for an event this far away.
Thursday, December 16, 2010
Evening Models Trending Snowier for the Weekend
You can't use the evening model runs alone to make a forecast but they can support a trend. And the trend is snow for the weekend.
Apparently, the National Weather Service is coming around to the idea as well. At 6 PM, they revised the forecast for our area to include a 50% chance of snow for Saturday night and Sunday.
So...game on. I'll be checking out the late-night runs tonight. Still 48 hours or so from the event - plenty of time for things to change for better or worse.
Early call would be: a decent chance of a "significant" winter storm Saturday night into Sunday. Storm track makes all the difference, as usual. Temperatures should easily support snow.
Apparently, the National Weather Service is coming around to the idea as well. At 6 PM, they revised the forecast for our area to include a 50% chance of snow for Saturday night and Sunday.
So...game on. I'll be checking out the late-night runs tonight. Still 48 hours or so from the event - plenty of time for things to change for better or worse.
Early call would be: a decent chance of a "significant" winter storm Saturday night into Sunday. Storm track makes all the difference, as usual. Temperatures should easily support snow.
This is What I Mean
Forecast for the weekend from the National Weather Service:
According to them, there's not enough model agreement to start worrying people about snow. I tend to agree.
But...
From the SAME National Weather Service, the precipitation forecast for Sunday:
Forget the models, even the humans aren't agreeing.
So we wait and watch. The evening models are running now but they're hardly ever used operationally (i.e. in service of an official forecast). Next big model runs late tonight - we'll see how they go.
Just a thought: the above map, if the forecast verifies, would put us well into the 5-inch range for snowfall on Sunday. Not the biblical snow of last year - but respectable all the same.
.SATURDAY...MOSTLY CLOUDY. HIGHS IN THE MID 30S. WEST WINDS AROUND 5 MPH...BECOMING SOUTHWEST AROUND 5 MPH IN THE AFTERNOON. .SATURDAY NIGHT...CLOUDY. LOWS IN THE LOWER 20S. .SUNDAY...PARTLY SUNNY. HIGHS IN THE MID 30S. .SUNDAY NIGHT...PARTLY CLOUDY. LOWS IN THE LOWER 20S.
According to them, there's not enough model agreement to start worrying people about snow. I tend to agree.
But...
From the SAME National Weather Service, the precipitation forecast for Sunday:
Forget the models, even the humans aren't agreeing.
So we wait and watch. The evening models are running now but they're hardly ever used operationally (i.e. in service of an official forecast). Next big model runs late tonight - we'll see how they go.
Just a thought: the above map, if the forecast verifies, would put us well into the 5-inch range for snowfall on Sunday. Not the biblical snow of last year - but respectable all the same.
Snow Falling Now, 1-3 inches, Models All Over the Place for the Weekend
Quick update. Snow has started falling in the DC Metro. If you're not seeing it, it's because you're in an office with no windows. Go outside!
Mostly light snow with occasional moderate bursts has been falling for a couple of hours now. Expect it to continue throughout the afternoon before winding up in the evening - maybe around 6 or 7.
The models still don't know what to do with the weekend storm. I've seen the local National Weather Service forecast add and remove snow chances for Saturday night about 5 times in the last 4 days. Almost all of the models show a low forming near the Gulf, almost all of them show it moving to the Carolinas and then some kind of model kegger breaks out and they all lose their minds. At least two show measurable snow for DC and the trend in the last two model runs is for a storm that is closer to us.
All we can do is watch. More to come when I know it.
Mostly light snow with occasional moderate bursts has been falling for a couple of hours now. Expect it to continue throughout the afternoon before winding up in the evening - maybe around 6 or 7.
The models still don't know what to do with the weekend storm. I've seen the local National Weather Service forecast add and remove snow chances for Saturday night about 5 times in the last 4 days. Almost all of the models show a low forming near the Gulf, almost all of them show it moving to the Carolinas and then some kind of model kegger breaks out and they all lose their minds. At least two show measurable snow for DC and the trend in the last two model runs is for a storm that is closer to us.
All we can do is watch. More to come when I know it.
Wednesday, December 15, 2010
Thursday Snow More Likely, Weekend Snow Less Likely
Quick post just to update the situation. Storm tomorrow (Thursday) looks to track far enough north so that we get some snow from it. 1-2 inches at the most.
The storm this weekend refuses to phase on 2 of the 4 model sets that I usually use. This leads to a solution where the developing low moves off the North Carolina coast and directly out to sea. Little or nothing for us.
Two other models - including the GFS - still have a coastal storm with somewhat more phasing present. Still need more model runs to determine the final story for the weekend.
Stay tuned.
The storm this weekend refuses to phase on 2 of the 4 model sets that I usually use. This leads to a solution where the developing low moves off the North Carolina coast and directly out to sea. Little or nothing for us.
Two other models - including the GFS - still have a coastal storm with somewhat more phasing present. Still need more model runs to determine the final story for the weekend.
Stay tuned.
Monday, December 13, 2010
Two Shots at Snow: Thursday and Late Weekend
Well, it's certainly cold enough to snow but we'll have to wait a bit for any moisture or possible storms to line themselves up. Still, the atmosphere is getting busy and there's plenty of energy to fuel storms that do form. Right now, in addition to some flurries and snow showers on Tuesday that shouldn't amount to much, we have two shots at significant snow in the next 7 days. The first is Thursday the second one is late in the weekend. That second storm is the one to watch closely.
But the storm Thursday could make a difference in the track of that second potential storm this weekend.
So let's talk about Thursday first.
A weak storm is on the models for Thursday moving northeastward from the southern U.S. and sliding just to the south of our area. The models have been fairly consistent with the track of the storm being to our south but a little inconsistent in how they describe the associated precipitation field.
Then the GFS and NAM runs late this afternoon showed a slightly more northern track which placed us in the precipitation area. According to these model, the atmospheric temperatures would be cold enough to support snow and indications are for about 1-2 inches of snowfall.
Still, the other models don't agree. They show accumulating snow for southern Virginia and North Carolina only.
I'm wary of trusting one model run - but here's the GFS:
As you can see, the track of the center is predicted by this run of the GFS to be more like along the VA/NC border instead of through central NC where it had been forecast earlier in the week. It bears watching but will almost certainly be a low-impact storm even if it does take a more northern track. You can see in the map above that over land, the maximum precipitation is between 0.10 and 0.25 inches which if we assume a 10:1 ratio (the rule of thumb) means about 1-2 inches.
Here's the crazy thing though: if you love snow (like I do), you should probably hope this doesn't happen.
I can see all of you snow lovers out there now:
Bear with me. Let's talk about the weekend storm potential first and then we'll come back to this crazy "if you love snow, you may not want it to snow Thursday" idea.
Long-range models have had a fairly large coastal storm for December 18-20 in our area for some time now. Ironic since, if it does happen, it will be right around one year to the day of the first huge snow in 2009. A big storm on the anniversary of the largest one-day December snowfall in Washington, DC history.
Anyway, when you're watching long-range models a few weeks out, here's the most important thing to look for until about a week before the predicted event: is the storm still there with each consecutive model run?
As far as that goes: so far so good.
Now that we're inside of a week away, we also start to look at storm track. Here's where the models are, to use technical terms, all over the damn place.
For now, we'll take the GFS as a fairly reliable long-range model. Here's what is shows for Saturday evening:
A developing storm moving from the northern Gulf of Mexico and across the southeastern US, emerging just off the coast of South Carolina. To the northwest, a solid push of cold air that shows the freezing line to our south. According to the GFS, still some 120 hours away from this forecast time, whatever is falling Saturday night will be snow.
6 hours later the GFS has the storm intensifying rapidly. Snow really gets going especially in central VA which looks to be the "bullseye" for this run of the GFS. Up here in the DC area we have less snow as the storm slides along the North Carolina coast - more eastward than northward.
6 hours more and the low is moving off the coast throwing some precipitation back into our area but the motion of the storm suggests lighter amounts than anything we saw on the 19th of December last year:
The east-northeast motion combined with the absence of any "bombing" or rapid intensification indicates that the GFS is predicting that the jet streams will not phase. I think I wrote about phasing last year but if not: here's a quick description:
Flow patterns in the upper atmosphere occur in sequences of waves. These waves can and do interact with one another sometimes in a way that is destructive, sometimes in a way that amplifies the original wave. When waves combine to form a larger wave, it is called "phasing". In meteorology, especially in the winter, we look for phasing of the northern and southern branches of the jet stream which tends to amplify existing storms. It's a tricky thing with respect to timing and it doesn't happen all that often, relatively speaking.
If the phase doesn't happen, you get a weak storm like the one predicted for Thursday. If it happens too early, a storm like the one above goes too close to the coast and we end up with rain. If it happens too late (like in the model runs above), it goes out to sea before it can get really wound up.
All that said: it's a good model run for us (remember "snow = good"). This far out, details like storm track and phasing are several runs away from being decided. We have the storm on the models still - that's one victory. The storm moves up the coast - that's another.
Now. What's bad about the Thursday storm?
Because of a complicated series of atmospheric interactions that have to do with something called the "Polar Vortex", the probability of an out-to sea solution for the December 19th storm (like the models show above) probably increases the further north the Thursday storm tracks. So if you want the potential for a big storm on Sunday, you want that Thursday storm to be as far to our south as possible.
Lots still to be worked out. We'll be closer to a solution for the Thursday storm tomorrow. I'll be keeping an eye on things!
Wouldn't a monster storm on the anniversary of a monster storm be cool?
Just to jog your memory: 12/19/2009:
But the storm Thursday could make a difference in the track of that second potential storm this weekend.
So let's talk about Thursday first.
A weak storm is on the models for Thursday moving northeastward from the southern U.S. and sliding just to the south of our area. The models have been fairly consistent with the track of the storm being to our south but a little inconsistent in how they describe the associated precipitation field.
Then the GFS and NAM runs late this afternoon showed a slightly more northern track which placed us in the precipitation area. According to these model, the atmospheric temperatures would be cold enough to support snow and indications are for about 1-2 inches of snowfall.
Still, the other models don't agree. They show accumulating snow for southern Virginia and North Carolina only.
I'm wary of trusting one model run - but here's the GFS:
As you can see, the track of the center is predicted by this run of the GFS to be more like along the VA/NC border instead of through central NC where it had been forecast earlier in the week. It bears watching but will almost certainly be a low-impact storm even if it does take a more northern track. You can see in the map above that over land, the maximum precipitation is between 0.10 and 0.25 inches which if we assume a 10:1 ratio (the rule of thumb) means about 1-2 inches.
Here's the crazy thing though: if you love snow (like I do), you should probably hope this doesn't happen.
I can see all of you snow lovers out there now:
 |
| "Crazy fool's done lost his freakin' mind!" |
Bear with me. Let's talk about the weekend storm potential first and then we'll come back to this crazy "if you love snow, you may not want it to snow Thursday" idea.
Long-range models have had a fairly large coastal storm for December 18-20 in our area for some time now. Ironic since, if it does happen, it will be right around one year to the day of the first huge snow in 2009. A big storm on the anniversary of the largest one-day December snowfall in Washington, DC history.
Anyway, when you're watching long-range models a few weeks out, here's the most important thing to look for until about a week before the predicted event: is the storm still there with each consecutive model run?
As far as that goes: so far so good.
Now that we're inside of a week away, we also start to look at storm track. Here's where the models are, to use technical terms, all over the damn place.
For now, we'll take the GFS as a fairly reliable long-range model. Here's what is shows for Saturday evening:
A developing storm moving from the northern Gulf of Mexico and across the southeastern US, emerging just off the coast of South Carolina. To the northwest, a solid push of cold air that shows the freezing line to our south. According to the GFS, still some 120 hours away from this forecast time, whatever is falling Saturday night will be snow.
6 hours later the GFS has the storm intensifying rapidly. Snow really gets going especially in central VA which looks to be the "bullseye" for this run of the GFS. Up here in the DC area we have less snow as the storm slides along the North Carolina coast - more eastward than northward.
6 hours more and the low is moving off the coast throwing some precipitation back into our area but the motion of the storm suggests lighter amounts than anything we saw on the 19th of December last year:
The east-northeast motion combined with the absence of any "bombing" or rapid intensification indicates that the GFS is predicting that the jet streams will not phase. I think I wrote about phasing last year but if not: here's a quick description:
Flow patterns in the upper atmosphere occur in sequences of waves. These waves can and do interact with one another sometimes in a way that is destructive, sometimes in a way that amplifies the original wave. When waves combine to form a larger wave, it is called "phasing". In meteorology, especially in the winter, we look for phasing of the northern and southern branches of the jet stream which tends to amplify existing storms. It's a tricky thing with respect to timing and it doesn't happen all that often, relatively speaking.
If the phase doesn't happen, you get a weak storm like the one predicted for Thursday. If it happens too early, a storm like the one above goes too close to the coast and we end up with rain. If it happens too late (like in the model runs above), it goes out to sea before it can get really wound up.
All that said: it's a good model run for us (remember "snow = good"). This far out, details like storm track and phasing are several runs away from being decided. We have the storm on the models still - that's one victory. The storm moves up the coast - that's another.
Now. What's bad about the Thursday storm?
Because of a complicated series of atmospheric interactions that have to do with something called the "Polar Vortex", the probability of an out-to sea solution for the December 19th storm (like the models show above) probably increases the further north the Thursday storm tracks. So if you want the potential for a big storm on Sunday, you want that Thursday storm to be as far to our south as possible.
Lots still to be worked out. We'll be closer to a solution for the Thursday storm tomorrow. I'll be keeping an eye on things!
Wouldn't a monster storm on the anniversary of a monster storm be cool?
Just to jog your memory: 12/19/2009:
Friday, December 10, 2010
Weekend Storm Likely to be Rain (Primarily) Then VERY Cold Next Week
After some model wavering, it seems a pretty good bet that except for relatively short periods at the beginning and the end, the Saturday-Sunday storm will be primarily rain.
Most of the models show a pretty strong low pressure system moving north from the south central US on Saturday. Clouds will increase rapidly Saturday afternoon but I think the precipitation holds off until evening. The temperatures will be warmer than we've seen, mid-40's, but they'll drop off in the evening - especially when the precipitation starts.
Because of that, I wouldn't be surprised if the precipitation started off as a bit of sleet which would change to rain as the night progressed and the low pressure center to our west draws warm air from the south over us as it moves northward. Temperatures overnight Saturday should stay in the mid-30's because of this storm track which means the second-worst thing winter can bring: cold rain.
Rain continues most of the day Sunday and because of the strength of the cold air behind the storm, I think it probably gets in here quickly enough to change our precipitation over to snow before it ends Sunday evening/night. It will be close but this is some serious cold air and it's going to be rushing in - so any precipitation that's left will change over.
Temperatures on Monday and Tuesday will spend the majority of the day in the 20's with Tuesday being the coldest day of the week, I think. Maybe not out of the 20's at all. By Wednesday we "warm" into the mid-30's.
Next chance for precipitation looks like late next week. Too soon to tell how much cold air is still around and how much moisture that system has.
Updates as I see 'em but for now, count on more rain than anything else from Saturday night through Sunday.
Most of the models show a pretty strong low pressure system moving north from the south central US on Saturday. Clouds will increase rapidly Saturday afternoon but I think the precipitation holds off until evening. The temperatures will be warmer than we've seen, mid-40's, but they'll drop off in the evening - especially when the precipitation starts.
Because of that, I wouldn't be surprised if the precipitation started off as a bit of sleet which would change to rain as the night progressed and the low pressure center to our west draws warm air from the south over us as it moves northward. Temperatures overnight Saturday should stay in the mid-30's because of this storm track which means the second-worst thing winter can bring: cold rain.
Rain continues most of the day Sunday and because of the strength of the cold air behind the storm, I think it probably gets in here quickly enough to change our precipitation over to snow before it ends Sunday evening/night. It will be close but this is some serious cold air and it's going to be rushing in - so any precipitation that's left will change over.
Temperatures on Monday and Tuesday will spend the majority of the day in the 20's with Tuesday being the coldest day of the week, I think. Maybe not out of the 20's at all. By Wednesday we "warm" into the mid-30's.
Next chance for precipitation looks like late next week. Too soon to tell how much cold air is still around and how much moisture that system has.
Updates as I see 'em but for now, count on more rain than anything else from Saturday night through Sunday.
Monday, December 6, 2010
Outlook for Winter 2010 and a Storm (probably rain) on Sunday
Welcome back to the blog! As crazy as it is during the Holidays, I'll try to start posting regularly again. Hopefully there will be something to post about!
Everybody wants to know the same thing: will this be a winter like 2009? Will it even snow? How much?
Those things are difficult to predict and long-range forecasting is really only slightly more accurate than throwing darts at index cards on the wall with snowfall totals and dates. But it is possible to make an educated guess and large-scale climate conditions make it a bit easier.
Almost everyone has heard of El Niño and has heard it blamed for everything from huge storms in the western United States to a string of car thefts in Kansas City. In reality, El Niño is defined as "warmer-than-normal sea surface temperatures in the western Pacific Ocean". This warm water causes a succession of atmospheric responses that affect the weather in the United States.
The exact opposite condition is known as La Niña. The cooler-than-normal sea surface temperatures in the western Pacific Ocean induce a low pressure zone there. Since everything in the atmosphere is interconnected, this unusually low pressure has side effects throughout North and South America.
Currently, there is a moderate La Niña occurring and it's forecast to strengthen a bit (which is an odd turn of phrase since it actually means that sea surface temperatures in the western Pacific will get a little colder) during the months of November-January.
So, what is the typical winter experience for the United States during a La Niña of the strength we're expecting?
In terms of precipitation:
First precipitation:
Green regions labeled "A" indicate above-average precipitation. The gradient lines are percentage probability of occurrence. Orange regions labeled "B" indicate below-average precipitation.
Again, keeping in mind this is an average, this forecast map suggests that more storms will pass to our west than to our east. If you like snow, you want coastal storms that pass offshore or move up the coast. Storms that pass to our west do two things in the winter: they bring rain or they bring ice. Both of those suck.
Now temperature:
Again, look at the trends. Washington isn't quite in the "above normal" 30% area but it's close. Generally, the forecast is for normal to slightly-above normal temperatures. That, in combination with the first map, suggests a winter where snow events are less frequent than last year and where many storms are deflected to the west of us.
This all adds up to the likelihood of a winter with normal to slightly below-normal snowfall in the DC area and temperatures that are normal to just above-normal.
About Sunday. A fairly large storm is expected to develop but most models are following the large-scale trends and suggest that the storm will pass to our west. This will probably lead to a very rainy and cold Sunday. It bears watching. If the track is further to the east, it could get much more interesting.
If this were 2009, I'd be excited about the possibility but, for now, it seems like this one is going to be wet and not frozen.
We'll see how the models trend as the week goes on.
Everybody wants to know the same thing: will this be a winter like 2009? Will it even snow? How much?
Those things are difficult to predict and long-range forecasting is really only slightly more accurate than throwing darts at index cards on the wall with snowfall totals and dates. But it is possible to make an educated guess and large-scale climate conditions make it a bit easier.
Almost everyone has heard of El Niño and has heard it blamed for everything from huge storms in the western United States to a string of car thefts in Kansas City. In reality, El Niño is defined as "warmer-than-normal sea surface temperatures in the western Pacific Ocean". This warm water causes a succession of atmospheric responses that affect the weather in the United States.
The exact opposite condition is known as La Niña. The cooler-than-normal sea surface temperatures in the western Pacific Ocean induce a low pressure zone there. Since everything in the atmosphere is interconnected, this unusually low pressure has side effects throughout North and South America.
Currently, there is a moderate La Niña occurring and it's forecast to strengthen a bit (which is an odd turn of phrase since it actually means that sea surface temperatures in the western Pacific will get a little colder) during the months of November-January.
So, what is the typical winter experience for the United States during a La Niña of the strength we're expecting?
In terms of precipitation:
- Above-average precipitation in the Pacific Northwest and Northern Rockies
- Above-average precipitation in the Ohio Valley
- Below-average precipitation in the south-central and southeastern U.S.
- "Normal" everywhere else
- Below-average temperatures in the coastal sections of the western states
- Above-average temperatures in the central and southern states
- "Normal" everywhere else
First precipitation:
Green regions labeled "A" indicate above-average precipitation. The gradient lines are percentage probability of occurrence. Orange regions labeled "B" indicate below-average precipitation.
Again, keeping in mind this is an average, this forecast map suggests that more storms will pass to our west than to our east. If you like snow, you want coastal storms that pass offshore or move up the coast. Storms that pass to our west do two things in the winter: they bring rain or they bring ice. Both of those suck.
Now temperature:
Again, look at the trends. Washington isn't quite in the "above normal" 30% area but it's close. Generally, the forecast is for normal to slightly-above normal temperatures. That, in combination with the first map, suggests a winter where snow events are less frequent than last year and where many storms are deflected to the west of us.
This all adds up to the likelihood of a winter with normal to slightly below-normal snowfall in the DC area and temperatures that are normal to just above-normal.
About Sunday. A fairly large storm is expected to develop but most models are following the large-scale trends and suggest that the storm will pass to our west. This will probably lead to a very rainy and cold Sunday. It bears watching. If the track is further to the east, it could get much more interesting.
If this were 2009, I'd be excited about the possibility but, for now, it seems like this one is going to be wet and not frozen.
We'll see how the models trend as the week goes on.
Monday, March 1, 2010
Another Miss
Seems like after about a month of every single storm hitting our area we have shifted the pattern somewhat.
This storm looks to stay to our south. I expect a rain/snow mix to start tomorrow evening, maybe changing to all snow overnight before ending Wednesday.
If the storm track continues as forecast, only minimal accumulations would occur. An inch perhaps.
It just doesn't appear that the jet streams will phase for this storm - which is what we need for the storm to turn northward and move along the coast.
This storm looks to stay to our south. I expect a rain/snow mix to start tomorrow evening, maybe changing to all snow overnight before ending Wednesday.
If the storm track continues as forecast, only minimal accumulations would occur. An inch perhaps.
It just doesn't appear that the jet streams will phase for this storm - which is what we need for the storm to turn northward and move along the coast.
Saturday, February 27, 2010
Tuesday - Wednesday
Tracking another storm system as it moves across the country, slides through the Southeast and then just off the Carolina coast on Tuesday night - Wednesday morning.
This one definitely bears watching although current model forecasts are for it to pass just to our south.
Here is the 84-hour GFS which is the forecast for Wednesday morning:
You can see the storm off the North Carolina coast. It moves essentially NE from here in later frames.
The storm is tossing back around 0.10-0.25 of precipitation - which is light: 1-3 inches of snow or so.
Two reasons this storm bears watching.
First: it's strong. A 989 mb low is fairly intense.
Second: this season, almost every storm this season has trended north and west from its position in model runs this far out.
If it moves more to the north and the west, this could be a big deal. For now, some snow Tuesday night into Wednesday morning seems likely - but light amounts also seem likely.
Don't cancel any plans - but stay tuned...
This one definitely bears watching although current model forecasts are for it to pass just to our south.
Here is the 84-hour GFS which is the forecast for Wednesday morning:
You can see the storm off the North Carolina coast. It moves essentially NE from here in later frames.
The storm is tossing back around 0.10-0.25 of precipitation - which is light: 1-3 inches of snow or so.
Two reasons this storm bears watching.
First: it's strong. A 989 mb low is fairly intense.
Second: this season, almost every storm this season has trended north and west from its position in model runs this far out.
If it moves more to the north and the west, this could be a big deal. For now, some snow Tuesday night into Wednesday morning seems likely - but light amounts also seem likely.
Don't cancel any plans - but stay tuned...
Thursday, February 25, 2010
Winter Precipitation Processes - Part 1: Pressure
I thought, while we have a break, that I would post a bit about the science of meteorology - especially as it pertains to the types of precipitation we see in the winter. Perhaps you have wondered why sometimes what falls from the sky is rain, sometimes sleet, sometimes snow. Maybe you've seen the nature of the snow that is falling change from tiny fluffy flakes (bad for snowballs) to wet, large flakes (great for snowballs) and wondered why that's happening.
And, maybe you don't care. If that's the case, I'd stop reading here.
But for the curious, let's talk for a moment about pressure. Hopefully at the end of this series, you'll start to see how this ties in to some of those forecast model maps I've posted in the past month or so.
ATMOSPHERIC PRESSURE AND UNITS
What is atmospheric pressure? Simply: it's the weight of the air in the column above any given point.
This can be measured in all sorts of ways. In the old days, somebody figured out that if you have put a pool of mercury into a reservoir and the reservoir extended upwards into a vacuum tube, you could measure how "heavy" the air above the mercury reservoir was by observing how high the mercury was forced up the tube.
For lack of anything better, this deflection was measured in inches - which is why you hear, even today, people referring to atmospheric pressure in "inches of mercury". Crazy and archaic, no?
Meteorologists switched to the metric (really, the "SI" - " le Système international d'unités" - or International Standard) system of units for a lot of things a long time ago. Atmospheric pressure is no exception.
The SI unit for pressure is a "Pascal". Sea level pressure is typically in the neighborhood of 100 kilopascals. Like other metric units, the prefix "kilo-" means "1000". So, we can say that sea level pressure is typically 100 X 1000 pascals. Or 100,000 pascals.
In the early 1900's, some dude named William Napier Shaw decided that nobody wanted to deal with a unit of measurement where the base value was going to be so high. Cleverly, he created a substitute unit called a bar. Since he was British, it's unlikely this has anything to do with him having a pint at the time he came up with it. Otherwise, we'd be walking around with pressure units called "pubs". But I digress...
A bar is equal to 100 kilopascals or, conveniently, typical sea level pressure.
Pressure decreases as you go higher into the atmosphere. Simple enough: there's less air between you and the top of the atmosphere (where air eventually runs out and the vacuum of space approaches), therefore the air column weighs less, therefore the pressure is lower.
Because so much of our weather is defined by what happens in the atmosphere, meteorologists knew they would have to use a fractional version of the bar to still be able to have manageable values as they spoke of different pressures in the atmosphere. So, they settled on the millibar as the unit of pressure measurement. The prefix "milli-" means 1/1000 for SI units. So, a millibar (abbreviated "mb") is 1/1000 of a bar and that, in turn, means that 1000 millibars is the same as 1 bar - or typical surface pressure.
Anytime you see pressure measurements on a forecast model map, they will always be in millibars. Some TV meteorologists have even started to introduce this standard to their audiences (although you'll still always see the pressure given in "inches of mercury").
CONSTANT PRESSURE SURFACES
Pressure is very important to what mets call the "sensible weather" - or the weather that you experience outside while walking down the street. In meteorology, we have defined what are called "constant pressure surfaces" as a way of understanding and modeling atmospheric processes that cause our weather.
Imagine, for a moment, an "idealized" or "perfect" atmosphere. This mythic world is one where the pressure at sea level is 1010 millibars (mb) and pressure decreases in a way that is perfectly proportional to how high you are in the atmosphere.
Now, let's say you are hooked onto a big balloon and you go straight up into the atmosphere. You're armed with a notepad and pencil, thermometer, a barometer, an altimeter (tells you how high up you are) and, hopefully, a parachute. Here's what you're asked to do by a meteorologist:
- "When your barometer reads 1000 mb, take a measurement of altitude and temperature".
- "Take the same measurements when your barometer reads: 850 mb, 700 mb, 500 mb, 250 mb and, if you can, 100 mb" (we'll assume, in the mythic world that we're discussing, that you don't need air to breathe and that you're impervious to cold).
Shortly after takeoff, your barometer reads 1000 mb. You take a measurement of 100 meters (300 feet) and a temperature of 15 C (59 F).
As you rise into the atmosphere on your balloon, you find that your barometer reads 850 mb at about 1500 meters (5000 feet) and that it's getting cold, about 5 C (41 F).
Up you go. The barometer reads 700 mb and you take a measurement: 3000 meters (10000 feet) and -5 C (23 F)
You continue taking measurements at the pressure markers the meteorologist mentioned. As you do, you notice that your balloon is getting larger and larger. Why? The pressure in the balloon has remained constant from sea level. But the pressure outside of the balloon is getting lower and lower. So the balloon is expanding outward. A short time after your barometer measures 100 mb and you take your measurements, your balloon pops. You release your parachute, fall gently to the surface and hand the meteorologist your pad which shows something like this:
Meteorologists think of the atmosphere as a series of constant pressure levels. In a perfect atmosphere, these would be standardized, i.e. your barometer would always read 850 mb at 5000 feet.
But in the real world, that's not true. And it's the difference in the height of those pressure levels, the amount of space between the pressure levels (referred to as "thickness") and the temperatures that make a difference.
For the purposes of winter precipitation, we'll concern ourselves with pressures from the surface up to 500 mb.
Next post: how the heights and thicknesses change and what that means for us on the ground.
Oh - and guess what? In my little example above, you were essentially a weather balloon. These are released by National Weather Service upper air stations (Dulles/Sterling is our closest one) twice a day (morning and evening) and record the information from the example as well as other information (dewpoint, winds, etc) on their way up. More about this in the next post, too!
And, maybe you don't care. If that's the case, I'd stop reading here.
But for the curious, let's talk for a moment about pressure. Hopefully at the end of this series, you'll start to see how this ties in to some of those forecast model maps I've posted in the past month or so.
ATMOSPHERIC PRESSURE AND UNITS
What is atmospheric pressure? Simply: it's the weight of the air in the column above any given point.
This can be measured in all sorts of ways. In the old days, somebody figured out that if you have put a pool of mercury into a reservoir and the reservoir extended upwards into a vacuum tube, you could measure how "heavy" the air above the mercury reservoir was by observing how high the mercury was forced up the tube.
For lack of anything better, this deflection was measured in inches - which is why you hear, even today, people referring to atmospheric pressure in "inches of mercury". Crazy and archaic, no?
Meteorologists switched to the metric (really, the "SI" - " le Système international d'unités" - or International Standard) system of units for a lot of things a long time ago. Atmospheric pressure is no exception.
The SI unit for pressure is a "Pascal". Sea level pressure is typically in the neighborhood of 100 kilopascals. Like other metric units, the prefix "kilo-" means "1000". So, we can say that sea level pressure is typically 100 X 1000 pascals. Or 100,000 pascals.
In the early 1900's, some dude named William Napier Shaw decided that nobody wanted to deal with a unit of measurement where the base value was going to be so high. Cleverly, he created a substitute unit called a bar. Since he was British, it's unlikely this has anything to do with him having a pint at the time he came up with it. Otherwise, we'd be walking around with pressure units called "pubs". But I digress...
A bar is equal to 100 kilopascals or, conveniently, typical sea level pressure.
Pressure decreases as you go higher into the atmosphere. Simple enough: there's less air between you and the top of the atmosphere (where air eventually runs out and the vacuum of space approaches), therefore the air column weighs less, therefore the pressure is lower.
Because so much of our weather is defined by what happens in the atmosphere, meteorologists knew they would have to use a fractional version of the bar to still be able to have manageable values as they spoke of different pressures in the atmosphere. So, they settled on the millibar as the unit of pressure measurement. The prefix "milli-" means 1/1000 for SI units. So, a millibar (abbreviated "mb") is 1/1000 of a bar and that, in turn, means that 1000 millibars is the same as 1 bar - or typical surface pressure.
Anytime you see pressure measurements on a forecast model map, they will always be in millibars. Some TV meteorologists have even started to introduce this standard to their audiences (although you'll still always see the pressure given in "inches of mercury").
CONSTANT PRESSURE SURFACES
Pressure is very important to what mets call the "sensible weather" - or the weather that you experience outside while walking down the street. In meteorology, we have defined what are called "constant pressure surfaces" as a way of understanding and modeling atmospheric processes that cause our weather.
Imagine, for a moment, an "idealized" or "perfect" atmosphere. This mythic world is one where the pressure at sea level is 1010 millibars (mb) and pressure decreases in a way that is perfectly proportional to how high you are in the atmosphere.
Now, let's say you are hooked onto a big balloon and you go straight up into the atmosphere. You're armed with a notepad and pencil, thermometer, a barometer, an altimeter (tells you how high up you are) and, hopefully, a parachute. Here's what you're asked to do by a meteorologist:
- "When your barometer reads 1000 mb, take a measurement of altitude and temperature".
- "Take the same measurements when your barometer reads: 850 mb, 700 mb, 500 mb, 250 mb and, if you can, 100 mb" (we'll assume, in the mythic world that we're discussing, that you don't need air to breathe and that you're impervious to cold).
Shortly after takeoff, your barometer reads 1000 mb. You take a measurement of 100 meters (300 feet) and a temperature of 15 C (59 F).
As you rise into the atmosphere on your balloon, you find that your barometer reads 850 mb at about 1500 meters (5000 feet) and that it's getting cold, about 5 C (41 F).
Up you go. The barometer reads 700 mb and you take a measurement: 3000 meters (10000 feet) and -5 C (23 F)
You continue taking measurements at the pressure markers the meteorologist mentioned. As you do, you notice that your balloon is getting larger and larger. Why? The pressure in the balloon has remained constant from sea level. But the pressure outside of the balloon is getting lower and lower. So the balloon is expanding outward. A short time after your barometer measures 100 mb and you take your measurements, your balloon pops. You release your parachute, fall gently to the surface and hand the meteorologist your pad which shows something like this:
| PRESSURE (mb) | APPROXIMATE ALTITUDE | TEMPERATURE |
| 1000 | 100 meters (300 feet) | 15 C (59 F) |
| 850 | 1500 meters (5000 feet) | 5 C (41 F) |
| 700 | 3000 meters (10000 feet) | -5 C (23F) |
| 500 | 5000 meters (18000 feet) | -20 C (-4 F) |
| 300 | 9000 meters (30000 feet) | -45 C (-49 F) |
| 200 | 12000 meters (40000 feet) | -55 C (-67 F) |
| 100 | 16000 meters (53000 feet) | -56 C (-69 F) |
Meteorologists think of the atmosphere as a series of constant pressure levels. In a perfect atmosphere, these would be standardized, i.e. your barometer would always read 850 mb at 5000 feet.
But in the real world, that's not true. And it's the difference in the height of those pressure levels, the amount of space between the pressure levels (referred to as "thickness") and the temperatures that make a difference.
For the purposes of winter precipitation, we'll concern ourselves with pressures from the surface up to 500 mb.
Next post: how the heights and thicknesses change and what that means for us on the ground.
Oh - and guess what? In my little example above, you were essentially a weather balloon. These are released by National Weather Service upper air stations (Dulles/Sterling is our closest one) twice a day (morning and evening) and record the information from the example as well as other information (dewpoint, winds, etc) on their way up. More about this in the next post, too!
Looks Like the Models Were Right...the First Time
The storm formed as expected, it snowed for a few hours after about 2 AM this morning and then dry air raced in behind and stopped the precipitation.
With the proximity of the storm, it still looks like on-and-off snow showers are possible - especially later tonight as the storm strengthens and retrogrades.
But the winds will be the principal focus.
This one was mostly a miss. Just to our north in Pennsylvania it's snowing this morning and will likely keep doing so - especially further north into the NYC area.
Let's see how far this thing comes back to the south as it gets stuck later today.
For now: sunshine! (where's my smiling sunshine graphic with sunglasses?)
With the proximity of the storm, it still looks like on-and-off snow showers are possible - especially later tonight as the storm strengthens and retrogrades.
But the winds will be the principal focus.
This one was mostly a miss. Just to our north in Pennsylvania it's snowing this morning and will likely keep doing so - especially further north into the NYC area.
Let's see how far this thing comes back to the south as it gets stuck later today.
For now: sunshine! (where's my smiling sunshine graphic with sunglasses?)
Wednesday, February 24, 2010
Downgraded to WWA - But Keep an Eye
So the NWS downgraded our Winter Storm Watch to a Winter Weather Advisory this afternoon, and I agree.
The models came west with the storm but the trend seems to have stopped for now. This will be a glancing blow for our area with the potential for 2-4 inches of snow with the 2-inch amounts in the DC area south and slightly more as you go north.
Just to clarify:
By NWS definition at this time of year, a "Winter Storm Watch (or Warning)" is defined as either the potential for at least 5 inches of snow in a 12-hour period or 7 inches of snow in a 24-hour period.
A Winter Weather Advisory is for less than 4 inches of snow.
This remains a complicated forecast - even here on the brink of the storm arrival in the DC area. A slight jog to the west - and I mean slight...perhaps 30 miles or fewer - and it changes forecast snow totals a great deal.
Also - even though the atmospheric temperatures support snow, it's likely that at least initially, surface temperatures will be too warm for it to stick. So we'll be "wasting" some of the snow to melting.
Finally, sometime tomorrow the flow in the atmosphere will grind to a halt and the storm will slow down dramatically - maybe even retrograde (a process where the storm actually heads back south and west instead of north and east as the typical flow would suggest).
As this happens, the storm will send QPF back over the Mid Atlantic and the northeast. How close? Do we get those bands of snow tomorrow evening and overnight? If so, then I can see the DC area getting closer to that 4-inch mark. If not, then 2 inches sounds more believable.
Will be watching the models to see how this plays out in the runs tonight.
The models came west with the storm but the trend seems to have stopped for now. This will be a glancing blow for our area with the potential for 2-4 inches of snow with the 2-inch amounts in the DC area south and slightly more as you go north.
Just to clarify:
By NWS definition at this time of year, a "Winter Storm Watch (or Warning)" is defined as either the potential for at least 5 inches of snow in a 12-hour period or 7 inches of snow in a 24-hour period.
A Winter Weather Advisory is for less than 4 inches of snow.
This remains a complicated forecast - even here on the brink of the storm arrival in the DC area. A slight jog to the west - and I mean slight...perhaps 30 miles or fewer - and it changes forecast snow totals a great deal.
Also - even though the atmospheric temperatures support snow, it's likely that at least initially, surface temperatures will be too warm for it to stick. So we'll be "wasting" some of the snow to melting.
Finally, sometime tomorrow the flow in the atmosphere will grind to a halt and the storm will slow down dramatically - maybe even retrograde (a process where the storm actually heads back south and west instead of north and east as the typical flow would suggest).
As this happens, the storm will send QPF back over the Mid Atlantic and the northeast. How close? Do we get those bands of snow tomorrow evening and overnight? If so, then I can see the DC area getting closer to that 4-inch mark. If not, then 2 inches sounds more believable.
Will be watching the models to see how this plays out in the runs tonight.
It's Gonna Be Close
So most all of the models shifted westward overnight with the storm track. However, that doesn't even remotely mean they all agree on the storm track.
Most have the track still fairly far off the coast. It makes this a complicated forecast because the cutoff line for significant and light snow (or what the NWS would call "warning-level" and "advisory-level" snow) will be very close to our area.
A slight shift in track eastward means less snow for us. A little further westward places us "safely" in the warning-level snow.
I need to see the model runs from this morning.
For now, expect the precipitation to start tonight, probably as a period of rain or mixed rain and snow. Eventually this will change over to all snow.
This will be a powerful storm once it's done strengthening over the Atlantic. So whether it snows 5 inches or 2 inches, it will be windy tomorrow afternoon and tomorrow night.
Most have the track still fairly far off the coast. It makes this a complicated forecast because the cutoff line for significant and light snow (or what the NWS would call "warning-level" and "advisory-level" snow) will be very close to our area.
A slight shift in track eastward means less snow for us. A little further westward places us "safely" in the warning-level snow.
I need to see the model runs from this morning.
For now, expect the precipitation to start tonight, probably as a period of rain or mixed rain and snow. Eventually this will change over to all snow.
This will be a powerful storm once it's done strengthening over the Atlantic. So whether it snows 5 inches or 2 inches, it will be windy tomorrow afternoon and tomorrow night.
Tuesday, February 23, 2010
Later Models Holding to the Westward Shift
The NAM still is significantly further offshore than the GFS in terms of storm track - but it, too, has come west from its earlier position.
This lends more credence to the 3-5 inch accumulation amounts forecast by the GFS earlier today.
Still watching...
This lends more credence to the 3-5 inch accumulation amounts forecast by the GFS earlier today.
Still watching...
It's Bigger...Possibly
Very difficult forecast situation developing for Wednesday - Thursday.
A low pressure system forming off the coast on Wednesday was forecast by most models to just brush the area with light precipitation before moving out to sea. Run after run of the models had confirmed this situation.
Until this morning.
This morning's run of the GFS shows a sudden shift to the west with the storm track, closer to the coast and in a position to bring us more snow than originally thought.
There is likely to be a very sharp cutoff to the areas receiving significant snow and "not much" snow. For now, we're in a zone where 4 inches are possible, more to our north and east.
It's very important that successive model runs stay with this idea and not to gravitate to one particular run. However, it is a fairly marked change from previous runs and will have to be watched closely.
Here's the NAM total precip forecast. When looking at this consider: the GFS is even further west with the track than the NAM which slides all of this further onshore here in the Mid Atlantic:
Note how close the blue is...just in northeastern Maryland. This particular run of the NAM also absolutely crushes NYC. We'll see.
I'll keep you up-to-date as the new model runs come in.
For now, there's a chance of significant snow in our area Wednesday night through Thursday.
A low pressure system forming off the coast on Wednesday was forecast by most models to just brush the area with light precipitation before moving out to sea. Run after run of the models had confirmed this situation.
Until this morning.
This morning's run of the GFS shows a sudden shift to the west with the storm track, closer to the coast and in a position to bring us more snow than originally thought.
There is likely to be a very sharp cutoff to the areas receiving significant snow and "not much" snow. For now, we're in a zone where 4 inches are possible, more to our north and east.
It's very important that successive model runs stay with this idea and not to gravitate to one particular run. However, it is a fairly marked change from previous runs and will have to be watched closely.
Here's the NAM total precip forecast. When looking at this consider: the GFS is even further west with the track than the NAM which slides all of this further onshore here in the Mid Atlantic:
Note how close the blue is...just in northeastern Maryland. This particular run of the NAM also absolutely crushes NYC. We'll see.
I'll keep you up-to-date as the new model runs come in.
For now, there's a chance of significant snow in our area Wednesday night through Thursday.
Monday, February 22, 2010
This Week: Rain, Some Snow
Two storms this week. The first one goes by today and tonight bringing rain throughout the area. Just not any cold air to work with - even though we are still in below normal temperatures. It's not going to be a huge rain producer, but it should continue the melting process that has been going on for the past few days.
The second storm - currently forecast to arrive Wednesday-Thursday is a bit more interesting.
Right now, it looks like a storm will approach from the south and a low pressure area will form off the coast and move up into the Northeast. At this point, it looks like the magical phase will happen too far north for us (or Philly or probably even New York City) to get serious snow out of this storm. But it does seem to be a big storm for mid to upstate New York and places in northern New England where winter has been a dud so far.
For us, I think some snow arrives Wednesday afternoon - possibly starting as rain depending on how early it gets started - but changing to snow overnight Wednesday into Thursday. Amounts seem light - at this point an inch or two seems to be the agreement between the forecast models.
If it turns into anything bigger, I'll let everyone know.
All meteorological eyes (at least in the mid Atlantic) are on the time period between about March 3-5 where long range models have a big storm affecting our area. Much can change between now and then, but that seems to be our next (last?) best shot at a major snowstorm in the DC area.
The second storm - currently forecast to arrive Wednesday-Thursday is a bit more interesting.
Right now, it looks like a storm will approach from the south and a low pressure area will form off the coast and move up into the Northeast. At this point, it looks like the magical phase will happen too far north for us (or Philly or probably even New York City) to get serious snow out of this storm. But it does seem to be a big storm for mid to upstate New York and places in northern New England where winter has been a dud so far.
For us, I think some snow arrives Wednesday afternoon - possibly starting as rain depending on how early it gets started - but changing to snow overnight Wednesday into Thursday. Amounts seem light - at this point an inch or two seems to be the agreement between the forecast models.
If it turns into anything bigger, I'll let everyone know.
All meteorological eyes (at least in the mid Atlantic) are on the time period between about March 3-5 where long range models have a big storm affecting our area. Much can change between now and then, but that seems to be our next (last?) best shot at a major snowstorm in the DC area.
Saturday, February 20, 2010
Rain...
Quick post because I don't have a lot of time but: here's how it looks for now.
The storm on Monday-Tuesday should be mostly rain for our area.
The storm on Thursday-Friday gives us a better chance of snow.
I'll post a more detailed analysis when I have the chance.
The storm on Monday-Tuesday should be mostly rain for our area.
The storm on Thursday-Friday gives us a better chance of snow.
I'll post a more detailed analysis when I have the chance.
Wednesday, February 17, 2010
Keep an Eye on Next Week
The forecast models have found the crack pipe and I can't get it away from them.
Currently, they are woofing about three possible upcoming storm situations:
February 22-23 (Monday-Tuesday)
February 26-27 (Friday-Saturday)
March 1-2 (Monday-Tuesday)
Now, based on climatological history, it gets more and more difficult for us to get buried by snowstorms as we move into March. The sun angle is higher, our days are longer ([our] nights are stronger than moooooonshine... +1 if you get the reference) and we're headed for Spring.
However, the change of seasons also brings a volatility to the atmosphere so if we can get enough cold air around, it certainly can snow and has. It snowed last year (for one of the few times - remember what that was like?) at the end of the first week in March and one need only look back to 1993 to find evidence that the atmosphere can produce big snowstorms for us after February.
Back to the models...
They can't seem to lock on to a solution this time. At first they couldn't even decide whether or not we were looking at three separate events. But it seems like at least the number of events is resolved, if not the tracks.
First, the possibility next Monday-Tuesday. Looks like a large storm, it also looks like we could have temperature issues which could cause this to be more of a "wintry mix" kind of storm - but those are details to be worked out later.
I got the GFS off the pipe long enough to show you these:
First, at hour 126 (sometime Monday evening) it shows the storm moving into southern West Virginia and spreading precipitation ahead of it. The blue line is the freezing line at 850mb (about 5000 feet up) which is a pretty good indicator of precipitation type (liquid or frozen). In this case, we're just barely north of the line which means this precipitation is forecast to be snow or sleet.
By 6 hours later (Hour 132), the low in West Virginia transfers its energy to the developing low off the coast of North Carolina. In response to the initial low, warmer air has surged over the region and changes our precipitation over to rain (we're south of the line...but just).
Another 6 hours and the coastal low has taken over completely. As it deepens, the counterclockwise flow brings cold air back over us and changes whatever precipitation is left back to snow before the low moves out to sea.
So - this is one to watch. Especially because we spend so much time right on the edge of the rain/snow line. If this were the definitive track for the storm we could start talking about details but since some models have the low pressure in West Virginia hanging on for the duration (which would mean rain for us...a bad thing considering how much snow is still out there) and others have the coastal low much stronger and moving right up the coast instead of out to sea (more snow for us), we just have to wait for more information.
Stay tuned!
Currently, they are woofing about three possible upcoming storm situations:
February 22-23 (Monday-Tuesday)
February 26-27 (Friday-Saturday)
March 1-2 (Monday-Tuesday)
Now, based on climatological history, it gets more and more difficult for us to get buried by snowstorms as we move into March. The sun angle is higher, our days are longer ([our] nights are stronger than moooooonshine... +1 if you get the reference) and we're headed for Spring.
However, the change of seasons also brings a volatility to the atmosphere so if we can get enough cold air around, it certainly can snow and has. It snowed last year (for one of the few times - remember what that was like?) at the end of the first week in March and one need only look back to 1993 to find evidence that the atmosphere can produce big snowstorms for us after February.
Back to the models...
They can't seem to lock on to a solution this time. At first they couldn't even decide whether or not we were looking at three separate events. But it seems like at least the number of events is resolved, if not the tracks.
First, the possibility next Monday-Tuesday. Looks like a large storm, it also looks like we could have temperature issues which could cause this to be more of a "wintry mix" kind of storm - but those are details to be worked out later.
I got the GFS off the pipe long enough to show you these:
First, at hour 126 (sometime Monday evening) it shows the storm moving into southern West Virginia and spreading precipitation ahead of it. The blue line is the freezing line at 850mb (about 5000 feet up) which is a pretty good indicator of precipitation type (liquid or frozen). In this case, we're just barely north of the line which means this precipitation is forecast to be snow or sleet.
By 6 hours later (Hour 132), the low in West Virginia transfers its energy to the developing low off the coast of North Carolina. In response to the initial low, warmer air has surged over the region and changes our precipitation over to rain (we're south of the line...but just).
Another 6 hours and the coastal low has taken over completely. As it deepens, the counterclockwise flow brings cold air back over us and changes whatever precipitation is left back to snow before the low moves out to sea.
So - this is one to watch. Especially because we spend so much time right on the edge of the rain/snow line. If this were the definitive track for the storm we could start talking about details but since some models have the low pressure in West Virginia hanging on for the duration (which would mean rain for us...a bad thing considering how much snow is still out there) and others have the coastal low much stronger and moving right up the coast instead of out to sea (more snow for us), we just have to wait for more information.
Stay tuned!
Sunday, February 14, 2010
Still North and West
Models have the storm tracking even further north and west which puts us in the warm sector and in the dry slot by late evening.
So - I think the forecast pattern will hold. Snow in the morning changing to a mix of rain and snow (or possibly all rain) changing back to snow by evening before ending.
1-3 inches should be the rule. Around 1-2 inches for us in the immediate DC area.
Should be a nothing storm - compared to what we've been experiencing lately.
After this - nothing on the long range models until the end of the month (2/26 or so). We'll see what happens!
So - I think the forecast pattern will hold. Snow in the morning changing to a mix of rain and snow (or possibly all rain) changing back to snow by evening before ending.
1-3 inches should be the rule. Around 1-2 inches for us in the immediate DC area.
Should be a nothing storm - compared to what we've been experiencing lately.
After this - nothing on the long range models until the end of the month (2/26 or so). We'll see what happens!
Saturday, February 13, 2010
Latest Models Further North
Which puts us in the warmer sector and creates a situation likely to be:
Some light snow in the morning on Monday mixing with rain in the afternoon, possibly all rain for a time and then changing back to snow late in the afternoon.
Based on the modeled position I think 2-4 inches with the 2" happening closer to the DC area, 4" further west.
I should mention that almost every snow-producing storm has shifted southward at the last moment. In the past, this has made the difference between, say, 20 and 24 inches. Not really noticeable.
A shift south here and we're back in the cold sector and, therefore, snowier.
Probably won't really know for sure until tomorrow night. I'm tempted to drop it to 1-3 inches based on tonight's models but experience has taught me not to follow any forecast model run too closely.
So I'll hold at 2-4 and we'll see what future model runs bring.
Some light snow in the morning on Monday mixing with rain in the afternoon, possibly all rain for a time and then changing back to snow late in the afternoon.
Based on the modeled position I think 2-4 inches with the 2" happening closer to the DC area, 4" further west.
I should mention that almost every snow-producing storm has shifted southward at the last moment. In the past, this has made the difference between, say, 20 and 24 inches. Not really noticeable.
A shift south here and we're back in the cold sector and, therefore, snowier.
Probably won't really know for sure until tomorrow night. I'm tempted to drop it to 1-3 inches based on tonight's models but experience has taught me not to follow any forecast model run too closely.
So I'll hold at 2-4 and we'll see what future model runs bring.
A Tough Little Storm
So this one will be a difficult forecast - hopefully things will begin to clear up later tonight or tomorrow.
For now we have this situation:
A low pressure system is coming south from Canada. These little storms are sometimes referred to as "Alberta Clippers" sometimes as "Manitoba Maulers" depending on their point of origin. Track is everything with these types of storms. If it tracks further to the south of DC, we see all snow. If it tracks further to the north, we run the risk of snow mixing with sleet or even rain.
Right now, the models have this tracking just barely south of the DC area. So, as it so often turns out, I believe the further north and west of the big cities you go, the more the precipitation that falls will be all snow.
To complicate things, these storms are usually very dry as they are continental in origin. This means that the storm's principle source of moisture (and, as it turns out, energy) will be the Atlantic Ocean. How quickly it intensifies and how close we are to the low center will make the difference between a little snow and some good accumulations.
This is not a similar scenario to the last two. This is a small storm with the potential to drop about 3-5 inches on the DC metro. Can these storms produce more snow? Sure. They have been known to produce 10-12 inches sometimes. But, for now, it looks like our snow totals will be compromised some by mixing and some by the strength of the storm when it arrives.
I'll go with 3-5 inches for now. More model runs are coming tonight which will hopefully lock on to the track.
It looks like it will snow, but not anything like what we've seen around here recently.
For now we have this situation:
A low pressure system is coming south from Canada. These little storms are sometimes referred to as "Alberta Clippers" sometimes as "Manitoba Maulers" depending on their point of origin. Track is everything with these types of storms. If it tracks further to the south of DC, we see all snow. If it tracks further to the north, we run the risk of snow mixing with sleet or even rain.
Right now, the models have this tracking just barely south of the DC area. So, as it so often turns out, I believe the further north and west of the big cities you go, the more the precipitation that falls will be all snow.
To complicate things, these storms are usually very dry as they are continental in origin. This means that the storm's principle source of moisture (and, as it turns out, energy) will be the Atlantic Ocean. How quickly it intensifies and how close we are to the low center will make the difference between a little snow and some good accumulations.
This is not a similar scenario to the last two. This is a small storm with the potential to drop about 3-5 inches on the DC metro. Can these storms produce more snow? Sure. They have been known to produce 10-12 inches sometimes. But, for now, it looks like our snow totals will be compromised some by mixing and some by the strength of the storm when it arrives.
I'll go with 3-5 inches for now. More model runs are coming tonight which will hopefully lock on to the track.
It looks like it will snow, but not anything like what we've seen around here recently.
Friday, February 12, 2010
More Snow...
Because we apparently still haven't been properly chastised for spending the last 4 years saying things like:
(in your best old-person voice)
"...I remember back in the 80's when we got lots of blizzards. But not anymore! It never snows like that here - and it might never snow like that again..."
"...I have Polaroids of back when it used to snow so that my kids can see what snow looked like..."
...first glance at the morning models for the upcoming storm look more promising for snowfall here in the DC Metro for Monday afternoon - Tuesday morning. An initial guess would be a 3-5 inch snowfall although storms of this particular type and the path it is taking have produced surprises in the past.
Here's the latest GFS, forecasting 84 hours from now (Monday night - Tuesday morning) clearly showing a deepening low just off the coast tossing back fairly respectable QPF over our area.
Variables will be track (as usual) and how quickly the storm intensifies off the coast before moving away.
(in your best old-person voice)
"...I remember back in the 80's when we got lots of blizzards. But not anymore! It never snows like that here - and it might never snow like that again..."
"...I have Polaroids of back when it used to snow so that my kids can see what snow looked like..."
...first glance at the morning models for the upcoming storm look more promising for snowfall here in the DC Metro for Monday afternoon - Tuesday morning. An initial guess would be a 3-5 inch snowfall although storms of this particular type and the path it is taking have produced surprises in the past.
Here's the latest GFS, forecasting 84 hours from now (Monday night - Tuesday morning) clearly showing a deepening low just off the coast tossing back fairly respectable QPF over our area.
Variables will be track (as usual) and how quickly the storm intensifies off the coast before moving away.
Wednesday, February 10, 2010
Don't Look Now
An Alberta Clipper is headed this way for Monday. It should be smaller than these last two storms, but it's currently forecast to take a track south of us which is a favorable track for getting the biggest bang-for-the-buck from systems that are generally dry compared to these coastal storms.
I haven't had time to look at any forecast models yet. I've got to shovel. Or go and drink a beer.
Or something.
I haven't had time to look at any forecast models yet. I've got to shovel. Or go and drink a beer.
Or something.
Where's My Toast Graphic?
There goes the all-time snowiest winter record. 2009-2010 now becomes the snowiest winter in the history of recording in Washington, DC.
At all three major airports.
I'll let the National Weather Service explain:
At all three major airports.
I'll let the National Weather Service explain:
...PRELIMINARY ALL-TIME SEASONAL SNOWFALL RECORDS SET AT THE THREE MAJOR CLIMATE SITES IN THE BALTIMORE-WASHINGTON AREA... AS OF 2 PM TODAY...WITH THE 9.8 INCH TWO-DAY SNOWFALL TOTAL MEASURED AT RONALD REAGAN WASHINGTON NATIONAL AIRPORT...THE SEASONAL SNOWFALL TOTAL IN WASHINGTON DC STANDS AT 54.9 INCHES. THIS WOULD BREAK THE PREVIOUS ALL-TIME SEASONAL SNOWFALL RECORD FOR WASHINGTON DC OF 54.4 INCHES SET IN THE WINTER OF 1898-99. OFFICIAL SNOWFALL RECORDS FOR WASHINGTON DC DATE BACK 126 YEARS TO 1884. AS OF 1 PM TODAY...WITH THE 11.9 INCH TWO-DAY SNOWFALL TOTAL MEASURED AT BALTIMORE/WASHINGTON INTERNATIONAL THURGOOD MARSHALL AIRPORT...THE SEASONAL SNOWFALL TOTAL IN BALTIMORE STANDS AT 72.3 INCHES. THIS WOULD BREAK THE PREVIOUS ALL-TIME SEASONAL SNOWFALL RECORD FOR BALTIMORE OF 62.5 INCHES SET IN THE WINTER OF 1995-96. OFFICIAL SNOWFALL RECORDS FOR BALTIMORE DATE BACK 118 YEARS TO 1893. FINALLY...AS OF YESTERDAY...THIS YEARS SEASONAL SNOWFALL TOTAL AT DULLES INTERNATIONAL AIRPORT STOOD AT 63.5 INCHES. THIS WOULD BREAK THE PREVIOUS SEASONAL SNOWFALL RECORD OF 61.9 INCHES SET IN 1995-96. AS OF 1 PM THIS AFTERNOON...THE TWO-DAY SNOWFALL TOTAL AT DULLES IS 8.5 INCHES...WHICH WOULD MAKE THIS YEARS SEASONAL SNOWFALL TOTAL 72.0 INCHES. OFFICIAL SNOWFALL RECORDS FOR DULLES DATE BACK 48 YEARS TO 1962.
Sitting and Spinning
If you look at the radar image in the last post (which is automatically updated because it links directly to the source), you can see that the area of precipitation has moved very little. As the storm off the coast strengthens (meteorologists also say "deepens" because the pressure is decreasing) it is actually retrograding slightly.
This means that the low is moving against the prevailing flow and is either stationary or even moving slightly south and west.
This has kept bands of precipitation from just south of DC northward for the past few hours and looks like it will continue into the mid-afternoon.
9 inches or so here in south Arlington although measuring has become a guessing game due to the wind.
This means that the low is moving against the prevailing flow and is either stationary or even moving slightly south and west.
This has kept bands of precipitation from just south of DC northward for the past few hours and looks like it will continue into the mid-afternoon.
9 inches or so here in south Arlington although measuring has become a guessing game due to the wind.
A For-Real Blizzard
Sometimes, the National Weather Service issues a Blizzard Warning because we in the Washington area are not used to heavy snow and strong wind. Rarely in the past has the weather actually reached blizzard conditions. Officially, those are:
- Sustained wind speeds of 35 mph or more, or frequent gusts to 35 mph or greater;
- Considerable falling and/or blowing snow that reduces visibility frequently to 1/4 mile or less.
Today, it's the real thing.
Winds are gusting to 43 MPH this morning, snow is falling at the rate of 1 inch per hour, whiteout conditions are occurring, National Airport is reporting visibilities of 1/10 mile, the air temperature has fallen all morning into the lower 20's and the windchill values are in the single digits: 6 at National Airport.
Have we moved to Buffalo?
The storm formed as the models promised and at around 2:45 AM, the sleet and light freezing drizzle that fell for a few hours last night changed back to snow. At around 6 this morning, the storm began to bomb off the coast and the snow started falling heavily around the area.
I have 8 inches here and reports are of generally 8-12 inches around the area. Measurement of snow becomes next to impossible in conditions like these - even for people not at National Airport.
The radar shows the back edge of the precipitation advancing slowly to the northeast. Still a few hours of moderate to heavy snow before things start to let up a bit. The winds will continue all day however.
Stay inside if you can!
What a winter.
So We're in the Dry Slot...Again
A lull in the precipitation here in the DC Metro area as the storm to the west shifts its energy to the storm off the coast. In between, a gap in the precipitation field has developed through most of Virginia although in the last few frames I see some snow is redeveloping in central VA.
I have just shy of 4 inches on the ground and we will see how this new storm develops and what track it takes.
A few things of note that were not forecast by the models:
- the dry slot has worked its way much further north than forecast. It actually pushed north of DC when it was originally forecast to be significantly to the southwest
- mixed precipitation and a changeover to sleet from snow has occurred much further north than forecast. DC was not expected to changeover and Baltimore was considered safely in the snow range. In the last hours, areas north of Baltimore have reported sleet and areas south of Baltimore have reported sleet or no precipitation for periods this evening as the dry slot worked in.
- the transfer of energy from the storm in the midwest to the storm off the coast has not happened as quickly or efficiently as forecast.
All of these suggest that the final snow totals will be somewhat less-than-forecast. A lot depends now on the next few hours and how the storm off the coast forms.
But I'm going to bed. The storm will do what it will do. I'll take another look later this morning.
Good night!
I have just shy of 4 inches on the ground and we will see how this new storm develops and what track it takes.
A few things of note that were not forecast by the models:
- the dry slot has worked its way much further north than forecast. It actually pushed north of DC when it was originally forecast to be significantly to the southwest
- mixed precipitation and a changeover to sleet from snow has occurred much further north than forecast. DC was not expected to changeover and Baltimore was considered safely in the snow range. In the last hours, areas north of Baltimore have reported sleet and areas south of Baltimore have reported sleet or no precipitation for periods this evening as the dry slot worked in.
- the transfer of energy from the storm in the midwest to the storm off the coast has not happened as quickly or efficiently as forecast.
All of these suggest that the final snow totals will be somewhat less-than-forecast. A lot depends now on the next few hours and how the storm off the coast forms.
But I'm going to bed. The storm will do what it will do. I'll take another look later this morning.
Good night!
Tuesday, February 9, 2010
Sleet Takes Over
A mixture of sleet and snow is falling in Arlington right now, I suspect more sleet than snow to the south and less sleet to the north. This should all change back over to snow after midnight when the storm off the coast really gets going.
The National Weather Service has officially decreased the forecast snow totals for the DC metro area to 8-12 inches.
The heaviest snow should fall tomorrow morning and with it, another serious concern: winds. It's possible that winds could gust to 45 mph tomorrow - which could cause big problems with trees and power lines.
The National Weather Service has officially decreased the forecast snow totals for the DC metro area to 8-12 inches.
The heaviest snow should fall tomorrow morning and with it, another serious concern: winds. It's possible that winds could gust to 45 mph tomorrow - which could cause big problems with trees and power lines.
Oh and...
you know how this morning the Winter of 2009-2010 ranked 3rd on the list of all-time snowiest winters ever officially recorded in Washington, DC? In second place was the Winter of 1995-1996 which featured a total of 46". As of this evening, that record is toast. Move over 1995-1996, we're in second place all-time with this season. And we still have a realistic chance of reaching #1 all-time...perhaps not in this storm (considering who will be officially measuring...see earlier posts for my rants about that)...but probably before this winter is over.
Moderate to Heavy Snow Continues
I have 3.25 inches here in Arlington since about 4:30 PM when the first flakes came down. Snow didn't really get started until after dark. Radar shows more bands of heavy snow - particularly south of DC and headed northward.
Light Snow Has Started, Storm is Forming
Reports of some intermittent light snow across the area, mixed with a bit of sleet to the south, as expected.
The map below is a plot of how many millibars the barometric pressure has changed in the last 3 hours. You can use this plot to see that in the southern states, the pressures are rising - indicating that storminess is moving out of that area.
You can also see that in the Great Lakes region, pressures are falling. That's the initial area of low pressure that is transferring its energy to the real storm which will be forming off the coast of the Carolinas. Evidence of the storm forming can be seen in this chart. Notice that the pressure is falling, quite rapidly, just off the coast.
I still think the NWS is on the high side of the forecast amounts. Their amended afternoon forecast is 10-14 inches. I'll stick with 8-12 here around Washington, more to the north and east, less to the south. We'll see how things go once the storm gets going later tonight.
The radar is filling in and snow should become fairly steady in the next few hours.
There is the chance that the snow will mix with sleet even as far north as Washington, DC - but I expect it to be more of an issue to the south of town.
Evening models coming in now. I'll let you know what I see.
The map below is a plot of how many millibars the barometric pressure has changed in the last 3 hours. You can use this plot to see that in the southern states, the pressures are rising - indicating that storminess is moving out of that area.
You can also see that in the Great Lakes region, pressures are falling. That's the initial area of low pressure that is transferring its energy to the real storm which will be forming off the coast of the Carolinas. Evidence of the storm forming can be seen in this chart. Notice that the pressure is falling, quite rapidly, just off the coast.
I still think the NWS is on the high side of the forecast amounts. Their amended afternoon forecast is 10-14 inches. I'll stick with 8-12 here around Washington, more to the north and east, less to the south. We'll see how things go once the storm gets going later tonight.
The radar is filling in and snow should become fairly steady in the next few hours.
There is the chance that the snow will mix with sleet even as far north as Washington, DC - but I expect it to be more of an issue to the south of town.
Evening models coming in now. I'll let you know what I see.
I'll Leave It Alone
For now, I'll leave my expectations at:
4-8 south of the city
8-12 from DC northward to Baltimore
12+ north of Baltimore
But based on what I've been seeing for the last hour or so, even that might be too high. Things could change, in either direction, once the storm actually forms. But for now, this looks like a big hit for our friends in New England - particularly north of New York City and into the Boston area.
We'll see how things evolve. I've left the official wording of the National Weather Service warning above. They are still calling for 12-16 inches in the DC area. I'll let you know if that changes.
4-8 south of the city
8-12 from DC northward to Baltimore
12+ north of Baltimore
But based on what I've been seeing for the last hour or so, even that might be too high. Things could change, in either direction, once the storm actually forms. But for now, this looks like a big hit for our friends in New England - particularly north of New York City and into the Boston area.
We'll see how things evolve. I've left the official wording of the National Weather Service warning above. They are still calling for 12-16 inches in the DC area. I'll let you know if that changes.
This is A Really Difficult Forecast - Maybe Less Snow?
Here we are, a few hours from the onset of precipitation and things aren't very clear yet. This sort of storm setup has so many things that could make a forecast go wrong.
This scenario is very complex and I don't envy the official forecasters at the National Weather Service who have to make a final call that will affect so many people.
I have some concerns based on the latest models and radar.
The newest model runs show the heaviest axis of precipitation shifting somewhat to the north and east. They also show the low pressure area itself forming closer to the coast - which could give us problems with mixed precipitation. Finally, some of the upper-level patterns suggest that we in the DC area could experience issues with the dry slot again (like we did early Saturday morning which cut down on accumulations somewhat).
On the other hand, the storm that is expected to develop will be intense and slow-moving. Models continue to put out a lot of QPF for us which increases the confidence for a major snowstorm.
I'm going to watch things develop for a bit. In no way am I suggesting that this storm will not happen. There's just much more uncertainty about exactly how all of the elements in the atmosphere will come together - a process that's happening even as I type this.
But I think there's a chance we get less snow than is forecast. Right now, the forecast is the same as it was last night:
8-12 inches from about Fredericksburg north to DC
12-16 inches from DC to Baltimore and points west
16+ from Baltimore northward to Philly
but after looking at things this morning, I think those amounts may need to be lowered some in the DC area and southward. Maybe to an average of 10 inches in the immediate DC metro, less south, more north.
In fact, if I had to do it right now, I'd just shift those values northward and put in a new range for south of DC. So:
4-8 from about Fredericksburg north to DC
8-12 from DC to Baltimore and points west
12-16 from Baltimore northward to Philly
More information when I have it!
This scenario is very complex and I don't envy the official forecasters at the National Weather Service who have to make a final call that will affect so many people.
I have some concerns based on the latest models and radar.
The newest model runs show the heaviest axis of precipitation shifting somewhat to the north and east. They also show the low pressure area itself forming closer to the coast - which could give us problems with mixed precipitation. Finally, some of the upper-level patterns suggest that we in the DC area could experience issues with the dry slot again (like we did early Saturday morning which cut down on accumulations somewhat).
On the other hand, the storm that is expected to develop will be intense and slow-moving. Models continue to put out a lot of QPF for us which increases the confidence for a major snowstorm.
I'm going to watch things develop for a bit. In no way am I suggesting that this storm will not happen. There's just much more uncertainty about exactly how all of the elements in the atmosphere will come together - a process that's happening even as I type this.
But I think there's a chance we get less snow than is forecast. Right now, the forecast is the same as it was last night:
8-12 inches from about Fredericksburg north to DC
12-16 inches from DC to Baltimore and points west
16+ from Baltimore northward to Philly
but after looking at things this morning, I think those amounts may need to be lowered some in the DC area and southward. Maybe to an average of 10 inches in the immediate DC metro, less south, more north.
In fact, if I had to do it right now, I'd just shift those values northward and put in a new range for south of DC. So:
4-8 from about Fredericksburg north to DC
8-12 from DC to Baltimore and points west
12-16 from Baltimore northward to Philly
More information when I have it!
Monday, February 8, 2010
Quick Note
Just a quick note for anyone who didn't know. You can click on any image I post and see it full-size.
And One More Graph to Illustrate This Winter
Thanks to Jessica for finding this. It shows various snow statistics for the DC area, including that the average snowfall is 15 inches...which we eclipsed in December. In one storm. To this chart you can now add the most recent storm which, officially at National Airport, recorded 17.8 inches. Ridiculously low (inaccurate and not-representative), but official is official.
A Record Winter
With this coming storm, we have a chance to break the all-time snow record for DC in one winter. Even with National Airport as the location of record, we are at 45 inches for the year. That's third behind
1898-1899: 54.4"
1995-1996: 46"
That 1995-1996 record is toast. Starting tomorrow, probably around 5 PM - but with a lot of February and all of March to go, that 1898-1899 record is reachable. It might even be gone by Wednesday.
And while this has been an amazing winter (we have 10 more inches, offcially, this year at Reagan National Airport than in the previous 4 winters...combined) it's very interesting to note that, based on climatological cycles, it may not have been too difficult to assume that this winter would be snowy (image below from the Washington Post).
We'll talk about why we seem to be in the bullseye this year and a bit about teleconnections (like El Niño) when we get some breathing time between storms...
1898-1899: 54.4"
1995-1996: 46"
That 1995-1996 record is toast. Starting tomorrow, probably around 5 PM - but with a lot of February and all of March to go, that 1898-1899 record is reachable. It might even be gone by Wednesday.
And while this has been an amazing winter (we have 10 more inches, offcially, this year at Reagan National Airport than in the previous 4 winters...combined) it's very interesting to note that, based on climatological cycles, it may not have been too difficult to assume that this winter would be snowy (image below from the Washington Post).
We'll talk about why we seem to be in the bullseye this year and a bit about teleconnections (like El Niño) when we get some breathing time between storms...
Another Big Storm
So this one started out looking average and winds up looking pretty impressive. Overall, this storm would be considered an unusually-strong winter storm for the DC area, except that we're kind of getting used to that now, aren't we?
Based on the model outputs and QPF's that are shown in the forecast, it seems like we're in line for at least a foot of snow from DC northward. As a first call:
8-12 inches from about Fredericksburg north to DC
12-16 inches from DC to Baltimore and points west
16+ from Baltimore northward to Philly
The NWS is going slightly lower, some TV mets slightly higher.
There is a somewhat lower-than-normal confidence with this forecast because although the components are in place, the storm has yet to form.
But look for snow to spread across the area beginning in the early afternoon and increasing in intensity. I think the majority of the snow will fall overnight tomorrow but we might have as much as 2-4 inches on the ground by nightfall tomorrow.
Mixed precipitation should not be an issue from DC northward but may be a factor south of DC - especially the further south you go.
I'm still going over model data so more posts to come. Stay safe, everyone!
Based on the model outputs and QPF's that are shown in the forecast, it seems like we're in line for at least a foot of snow from DC northward. As a first call:
8-12 inches from about Fredericksburg north to DC
12-16 inches from DC to Baltimore and points west
16+ from Baltimore northward to Philly
The NWS is going slightly lower, some TV mets slightly higher.
There is a somewhat lower-than-normal confidence with this forecast because although the components are in place, the storm has yet to form.
But look for snow to spread across the area beginning in the early afternoon and increasing in intensity. I think the majority of the snow will fall overnight tomorrow but we might have as much as 2-4 inches on the ground by nightfall tomorrow.
Mixed precipitation should not be an issue from DC northward but may be a factor south of DC - especially the further south you go.
I'm still going over model data so more posts to come. Stay safe, everyone!
It's Actually the Precipitation Fields
The low track looks fairly consistent from model to model, but the strength of the low that develops after phasing is intense. The models have responded by extending the precipitation field southward. New model runs coming in now...but the midday runs aren't usually the most accurate.
Still, most models have us in the 8-12 inch range and this is looking more likely.
Oh - and just as I was typing this, the NWS has put us under a Winter Storm Warning.
In the text it warns of 10-20 inches...the lower end of that range happens the further south you go. So in Arlington, that argues for 10-14 inches.
Still trying to sort it all out - but right now, significant snowfall - at least 8-10 inches and possibly a foot or more seems probable for a time period beginning tomorrow afternoon and ending in the afternoon on Wednesday.
QPF Plot for this storm. I expect ratios slightly above the 10:1 rule...
Still, most models have us in the 8-12 inch range and this is looking more likely.
Oh - and just as I was typing this, the NWS has put us under a Winter Storm Warning.
In the text it warns of 10-20 inches...the lower end of that range happens the further south you go. So in Arlington, that argues for 10-14 inches.
Still trying to sort it all out - but right now, significant snowfall - at least 8-10 inches and possibly a foot or more seems probable for a time period beginning tomorrow afternoon and ending in the afternoon on Wednesday.
QPF Plot for this storm. I expect ratios slightly above the 10:1 rule...
More Snow Coming
I'm checking out the morning/afternoon models. Things look a little stronger and a little further south at first glance. Many models now suggesting at least 8 inches of snow, some hint at a foot.
I'll post more once I've had the chance to look them over...
I'll post more once I've had the chance to look them over...
The Game is Still On
And I don't mean the Super Bowl.
The nighttime run of the GFS still holds on to the timing of the storm. This is going to be a big deal for southern New England - from Philly to New York City. They could get a storm much like the one that just hit us this past weekend.
But it won't be negligible for us - not if the timing and positioning continues the way it looks like it will. Here's the GFS in hour 72 showing the total QPF over us for this storm:
This puts us between 0.75 and 1.00 inches of QPF. To the GFS, that translates to the following snow forecast:
Overall, the models seem to show us in the 6-10 inch range with higher totals northward to Baltimore - where they could see another foot.
Unlike the previous storm where the energy could be tracked as it moved across the country, this storm won't even form until the phasing takes place. So even though it's only a couple of days away, there's still a fair amount of uncertainty with this storm. A slight shift to the south puts us in the heavier snow. A shift to the north and we have less snow and more mixing issues (with sleet or rain). And all of it depends on the timing of the jet stream phase.
*whew*
So - for now, it looks as though snow will start falling sometime in the second half of Tuesday - maybe late afternoon. The heaviest snow will fall overnight and into the morning on Wednesday and then continue with decreasing intensity through Wednesday afternoon.
6-10 inches seems like a good forecast at this point. And we are under a Winter Storm Watch which, for the NWS, means the risk of 5 or more inches of snow.
I'll be watching the forecast models...
Good night.
The nighttime run of the GFS still holds on to the timing of the storm. This is going to be a big deal for southern New England - from Philly to New York City. They could get a storm much like the one that just hit us this past weekend.
But it won't be negligible for us - not if the timing and positioning continues the way it looks like it will. Here's the GFS in hour 72 showing the total QPF over us for this storm:
This puts us between 0.75 and 1.00 inches of QPF. To the GFS, that translates to the following snow forecast:
Overall, the models seem to show us in the 6-10 inch range with higher totals northward to Baltimore - where they could see another foot.
Unlike the previous storm where the energy could be tracked as it moved across the country, this storm won't even form until the phasing takes place. So even though it's only a couple of days away, there's still a fair amount of uncertainty with this storm. A slight shift to the south puts us in the heavier snow. A shift to the north and we have less snow and more mixing issues (with sleet or rain). And all of it depends on the timing of the jet stream phase.
*whew*
So - for now, it looks as though snow will start falling sometime in the second half of Tuesday - maybe late afternoon. The heaviest snow will fall overnight and into the morning on Wednesday and then continue with decreasing intensity through Wednesday afternoon.
6-10 inches seems like a good forecast at this point. And we are under a Winter Storm Watch which, for the NWS, means the risk of 5 or more inches of snow.
I'll be watching the forecast models...
Good night.
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