Tuesday, January 25, 2011
GFS Total Precipitation
A quick map: GFS total precip for the storm. We're well in the 1-inch range which is something along the lines of 7-9 inches of snow equivalent. We'll lose some (.25?) to slop, some more (.25?) to melting when the changeover happens and that leaves around .50 - .75 of liquid equivalent or around 4-6 inches with the bad snow ratios I expect. Usually there's a 10 to 1 rule (1 inch of liquid precipitation is 10 inches of snow). If the snow is exceptionally dry (cold atmosphere) the ratios can go up to 15 or 20 to 1. With the borderline atmosphere tomorrow, I'd expect less than 10 to 1.
Things Get MUCH More Interesting
Washington DC and points north and west are now under a Winter Storm Watch for tomorrow night through Thursday morning. I imagine the rest of us will be under some kind of advisory eventually as well.
Latest models in with 4+ inches for our area - the average forecast is 6 and the extremes are 2-3" on the low side to over 10 on the high side. I think 4-6 is a good first call.
The time of the changeover is crucial. Looks like "early evening" for us here and if so, there will still be plenty of energy and moisture to work with. Because of the preceding slop (rain/snow/sleet) we will probably lose a couple of inches to melting (snow doesn't stick to water) but the snow could fall rather heavily tomorrow night into the early hours of Thursday morning.
Still a lot of data to ponder. Thermal profiles are still borderline and any little shift in track or timing could change the whole thing - one way or the other.
For now - a bit of snow in the morning, a bit of slop in the afternoon and some significant snow tomorrow night seems to be the trend.
Still watching the data come in - more to come.
Latest models in with 4+ inches for our area - the average forecast is 6 and the extremes are 2-3" on the low side to over 10 on the high side. I think 4-6 is a good first call.
The time of the changeover is crucial. Looks like "early evening" for us here and if so, there will still be plenty of energy and moisture to work with. Because of the preceding slop (rain/snow/sleet) we will probably lose a couple of inches to melting (snow doesn't stick to water) but the snow could fall rather heavily tomorrow night into the early hours of Thursday morning.
Still a lot of data to ponder. Thermal profiles are still borderline and any little shift in track or timing could change the whole thing - one way or the other.
For now - a bit of snow in the morning, a bit of slop in the afternoon and some significant snow tomorrow night seems to be the trend.
Still watching the data come in - more to come.
Things Get More Interesting
The general forecast continues essentially on track with the thinking from yesterday: a period of snow to start tomorrow morning, mostly a rain/snow mix (or all rain) during the day Wednesday and a changeover back to snow tomorrow night as cold air gets pulled into the storm.
But...
Remember how the NAM yesterday showed a fairly good chunk of moisture passing through after the temps get cold enough for snow tomorrow night? That idea has been picked up by many of the models and it looks like there could be some accumulation of snow tomorrow once the changeover takes place - if indeed the upper level energy has enough moisture to work with.
I'm checking out the 12Z models as they come in today.
Just a heads up though: if this changeover takes place early enough and if enough cold air can hang around and if there's enough moisture associated with the upper level energy (if, if, if) - we could be looking at some heavy snow tomorrow night. Some of the models are saying some ridiculous stuff right now. I'm trying to sort it out.
More soon.
But...
Remember how the NAM yesterday showed a fairly good chunk of moisture passing through after the temps get cold enough for snow tomorrow night? That idea has been picked up by many of the models and it looks like there could be some accumulation of snow tomorrow once the changeover takes place - if indeed the upper level energy has enough moisture to work with.
I'm checking out the 12Z models as they come in today.
Just a heads up though: if this changeover takes place early enough and if enough cold air can hang around and if there's enough moisture associated with the upper level energy (if, if, if) - we could be looking at some heavy snow tomorrow night. Some of the models are saying some ridiculous stuff right now. I'm trying to sort it out.
More soon.
Monday, January 24, 2011
It's Been One of Those Winters
So I've held off posting until now in the hopes that, given time, the picture of what is to happen with this mid-week storm would clarify. I hoped that the models would arrive at a consensus and that the biggest problem with the forecast: the absence of a strong cold air source during the start to middle of the storm - would also present itself more clearly and allow me to make solid conclusions.
But - nothing like that has happened. Nothing.
To have a good "Miller A" Nor'easter - the type of snowstorm that is likely to dump 6+ inches of snow (or more) on our area, you need a few ingredients:
- A persistent cold air source. This is provided by a strong high pressure over New England of the sort we've had for the past few days. The problem? It's moving east. The further east it moves the more the clockwise flow around the high stops funneling cold air from the north and begins to funnel warmer air from the east and southeast. So this is something of a question right now.
- A surface low pressure system near or over the Gulf Coast. This we have - no problem here.
- An upper-level low moving out of the Plains states and phasing with the surface low somewhere in the southeastern United States. Most all of the models agree this will happen although there are variations with respect to timing. All of the important models have the phase happening well before the storm enters our region. This is a good thing. You'll remember that the majority of the problems we've had with storms and forecasts this year have been due to the difficulty in predicting the exact time and location of the phase and that the phasing has happened almost overhead which has caused dryslot problems. Not this time.
- Favorable storm track. Too close to the area and you have mixing problems: sleet or even rain mixes in with the snow and keeps accumulations down. Too far off the coast and you get missed by the juiciest part of the storm and that keeps snow totals down. The models are still in disagreement about the final track except that all the important ones agree that the track will be somewhere east of here. In this case especially - with the cold air and temperatures being borderline at best - a small change in storm track will probably mean a huge change in what we experience.
So - where are we with this thing?
To begin with, the storm forecast positions are slowing down. What was once a Tuesday night into Wednesday storm now looks like a Wednesday morning - Thursday morning storm. This has been a trend for the last couple of days so I wouldn't be surprised to see things slow down even further as we get closer to the event.
Now lets look at some of the big players in the model world and where they stack up.
We'll begin with the forecast frame for 30 hours from now (36 hours from 12Z - the run time of the model) or about 7 PM EST Tuesday night. Here are both the NAM and the GFS for comparison at the same time frame. The NAM will be on the top, the GFS on the bottom. Remember that you can click on any image in this blog to see the full-size version.
I want you to keep an eye on two things. First, notice that the position of the low (NAM just south of the Florida Panhandle, GFS right over it) is pretty similar in each model as is the precipitation field. Heavy rains are breaking out from Georgia into South Carolina and severe thunderstorms are on their way into Florida. Clearly there is a lot of energy and moisture available to this storm.
But notice, also, that blue line that runs from the Missouri/Arkansas area northeastward towards the mid Atlantic. That's the 0-degree isotherm and it can be used to approximate where the precipitation would remain snow (north and west of that line) and where it might change to rain (south and east of that line).
Notice that the NAM is quite warm - forecasting our entire area to be above freezing even at this frame while the GFS is slightly colder and actually paints the 0-degree line right over our area.
Onward to 42 hours - around 1 AM Wednesday morning.
The storm is strengthening and is moving to the northeast. Heavy precipitation is falling in Florida and along the Georgia and Carolina coasts. We still have no precipitation in either model though it's beginning to break out just to our south in the GFS version of events.
Again the low position is pretty similar in both models but the precipitation field is more inland on the NAM bringing heavier rains to interior Georgia and South Carolina. For us, note the 0-degree line. The NAM now has it right over us. The GFS continues with a colder forecast showing the line now sagging south of us putting us in a regime where snow could be expected once the precipitation starts falling.
6 more hours and its 7 AM Wednesday morning:
In the NAM, precipitation is still not quite over us but in the GFS it has begun to do something in our area. Again the 0-degree line is right overhead in both models making the precipitation type a very difficult call. The storm positions are a little different now with the NAM showing the center around the GA-SC border on the coast and the GFS slightly faster: along the NC-SC border at the coast.
6 more hours and it's 1 PM Wednesday afternoon.
We have ourselves a storm. Both models agree on that and both have a fairly powerful one located somewhere near the VA-NC coast. However, the other thing both models have for us is precipitation type issues. The NAM is really warm with the 0-degree line now bisecting Pennsylvania while the GFS has it closer to us but still to the north. This would argue for at least a mix of snow and rain if not all rain.
To 7 PM Wednesday evening we go:
The storm is quite intense now and just to our east. It's powerful enough that it's drawing its own cold air down with the counterclockwise flow around the low and the 0-degree line creeps southward again. Significantly more precipitation exists over us in the GFS forecast when compared to the NAM at the point where this takes place.
6 hours later - 1 AM Thursday morning:
Here we see a bomb of a storm with the GFS center position slightly to the west of the NAM position. However, the NAM keeps the precipitation going for us with as the upper level energy rotates through. The 0-degree isotherm hovers right overhead on the NAM however. The GFS is colder - the 0-degree line is to our south - but much drier with just a little leftover precipitation in our area.
So, as you can see, even with a reasonable consensus on track and strength, two of the biggest models are still quite different in terms of thermal and precipitation profiles. Following either of these models strictly is probably a bad idea but the general idea in either case is for a day when snow may begin in the morning on Wednesday and change to or mix with various other types of precipitation including sleet and rain. Towards the evening, precipitation would change back to snow and if that upper energy swings through like the NAM suggests, we could see snow falling through most of Wednesday night into early Thursday morning.
Thermal profiles are tough for models to forecast and a lot is probably going to depend on your locale. Further north and west is the better chance for this to remain all snow however it's also a place where less precipitation may fall.
Accumulations are difficult to predict as some of the potential could be lost to liquid precipitation instead of accumulating snow.
I should note that other models have a similar story but a couple of them hold onto the snow longer. One thing is certain: any snow that does fall will be wet and heavy.
This has been a difficult year for forecasting. In every case it seems that one element has been missing: phasing, precipitation field or, in this case, source of cold air. That makes for difficult forecasts and this is one of the toughest.
The afternoon models are running (as is the 12Z Euro). If they contain any surprises, I'll post them.
But - nothing like that has happened. Nothing.
To have a good "Miller A" Nor'easter - the type of snowstorm that is likely to dump 6+ inches of snow (or more) on our area, you need a few ingredients:
- A persistent cold air source. This is provided by a strong high pressure over New England of the sort we've had for the past few days. The problem? It's moving east. The further east it moves the more the clockwise flow around the high stops funneling cold air from the north and begins to funnel warmer air from the east and southeast. So this is something of a question right now.
- A surface low pressure system near or over the Gulf Coast. This we have - no problem here.
- An upper-level low moving out of the Plains states and phasing with the surface low somewhere in the southeastern United States. Most all of the models agree this will happen although there are variations with respect to timing. All of the important models have the phase happening well before the storm enters our region. This is a good thing. You'll remember that the majority of the problems we've had with storms and forecasts this year have been due to the difficulty in predicting the exact time and location of the phase and that the phasing has happened almost overhead which has caused dryslot problems. Not this time.
- Favorable storm track. Too close to the area and you have mixing problems: sleet or even rain mixes in with the snow and keeps accumulations down. Too far off the coast and you get missed by the juiciest part of the storm and that keeps snow totals down. The models are still in disagreement about the final track except that all the important ones agree that the track will be somewhere east of here. In this case especially - with the cold air and temperatures being borderline at best - a small change in storm track will probably mean a huge change in what we experience.
So - where are we with this thing?
To begin with, the storm forecast positions are slowing down. What was once a Tuesday night into Wednesday storm now looks like a Wednesday morning - Thursday morning storm. This has been a trend for the last couple of days so I wouldn't be surprised to see things slow down even further as we get closer to the event.
Now lets look at some of the big players in the model world and where they stack up.
We'll begin with the forecast frame for 30 hours from now (36 hours from 12Z - the run time of the model) or about 7 PM EST Tuesday night. Here are both the NAM and the GFS for comparison at the same time frame. The NAM will be on the top, the GFS on the bottom. Remember that you can click on any image in this blog to see the full-size version.
I want you to keep an eye on two things. First, notice that the position of the low (NAM just south of the Florida Panhandle, GFS right over it) is pretty similar in each model as is the precipitation field. Heavy rains are breaking out from Georgia into South Carolina and severe thunderstorms are on their way into Florida. Clearly there is a lot of energy and moisture available to this storm.
But notice, also, that blue line that runs from the Missouri/Arkansas area northeastward towards the mid Atlantic. That's the 0-degree isotherm and it can be used to approximate where the precipitation would remain snow (north and west of that line) and where it might change to rain (south and east of that line).
Notice that the NAM is quite warm - forecasting our entire area to be above freezing even at this frame while the GFS is slightly colder and actually paints the 0-degree line right over our area.
Onward to 42 hours - around 1 AM Wednesday morning.
The storm is strengthening and is moving to the northeast. Heavy precipitation is falling in Florida and along the Georgia and Carolina coasts. We still have no precipitation in either model though it's beginning to break out just to our south in the GFS version of events.
Again the low position is pretty similar in both models but the precipitation field is more inland on the NAM bringing heavier rains to interior Georgia and South Carolina. For us, note the 0-degree line. The NAM now has it right over us. The GFS continues with a colder forecast showing the line now sagging south of us putting us in a regime where snow could be expected once the precipitation starts falling.
6 more hours and its 7 AM Wednesday morning:
In the NAM, precipitation is still not quite over us but in the GFS it has begun to do something in our area. Again the 0-degree line is right overhead in both models making the precipitation type a very difficult call. The storm positions are a little different now with the NAM showing the center around the GA-SC border on the coast and the GFS slightly faster: along the NC-SC border at the coast.
6 more hours and it's 1 PM Wednesday afternoon.
We have ourselves a storm. Both models agree on that and both have a fairly powerful one located somewhere near the VA-NC coast. However, the other thing both models have for us is precipitation type issues. The NAM is really warm with the 0-degree line now bisecting Pennsylvania while the GFS has it closer to us but still to the north. This would argue for at least a mix of snow and rain if not all rain.
To 7 PM Wednesday evening we go:
The storm is quite intense now and just to our east. It's powerful enough that it's drawing its own cold air down with the counterclockwise flow around the low and the 0-degree line creeps southward again. Significantly more precipitation exists over us in the GFS forecast when compared to the NAM at the point where this takes place.
6 hours later - 1 AM Thursday morning:
Here we see a bomb of a storm with the GFS center position slightly to the west of the NAM position. However, the NAM keeps the precipitation going for us with as the upper level energy rotates through. The 0-degree isotherm hovers right overhead on the NAM however. The GFS is colder - the 0-degree line is to our south - but much drier with just a little leftover precipitation in our area.
So, as you can see, even with a reasonable consensus on track and strength, two of the biggest models are still quite different in terms of thermal and precipitation profiles. Following either of these models strictly is probably a bad idea but the general idea in either case is for a day when snow may begin in the morning on Wednesday and change to or mix with various other types of precipitation including sleet and rain. Towards the evening, precipitation would change back to snow and if that upper energy swings through like the NAM suggests, we could see snow falling through most of Wednesday night into early Thursday morning.
Thermal profiles are tough for models to forecast and a lot is probably going to depend on your locale. Further north and west is the better chance for this to remain all snow however it's also a place where less precipitation may fall.
Accumulations are difficult to predict as some of the potential could be lost to liquid precipitation instead of accumulating snow.
I should note that other models have a similar story but a couple of them hold onto the snow longer. One thing is certain: any snow that does fall will be wet and heavy.
This has been a difficult year for forecasting. In every case it seems that one element has been missing: phasing, precipitation field or, in this case, source of cold air. That makes for difficult forecasts and this is one of the toughest.
The afternoon models are running (as is the 12Z Euro). If they contain any surprises, I'll post them.
Thursday, January 20, 2011
By The Way
Long-range models are showing the first credible shot at a snowstorm around here since the Boxing Day Storm that just missed us for mid-next week (Wednesday-Thursday as it looks now).
Still a LONG way to go (and you remember how the Boxing Day storm waffled back and forth before giving us nothing) - so don't get too excited.
One at a time, I guess. Let's wait for this one to pass before we get excited about next week.
Still a LONG way to go (and you remember how the Boxing Day storm waffled back and forth before giving us nothing) - so don't get too excited.
One at a time, I guess. Let's wait for this one to pass before we get excited about next week.
Quick Update
Haven't had all that much time to look at the models but it seems like it's GFS vs NAM. The GFS model suggests a more southerly track to the low across VA this evening which would give us some snow - mostly from midnight into the early morning hours tomorrow. An inch to slightly more is probably all we can look for from this.
The NAM is even worse. A more northerly track almost over us decreases the amount of moisture available, introduces the dry slot and makes atmospheric temperatures questionable with regards to the rain/snow line.
Based on the morning model runs, I'd say around an inch or two - but I'll take a look at the afternoon models as well.
Either way - shouldn't be a big deal and I don't expect it will start until closer to midnight tonight - maybe even later.
The NAM is even worse. A more northerly track almost over us decreases the amount of moisture available, introduces the dry slot and makes atmospheric temperatures questionable with regards to the rain/snow line.
Based on the morning model runs, I'd say around an inch or two - but I'll take a look at the afternoon models as well.
Either way - shouldn't be a big deal and I don't expect it will start until closer to midnight tonight - maybe even later.
Tuesday, January 18, 2011
Storm Watch: Thursday Night - Friday
First of all: sorry about not posting for this latest storm. I was out of town with no Internet connection. This was an interesting storm and at some point on Monday night I think I saw every possible type of winter precipitation possible: snow, sleet, graupel and freezing rain.
Before we get into the possible storm Thursday night into Friday, I thought it might be good to talk about the different types of precipitation and how they form.
Snow - this is the easy one. Temperatures in the clouds are cold enough to produce ice crystals that we recognize as snow. As it falls from the clouds, the atmosphere is sufficiently cold all the way from cloud base to the ground (or almost the ground) for the ice crystals to remain intact.
Variants:
- "wet" snow which we sometimes see when the air between the cloud and the ground is hovering around freezing. Some melting takes place which makes the snow "sticky". You know this because snow with a higher water content makes better snowballs than dry snow which we get when it's very cold. The snow flakes collide with each other and stick together making for the streaky, sometimes large flakes we see during "wet" snow.
- "graupel" which is a phenomenon that almost always happens when the precipitation type is transitioning (e.g. from snow to sleet) and is caused by snowflakes hitting droplets on the way down that are colder than 32 degrees but remains in a liquid state. This is called supercooling. The snowflake develops a coating of ice which continues accumulating (accreting) around the ice crystal until it resembles a small ball rather than a snowflake. This is not the same as sleet.
Sleet - as snow falls from a cloud into air that is too warm to support ice, the snowflake melts into a water droplet. At some point in its travel toward the ground the air cools again for enough distance to refreeze the droplet of water into an ice ball which we recognize as sleet.
Freezing Rain - as snow falls from a cloud into air that is too warm to support ice, the snowflake melts into a water droplet. The temperature near the surface is at or below freezing, however (meteorologists refer to this as the cold air being "shallow"). Therefore the liquid water freezes on contact with anything on the surface that is below freezing especially cars and trees. Over time, the ice accumulates (we say "accretes") and produces that glazed look like the one we saw this morning.
Ok - class time is over! On to the possible Thursday storm.
As has become the norm this year, the models disagree on the track - but moreso on the intensity - of this storm. We'll look at the NAM and the GFS for this discussion.
Oh, and a quick note: you can click on any of the images in the blog and get see the full-sized version. This can help if you're having trouble seeing details in the smaller maps.
We'll start about 54 hours from now: Thursday morning.
Here's the NAM forecast for that time:
You can see our storm in the mid-Mississippi Valley. The NAM already has a strong low - especially when compared to the GFS at the same time which forecasts a weaker and more "elongated" area of precipitation:
Later that night (12 hours later) the NAM shows the storm really getting its act together. Precipitation extends from the Northeast into the southern states - although not quite in our area yet. You can see a storm center forming as the precipitation is forecast to be heavier in Kentucky and Tennessee at this point.
The GFS is not quite as excited about the storm although it does "close off" the low and shows its position in southwest Virginia at this forecast time. You can tell the intensity isn't as high mostly because the precipitation field is less impressive - especially near the center. However, unlike the NAM, the GFS has light precipitation breaking out over our area by this time.
Moving ahead 6 hours, the NAM has a pretty nice system starting its turn up the coast. We are in a region of moderate precipitation and this would argue for a few inches of snow overnight Thursday into Friday morning.
The GFS has a similar solution for this time frame - the storm is wrapping itself up over the Atlantic - but the overall precipitation field is somewhat lighter especially from our area northeastward into Philadelphia.
So - the timing of this storm between the two models is fairly close. I think we can expect any snow that does fall to happen sometime after sunset on Thursday night. The models would argue more towards midnight.
We'll need another few models runs before talking seriously about possible accumulations and snow totals. For now, I'd say the outlook is favorable for snow on Thursday night into Friday morning and there's a 50/50 chance that the snow would be "significant" - i.e. 2 inches or more.
This isn't a big storm for us by any stretch - no Snowpocalypse this time. But it could be a nice, quick snowfall of a few inches if things come together over the next day or two.
Stay tuned...
Before we get into the possible storm Thursday night into Friday, I thought it might be good to talk about the different types of precipitation and how they form.
Snow - this is the easy one. Temperatures in the clouds are cold enough to produce ice crystals that we recognize as snow. As it falls from the clouds, the atmosphere is sufficiently cold all the way from cloud base to the ground (or almost the ground) for the ice crystals to remain intact.
Variants:
- "wet" snow which we sometimes see when the air between the cloud and the ground is hovering around freezing. Some melting takes place which makes the snow "sticky". You know this because snow with a higher water content makes better snowballs than dry snow which we get when it's very cold. The snow flakes collide with each other and stick together making for the streaky, sometimes large flakes we see during "wet" snow.
- "graupel" which is a phenomenon that almost always happens when the precipitation type is transitioning (e.g. from snow to sleet) and is caused by snowflakes hitting droplets on the way down that are colder than 32 degrees but remains in a liquid state. This is called supercooling. The snowflake develops a coating of ice which continues accumulating (accreting) around the ice crystal until it resembles a small ball rather than a snowflake. This is not the same as sleet.
Sleet - as snow falls from a cloud into air that is too warm to support ice, the snowflake melts into a water droplet. At some point in its travel toward the ground the air cools again for enough distance to refreeze the droplet of water into an ice ball which we recognize as sleet.
Freezing Rain - as snow falls from a cloud into air that is too warm to support ice, the snowflake melts into a water droplet. The temperature near the surface is at or below freezing, however (meteorologists refer to this as the cold air being "shallow"). Therefore the liquid water freezes on contact with anything on the surface that is below freezing especially cars and trees. Over time, the ice accumulates (we say "accretes") and produces that glazed look like the one we saw this morning.
Ok - class time is over! On to the possible Thursday storm.
As has become the norm this year, the models disagree on the track - but moreso on the intensity - of this storm. We'll look at the NAM and the GFS for this discussion.
Oh, and a quick note: you can click on any of the images in the blog and get see the full-sized version. This can help if you're having trouble seeing details in the smaller maps.
We'll start about 54 hours from now: Thursday morning.
Here's the NAM forecast for that time:
You can see our storm in the mid-Mississippi Valley. The NAM already has a strong low - especially when compared to the GFS at the same time which forecasts a weaker and more "elongated" area of precipitation:
Later that night (12 hours later) the NAM shows the storm really getting its act together. Precipitation extends from the Northeast into the southern states - although not quite in our area yet. You can see a storm center forming as the precipitation is forecast to be heavier in Kentucky and Tennessee at this point.
The GFS is not quite as excited about the storm although it does "close off" the low and shows its position in southwest Virginia at this forecast time. You can tell the intensity isn't as high mostly because the precipitation field is less impressive - especially near the center. However, unlike the NAM, the GFS has light precipitation breaking out over our area by this time.
Moving ahead 6 hours, the NAM has a pretty nice system starting its turn up the coast. We are in a region of moderate precipitation and this would argue for a few inches of snow overnight Thursday into Friday morning.
The GFS has a similar solution for this time frame - the storm is wrapping itself up over the Atlantic - but the overall precipitation field is somewhat lighter especially from our area northeastward into Philadelphia.
So - the timing of this storm between the two models is fairly close. I think we can expect any snow that does fall to happen sometime after sunset on Thursday night. The models would argue more towards midnight.
We'll need another few models runs before talking seriously about possible accumulations and snow totals. For now, I'd say the outlook is favorable for snow on Thursday night into Friday morning and there's a 50/50 chance that the snow would be "significant" - i.e. 2 inches or more.
This isn't a big storm for us by any stretch - no Snowpocalypse this time. But it could be a nice, quick snowfall of a few inches if things come together over the next day or two.
Stay tuned...
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