Forecast on Track, Winter Storm Warnings Are Up

The models have come into better agreement for this quick but potent storm that should be starting in a few hours.  I'd expect snow to begin around daybreak and really pick up in intensity between about 7 AM and 10 AM.  3-5" is forecast by the National Weather Service and that seems reasonable to me. 

Banding is going to set up as I mentioned in a previous post and that could create locally higher snowfall amounts.  Thundersnow is not out of the question!

There is still a chance that most of the precipitation gets pushed south of our area which would result in less snow overall but that doesn't seem likely at this point.

Round 2

For a few days now, both the GFS and the NAM (which is a good short-term model) have been suggesting a reinforcing shot of cold air would come through our region overnight and into the day tomorrow along with enhanced upper level dynamics.  This combination was indicative of a second burst of snow that would begin early tomorrow morning and continue into the afternoon.

It now looks more likely that this will happen.

The only thing that gives pause is the Euro model which has insisted for a while now that any precipitation will be pushed south of our region.

The primary difficulty in this forecast is less what type of precipitation will fall (as it was in the last storm) but how much.  A lot of the snow totals could largely depend on small localized banding events.  If one of those bands forms overhead, it could mean the difference between 1-3 inches and 3-6 inches of snow.

One thing we have going for us is the push of cold air aloft which should mean enhanced liquid-to-snow ratios. 

Here's the GFS snow total map for this storm:

you can see the bullseye in the Baltimore area but we in the DC metro are still doing pretty well - in the 3-6 inch range, generally.  When I look at the projected temperatures at the 850 mb level:

I see a lot of -4 and -5 degree readings overhead which suggest to me a liquid-to-snow ratio a little higher than the standard 10:1.  Maybe as much as 15:1.  This means that whatever falls will accumulate and quickly.

The DC metro is under a Winter Storm Watch for tomorrow morning for 3-5+ inches of snow.  I think this is reasonable based on the GFS and NAM model forecast.  I'm going to wait to see the Euro before I get too excited but, in summary, prepare for more snow starting early tomorrow morning and accumulating quickly.

Snow, Then Ice, Then Rain and Some Combination of Everything in Between

I thought about just posting that headline and calling it a day because that's pretty much what the forecast boils down to.

There's a good chance that we'll see every possible winter precipitation type tomorrow at some point.  As to exactly when the changeover occurs and how much precipitation will be falling at what times - that's where the complexity of the forecast really lies.

A couple of things:

- The models have been trending towards less overall moisture for this storm but have also been shifting the axis of heaviest precipitation right over our region.

- The NAM has a wet bias so I wouldn't get too excited about snow amount forecasts and instead concentrate on the area modeled for maximum precipitation.

- Models in general are pretty bad at forecasting overrunning events like this one (cold air at surface hemmed in by the mountains, warm moist air aloft).


Here's the NAM clown map from this morning:

again, focus less on the amounts for now and more on that stripe of heavier precipitation that goes right over us.  This modeled local maximum in precipitation has been shifting southward with consecutive runs of the models and appears to be in pretty good position to give us some precipitation.

Now, what kind will fall?

My best guess is that the precipitation will start mid-morning in the DC area as snow and it will snow for the rest of the morning into the afternoon.  The changeover will begin from east to west, first as sleet (which could be heavy for a bit), then to freezing rain (liquid precipitation that freezes on contact with the surface) then to plain rain as temperatures even at the surface rise above freezing.

I don't think temperatures get much higher than 32 tomorrow or so - maybe less if enough snow and sleet fall in the morning.

By Monday morning, it's all rain and then we'll have to watch and see what happens.  Some models suggest a continuation of the moisture flow into our region and with the cold air firmly in place by Tuesday, we could have another shot at some snow then.

This graphic (called a meteogram, tells some of the story):

Since this is built off of the NAM run, we should probably take the precipitation totals with a grain or two of salt (if this verified, for instance, it suggests 6-8 inches of snow by Tuesday night).  But check out the precipitation types and the difficulty the model has in resolving them.  Blue bars with an "S" caption is snow, red bars with a "P" caption is sleet, orange bars with a "Z" caption is freezing rain and the green bars with an "R" caption is plain rain.

You can see the secondary burst of snow Tuesday morning over there on the right of the graph.

At this point, I think a Winter Storm Warning will be needed for points north and west of DC and a Winter Weather Advisory for DC and points east. 

This should be the type of storm which will keep you inside on a Sunday and, since the Redskins are 3-9 and playing a 9-3 team, should keep attendance at FedEx field on Sunday to about a dozen of Daniel Snyder's family members.

More updates if the afternoon model runs show anything different.

Model Runs Trending a Bit Colder - Still a Mess of a Forecast for DC Area

As Paul pointed out in his comment on my last post, the models have been trending colder overall with this storm - especially with regards to the depth of the cold air and the time it will take the warm air to erode the dome.

Other forecast elements still in place:  high pressure funnels cold air southward starting Friday night.  Saturday will be noticeably colder.  The cold air is in place as warmer air starts to overrun early Sunday morning.

At this point, it looks like a shot of snow followed by sleet, possibly significant freezing rain before changing to all rain late Sunday night into Monday.

There are still a lot of questions about timing and how much front-end precipitation will fall but it seems somewhat more likely today that frozen precipitation will fall on Sunday and we could have some icing problems before things changeover to rain.

Here's this morning's Euro snowfall forecast:

which still suggests a few inches of snow for the DC area.

More models are coming in - I'll post later today.

First Significant DC Metro Winter Precipitation Possible on Saturday-Sunday

Humm.  Good to be awake again here on the PV blog! 

That was a strange offseason.  No real summer to speak of and then a cold autumn which seems to want to transition to winter as soon as possible.

Fine then.  Let's begin.

We've had our share of cold weather but precious little wintry precipitation.  I'm loathe to credit the seventeen flakes of snow that fell here just before Thanksgiving as "wintry precipitation" but either way, it certainly wasn't significant.

Saturday night into Sunday could be a different story.

Let's set up the scenario and I'll chat about the issues and complexity of the forecast.

First off, we have a couple of storms.  The first one goes through here Thursday night through Friday as a front passes through the area and brings colder air.  Could it end as a bit of snow?  It could.  But it won't amount to much.  The majority of what falls around here will be rain so, for the purposes of this blog, we'll ignore it, no matter what the "incredibly thoughtful and clever" folks at The Weather Channel have decided to call it. 

And by "incredibly thoughtful and clever" I mean, "galactically moronic lunatics desperate for ratings".

What the hell is a "Cleon", anyway?

Onward.  The second storm (I can think of a compound "d" word the Weather Channel can use if it needs a suggestion) is the more interesting one around here.

The synoptic situation is this: 

A strong high pressure will be moving east over New England but will have enough influence on our region to pump cold air southward.  This cold air will funnel its way into our area - channeled against the mountains like a quarter pipe.  This is a classic setup for what we call "cold air damming".  Essentially, since cold air sinks, the air just sits in a pool bounded by the mountains to the west and the warmer ocean to the east. 

Now here comes the storm.  The main low pressure center is forecast to take a path well to the west of here through the upper Midwest and into the Great Lakes region.  This is normally a horrible setup for us if we want any kind of frozen precipitation.  The counterclockwise flow around the low brings air from the south which is quite moist but also quite warm.

However.  With the cold air settled in a pool, the warm air won't dislodge it right away.  In fact, initially it will ride over top of the cold dome, condensing along the way.  If the cold air is deep enough, we'll see snow or sleet for a time Saturday night into Sunday.  As the warm air erodes the cold air below it, any snow will change to sleet and any sleet will change to rain. 

Here at the surface, the temperatures might stay cold enough for some (most?) of Sunday that any liquid precipitation will freeze on contact with the surface.  That's the dreaded freezing rain which would leave a glaze on things.  No fun.

At some point on Sunday evening, the warm air will begin to win and by Monday, plain rain will be falling with much warmer temperatures as the last of the cold air is scoured out.

QUESTIONS

- How deep will the cold air be?
- How long will the cold air last?
- How much precipitation will fall while the cold air is still around?

These will make all the difference between a forecast that is just for a bit of sleet followed by rain and a forecast that includes every possible frozen precipitation type followed by rain later Sunday night and Monday.

Most of these details have yet to be resolved.


MAPS

Here's this morning's Euro model output for total snowfall from this storm:

as you can see, the DC metro falls between the 3-6 inch contour.  This represents what I believe to be fairly ideal conditions.  Deep cold air, lots of precipitation on the front end of the storm.

I'd say, at this point, there's a 20% chance of verifying in the DC metro.  A greater chance as you move north and west of the city.

I'm concerned about how much cold air there will be and how much precipitation will be out on the front-end of this storm.  But I expect we'll see some frozen precipitation on Saturday night into Sunday.  How much and for how long remains to be seen.

I'll update when new information comes in.

Welcome back!

Radar Shows the Problem

This radar I "borrowed" from the CWG page shows the issues:

Look at that rain/snow line.

Keeps bouncing back and forth in the District, all snow to the west, all (or mostly) rain to the east.

In the City and East - A Different Story

In the city and points east, the dreaded temperature issues are having an effect.  Rain is falling throughout most of southern Maryland and the eastern Shore.  Very wet and non-accumulating snow is falling in parts of the District itself.  I think these areas don't do well in this storm although a changeover should occur as the storm moves to our north.

Expect the NWS to lower snow total forecasts for these areas and, really, anywhere east of central Fairfax County.

Still, a snowy day to "end" the winter isn't all that bad. 

Things are On Track - West of the City

For most of the area west of DC, things are on track.  Snow has been falling overnight and mixing with (even changing to) rain as expected for areas in the city and eastward.  It's been all snow to the west - even in Annandale just outside of the Beltway.

Snow was light to moderate overnight and had the expected difficult time sticking due to the warm temperatures yesterday.

Today snow will be heavy at times and the bulk of our accumulations will fall during the daytime hours.  I think.

The snow will have a harder-than-usual time accumulating because of the temperatures and its consistency - especially on the roads and sidewalks and we have to watch that rain/snow line.

I think the city and points east will have a hard time getting to the forecast snow amounts.  Westward, things should be better.

Here's the NAM clown map which is more realistic than previous NAM versions.

Enjoy whatever snow does fall and be safe!

Winter Storm Warning

A Winter Storm Warning has been issued for most of the DC metro.  This is the latest forecast from the NWS.  Note that the snowfall totals are shifted west somewhat indicating less snow around the city but still a significant snowstorm.  NWS is expecting some problems with mixing during the first half of Wednesday.

Forecast Mostly On Track

As it has looked from the beginning, this is going to be an issue of temperatures and whether the intensity of the storm and the dynamic cooling processes in the atmosphere can overcome the marginal cold air and the complete lack of a strong cold air source.

This storm and the forecasts around it have high bust potential.

Still - climatology and atmospheric analysis suggests that the DC metro area and especially points west and north have a good chance to end a 2-year snow drought in spectacular fashion.

The NAM model run from this morning is one of the more bullish solutions for this storm.  Here is the clown map it rendered from the morning run:

Now the NAM does have a precipitation bias - it can overdo the precipitation forecast.  Still, if this were to verify, it suggests widespread totals of a foot or more in DC and especially westward where totals could exceed 15 inches.

The GFS is slightly less excited but still shows our region in the bullseye of the storm:

Although as you can see, the gradient of precipitation is quite tight on the eastern side and the snowfall amounts drop off rapidly as you go east.

Track is often the most important thing with winter storms but in this case, the track is almost the entire story.  There's not much question about precipitation falling tonight, tomorrow and tomorrow night.  The question is, "what type?"

A small shift - and we're talking maybe 50 miles - southward and this becomes a snowstorm with 10-15 inches of snow even inside the Beltway.  The same shift northward dramatically decreases the snowfall totals in the region and pushes everything west.

My sense is that snow will fall and accumulate and that we'll end the 2-year snow drought tonight and tomorrow.  But I do worry about the rain/snow line and how far westward it will move.  I also worry about how much precipitation will fall before it changes to snow.

Like I said:  high bust potential.

Right now, it seems reasonable to go with this forecast:

Tonight:  Precipitation begins as rain, probably in the evening, 5-6 PM is a good guess for the immediate DC metro.  Earlier to the west, of course.  The rain should fairly quickly start to mix with snow.  Overnight, closer to midnight, the rain/snow mix will changeover to all snow and the precipitation rates really get going.  If everything works out, we could see 2 or 3 inches on the ground by the time we wake up in the morning.

Tomorrow:  Snow for most of us but more mixing issues will be in store the further east you go in the region.  Out in Loudoun I expect they stay all snow but there probably be mixing issues east of the city.  The precipitation will be heavy both in the "intensity" sense and the "weight" sense.  This will be a wet snow which, combined with the expected winds, could cause power outages. 

Tomorrow Night:  Snow continues but starts to end from west to east overnight.  Only the far northeast suburbs should see any lingering light snow Thursday morning.

This NWS map seems reasonable at this time:

I'll update if anything changes!

Worrisome Trend?

The GFS has been trending further and further northwest with each run.  This was an "ok" trend at first because it brought the heaviest core of precipitation northward towards us.  However, it's starting to become a touch uncomfortable because a northern track is also a warmer track which we don't want. 

Temperatures for this event were going to be marginal anyway.

The official NWS call for the DC area is 2-4 inches east of I-95 and then a fairly tight gradient of about 6-8 inches just west of DC to over a foot for the western suburbs.

We need to keep a southern component to the track or we'll definitely have precipitation type issues.

Most March storms are like this but the snowfall forecasts are too close for comfort:

I should note that the GFS has the furthest northward track of any model and has been leading the northward charge for the last few cycles.  In fact, the Euro - which is considered a better model - insists on a more southerly track.  The disparity between the models - especially after the storm gets off the coast - is quite remarkable at this timeframe and highlights the complexity of the atmosphere and the interactions between pressure systems.

All I can say is "stay tuned".  Snow still seems likely but we're getting close - too close - to having rain mixing problems, especially near the city itself.

The GFS Model Agrees

NAM Total Precipitation Forecast

You can see we're in the "bullseye" for the NAM which shows 1.5-2+ inches of liquid precipitation from this storm.  Even losing some of that to rain/slop and assuming low snow-liquid ratios, that suggests at least 6 inches of snowfall.

Snow Seems Likely

It's down to a question of "how much?" which will be strongly driven by the ultimate storm track and the intensity of the storm as it passes through.  We could "lose" some precipitation to rain at the beginning (Tuesday afternoon/evening) but the changeover to snow should start from west to east overnight Tuesday.

The closest analog to this storm and the pattern seems to be February 5-6, 2010 which dumped 18-32 inches of snow in the DC area.  This will not perform as well for two reasons:

1 - Lack of abundant cold air at the surface.  This storm will depend on dynamic processes to cool the air and change initial precipitation to snow.  The forecast high for DCA on Tuesday is 47 so there will be some distance to go - even at night - before the atmosphere is fully conducive to supporting snowfall.

2 - Lower liquid-snow ratios.  The rule-of-thumb is 1 inch of liquid = 10 inches of snow.  Because of the marginal temperatures, the ratios are likely to be more like 8:1.

Still, most of the models produce significant QPF (precipitation forecast amounts) for our area - well over an inch in many cases.  Those of you along and east of I-95 stand the best chance of having precipitation problems with rain mixing in even on Wednesday which may keep totals down.  To the west, the precipitation is more likely to remain snow.

The timing for this is:

Tuesday afternoon/evening:  rain starts and changes to snow overnight.

Tuesday night:  rain/snow changing to all snow by morning

Wednesday:  snow (possibly mixed with rain to the east)

Wednesday night:  snow ending from west to east


Here is the NAM clown map from this morning's run:

Looks like 6-12 inches from DC westward, over a foot in the mountains.  Less as you go east.  The gradient is pretty tight along I-95 so this will be a tough call.  I can see scenarios where only rain falls in the I-95 corridor and I can see others where all snow falls.

It's a tough forecast as almost all March snowstorms tend to be.

I'll say a general 6-inch snowstorm is likely now for the DC metro with greater amounts to the north and west and lesser amounts to the east.

We'll see what the models have to say this afternoon.

Keep an Eye on Tuesday-Wednesday

As you've probably heard by now, the potential for significant snow exists for Tuesday night through Wednesday night in our area.  This is a potentially serious situation because the precipitation, if it indeed falls as all snow, would be wet and heavy and according to some models I'm looking at now - there could be a lot of it.

Here's the GFS "clown map" for example:

showing our area "bullseyed" with 12-16 inch snows.  The GFS does some strange things in this run so we'll have to watch to see if it's repeated in tomorrow's run and if the other models agree.

There's a ton of data to look over and most importantly, the track is going to be a big big deal.  In March, the sun angle is higher and it's going to be hard to get temperatures down to the level where everything sticks (or even falls as snow).  But if the track is just right, this could happen.

I'm still checking things but let's just say that the chance of significant accumulating snowfall starting Tuesday night and continuing through Wednesday night is better than 50%.

The rain/snow line is more of a question now rather than the question of whether or not anything will fall.

Friday-Saturday Snow Chances Just Got More Interesting

I hadn't posted because, really, there's not much to get excited about when Snow Chance #1 pretty much slopped for 3 hours and went away.

Friday night's chance looks like that's still a possibility but a few models have a different, more significant idea for Friday night into Saturday.

It's a mess of temperature issues and whether or not the storm can get going off the coast before it moves too far away.  If it can do this - and it's still a big "if" - we could have a better chance at accumulating snow here in the DC area.

I'll post again if there's anything to it...

Review of Big Storm, Three Upcoming Chances for DC

Hopefully all of our friends and family in the northeast - especially Connecticut and Massachusetts - are doing well after the historic storm last week.  Truly amazing amounts of snow fell in parts of northern New England.

If you haven't seen this video yet, give it a look.  This is a time-lapse video of the snowfall near Hartford, CT.

When you consider NYC even got in on the action at 8-12 inches, why didn't we see any snow here?  It sleeted/snowed for about an hour on Thursday night and then rained a bit into Friday morning. 

Boring!

Let's have a look at what happened because this has bearing on the upcoming possibilities.  To save you from sensationalistic plugs that make you read an entire post before you get what you're looking for, I present a digest:

3 chances in the next 7-10 days: 

- Wednesday 2/13
- This coming weekend
- Next Monday-Tuesday or so

Those of you with short attention spans can now go merrily on your - LOOK!  SOMETHING SHINY!

Onward...

It's all about storm type and phasing.  There were two systems moving eastward in the two main branches of the jet stream.  The Northern Stream storm would head on a path across the Upper Midwest and to the north of our area. 

Since winds move counterclockwise around a low pressure system, this created a southerly flow around our area that quickly changed frozen precipitation to liquid overnight on Thursday.

This was a Miller B type storm (as they all have been this year) and as the Northern Stream system moved eastward it transferred its energy and phased with the Southern Stream system which had moved out of the southeastern US and up the Atlantic Coast.

Now, storms moving along the coast from the south in the winter is usually a good thing for us.  However, with that Northern Branch system moving north of us and the phase happening just to our northeast, we were caught in the middle - in an area between two phasing low pressure systems.  This is almost always a zone of minimum precipitation and it's a big problem with Miller B storms that don't phase to our south.


The Southern Stream storm would move from the southeast and then up the coast, phasing with the Northern Stream late in the afternoon on Friday - well to our northeast.  The precipitation gradient was extreme with parts of the Eastern Shore seeing heavy rain all day Friday while we in DC saw some sunshine in the afternoon.

Here are the two storms last Thursday morning as they began to phase.  

Watch the following animation and see the phase as the southern system absorbs the northern system.

Notice a couple of things here.  First, the northern storm starts to weaken (the 1008 mb contour disappears from the center of that storm towards the end) and the storm on the coast significantly intensifies as it moves up the coast (the last few frames show a center contour of 996 mb).  Where are we?  

Right in the middle.

Ok - congratulations to our friends up north.  What about our snow?  Let's talk about the first chance.

WEDNESDAY 2/13 - THURSDAY 2/14

The models have been showing a "Valentine's Day Storm" for a while now and it's encouraging that this close to the event, they still have it.

What's discouraging is that they seem to have little consensus about what path its going to take and whether temperatures in our area will be suitable for frozen precipitation.  

At the moment, I would classify the event as marginal as we're in one of those situations where a more northern track would produce heavier precipitation but also warmer temperatures (rain) and a southern track would produce lighter precipitation but colder temperatures (snow).  In this case (ironically), the best place for us to be would be in the middle of those track solutions.

Here's the Euro at hour 72 (Wednesday night) showing the storm center in the southeastern US:

And then off the coast by hour 96 (Thursday morning)

The GFS is in favor of a more northward track:

 which, as you can see, would move the higher axis of snow accumulations northward.  

This bears watching and until there's a track consensus, anything is possible from warning-criteria snow to a rain event here.

We'll see what the evening model runs hold.

I'll post later about snow chances on the weekend and then a bigger chance early next week.

 

Forecast On Track

Not much change in the thinking.  A quick-moving storm will produce about 6 hours of snow tomorrow starting in the early afternoon most likely.  I expect 1-2 inches but depending on where small-scale bands set up, more is possible in some areas, less as you go north.

The "storm" should be out of here by nightfall.

The weekend will be cold again but temperatures begin to moderate by Monday and we could be near 60 again by Wednesday before the next cold air arrives.

Rain probable next week with an outside chance of snow as the cold air arrives Wednesday night.  But the next real chance for snow shows up on the models for the February 2-4 timeframe.  Plenty of time to watch that one.

Enjoy whatever snow falls tomorrow.  I'll post an update if there's any change.

NAM "Clown Map" For Friday

Hot off the presses, the "Clown Map" for Friday which shows forecast snowfall amounts.  You can see that the forecast from this model is between 1 and 2 inches for the DC metro.  The interesting (and somewhat sad, given the last two winters) is that this is actually an improvement over previous NAM runs.

Maybe we're headed in the right direction...

One Shot at Light Snow Tonight, Another on Friday

At the moment, it doesn't look like any major storms are in our near future.  The best chance we had of a moderate storm was Friday but at this point, the storm looks like it will be weaker and will not produce all that much precipitation.

Let's look at where we are - first with tonight's event.

A weak "clipper" system is poised to move over our area overnight - probably after midnight and into the hours around dawn.  The GFS and the NAM both show this clipper and show a decent amount of energy (for a storm like this) passing over our region.  We can expect a dusting to perhaps an inch and the enjoyment of watching snow fall if you're up late enough (or early enough).  The track is far enough south that areas around Fredericksburg could see the most snow - but even there, an inch or so is about as good as it will probably get.

GFS model output showing light band of snow across the region overnight.  Liquid equivalent precipitation forecasts are around 0.05 - 0.1 inches which translate to around an inch of snow.

Because of the very cold airmass that is currently locked in our region, the snow ratios should be higher than the standard rule-of-thumb 10-1.  So even though the amount of available moisture is small, the chances of snow accumulation are pretty good.  I'd say in the DC metro area we stand a 60-70% chance of seeing around an inch of snow tonight. 

If the snow should continue into the morning rush hour, expect the NWS to put out Winter Weather Advisories.

This is a fairly narrow band of energy (as you can see from the model output above) so if we don't see much of anything on the ground and just a few flurries flying, I wouldn't be that surprised.

 On to Friday:

The models are slowly coming into agreement and the news for snow-lovers isn't that good.  The phasing I discussed in my previous post seems unlikely to happen as the southern stream energy gets shunted south and sheared apart.  That means the northern stream energy has to work with whatever moisture it has and since it's continental in origin, that's not much.

Also, the storm will be a Miller B type scenario.  Miller B storms are characterized by a primary low moving over land that weakens and transfers energy to a secondary low - usually off the coast.  Sometimes this is a really good thing.  For example, the Snowmageddon storm was a Miller B.

But.

In that case the energy transfer happened earlier and the southern stream low pressure system was much stronger (and had its origins in the Gulf Coast).  As the storm moved up the coast, it actually almost stopped to our east before proceeding up the coast because of blocking patterns.

Radar composite image from February 5-6, 2010 showing strong southern stream low receiving energy from a weaker low over the Tennessee Valley before moving up the coast.

In this case, however, the southern stream energy is weak and the energy transfer may happen right over our area.  These are bad things.  If the energy transfer happens directly over us, we could be caught in the middle - what meteorologists call a "snow hole".  That would result in us seeing very little snow at all.

The models are still in a bit of disagreement as to the path and the amount of moisture available to the system.  At this point, it's unlikely that the storm will be anything to get excited about but we'll see how the models look this afternoon.

For now, there's a pretty good chance of light snow on Friday that could maybe result in 1-3 inches of snow if we're lucky.

Cold! Then Another Chance of Wintry Precipitation Friday

Cold is easier to forecast than snow, so let's start there.

It's cold.  It's going to be cold - until Sunday, at least.  The worst of the cold weather will be today through Friday.  Highs in the 20's, lows in the teens and a noticeable wind which will make it feel even colder.  Bundle up!

Now - let's have a look at Friday...

Most of the forecast models show a fast-moving low pressure system moving out of the southeast on Thursday night and making it here on Friday morning.  That's about all they agree on however.

A few (GFS, Euro) show the storm tracking just to our south before reforming off the coast and moving out to sea.  This would give us a relatively quick shot of precipitation but would keep it all snow. 

Others (UKMET, NOGAPS) show the storm tracking just over us or just to our north.  This would create an "overrunning" situation where warm air on the southeast side of the low slides over the cold air at the surface.  Depending on the depth of the cold air, this can create the "mixed bag" of precipitation which starts and snow then changes to sleet or possibly freezing rain.  I'd consider this the least likely scenario at this point.

The big questions as usual are:

• Storm track.  Half the models are still forecasting a northward track.
• Phasing.  There is a scenario where energy from the Pacific phases with the southern stream jet which then moves across our area.  This scenario, by far, would produce more precipitation.

An extra component to this situation is the cold air mass over our area.  This cold will produce higher-than-normal snow ratios which means we can squeeze a bit more snow out of the same amount of precipitation.

Here is the GFS model showing precipitation overspreading the area on Friday:

You can see that the freezing line (blue) is quite a bit south of here which indicates a mostly snow event.

The storm remains a quick-mover and doesn't really get going until it's well off the coast.  An event like this could produce 2-4 inches but what we want is a phase.  In this scenario, the southern energy is sheared away which produces a weaker low.

The energy that will be producing this precipitation isn't even on the West Coast yet so there's plenty of time for things to change.

This is more of a "heads-up" that some snow is likely on Friday and it looks like there will be multiple parameters to consider before a solid forecast can be made.  We'll see if there is any consensus from the models and what the energy looks like when it finally arrives.

Stay tuned.

Southward Track - No Significant Snow Expected

The southward track of the storm seems to be verifying and radar trends (and, frankly, the sky appearance) are not favorable for significant snowfall here in the DC area.  I think some snow will fall, maybe a coating here and there on grassy surfaces - but nothing memorable.

This is another case of a storm which looked like nothing a few days ago, looked like something significant for two model runs yesterday and then beginning yesterday evening, started to look like nothing again.  Inconsistencies in model runs like that within 24-36 hours of the start of the storm can cause forecasting havoc.  This one sure did.  When I stopped looking last night, the NWS was still forecasting 3-5 inches for the Fairfax area.

We're now 2 years into our snow drought.  Cold air returns for the beginning to middle of next week but very little moisture will be passing by.  I think there's a chance of snow showers on Inauguration Day but apart from that, looks like we'll have to wait a little longer for significant snow.

More Questions Than Answers

The NAM says "no snow".  The Euro says "no snow".  The GFS and several short-term models insist on at least 2 inches of snow for the DC area.

At this point, there are more forecast questions than answers.

The official NWS forecast still calls for a storm with snow totals ranging from 2-6 inches across DC and points south.  The snow numbers increase as you go further south with the "bullseye" across central and southwest Virginia.  Locations to the north of DC are unlikely to see much, if any, snow at all if the current storm track holds.

As a forecaster, I would like to have more consensus from the models by now.  I'd also like to NOT be right in the middle of the gradient of snow amounts - which means very small deviations in track could affect the outcome.  To add complexity to an already-complex situation, small-scale events and "banding" could create localized areas of heavier snowfall just about anywhere from DC south.

Here is the Hydrometerological Prediction Center forecast map which shows the probability of seeing more than one inch of snow tomorrow until early evening.  This is from a branch of NOAA which means they have considered all available model data from this evening when making this forecast:

You can see that the DC area is in the 60-70% range to see at least an inch of snow accumulation.

Change that to a parameter of at least 4 inches and the map changes dramatically:

This forecast has us in a very low probability - 5-10%.

Now this only applies until tomorrow early evening.  If some of the models pan out, the snow could continue into the nighttime hours and more accumulation is possible.  But based on this, I think a conservative forecast is in order with the usual caveats about storm track.  Any shift to the north, and this becomes a bigger deal.  Any shift to the south and we could be talking about another miss.

For now, I'll say 1-3 inches in the DC area, 2-4 points south of DC. 

More model data is coming in and if there's any substantive change, I'll be sure to post it.

Those of you who love snow:  hope for a northward shift which would get us in the "good stuff".

NAM Comes in South

The 18Z NAM model run has come in further south with the storm which isn't good for snow lovers up here in the DC area.  There's likely to be a pretty sharp gradient of total snowfall north of the center. 

Let's hope this is just a one-run problem because the trend had been more favorable until now.

I will watch the other models as they come in.

Winter Finally Arrives?

I'm watching a storm system that has become significantly more interesting in the past 24 hours.  A low pressure system tracking from the southeast had previously been forecast to move quite far to the south, making any adverse effect on our region unlikely.  In a "winter" full of near misses and no cold air, this didn't seem especially surprising.

However, in the last 24 hours, models have shifted their forecasts significantly northward.  So much so, that we're approaching the "sweet spot" of the storm where cold air and moisture are co-located.

The result?  Snow seems more likely now - and it could be a nice shot of snow during the daytime hours tomorrow.

The NWS office in Sterling has raised a Winter Storm Watch.  The official forecast now is for 2-5 inches in the advisory area which covers most of the DC area.  Because of the storm track, the totals actually increase to the south and areas around Fredericksburg could see more like 4-8 inches of snow.

If the storm track continues to shift north, some of that heavier snow could fall in the DC area.

Let's look at the maps:

Here's the GFS total forecast precipitation for the storm.  This is from last night's model run...we're still waiting for the data from this morning.

You can see the precipitation is heaviest in central Virginia but this is a significant northward shift from previous runs of the GFS.  That trend has continued and those darker areas which represent heavier snow are inching this way.

We should wait for the data from the morning model runs and the ones later this evening before making any solid forecasts.  For now, let's say the chance of accumulating snow tomorrow is better than 75%, the chance of more than an inch is well over 50% and it's safe to assume 2 inches or so will fall in the DC area with more certainly possible depending on storm track...
 
I'll update when more data comes in.

For what it's worth, here's the current National Weather Service Snow Total Forecast Map.  Our local NWS is well known for being conservative with snow storm forecasts so it's a bit of a surprise that they're really bullish about this storm.  Maybe they've seen model data I haven't yet.