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Posted at 01:53 PM ET, 02/16/2012

Sunday storm chances steady, snow odds still iffy

We continue to see the potential for a winter storm Sunday and at least a chance that some precipitation falls as snow.

The big forecast problem still is whether the air in the lowest part of the atmosphere will be cold enough to support snow, and if it does become cold enough, what percentage of the precipitation will be snow rather than rain.

Unfortunately, the answer to that latter question will probably not be known until at least some time tomorrow, possibly even later. Right now, I think odds favor a period of snow towards the end of the event and that an all rain event is less likely. On the other hand, it doesn’t look like an all snow event (unless you live along and west of I-81), and there’s a chance the storm just grazes us.

Having said all of that, accumulating snow is a possibility in the D.C. area. For that reason, we’re introducing the snow lover’s crystal ball. Changes in storm track, how quickly and where the storm develops, and the intensity of the precipitation all could impact on the surface temperatures and accumulation potential of the storm.

Next chance of accumulating snow: Sunday, February 19
Percent chance of accumulating snow (1” or more):
30-50% (east to west)
Snow timing: Sunday into Sunday evening (best chance Sunday afternoon and evening)
Forecast confidence: Low-Medium

For snow lovers, this is our best chance yet of getting a decent snowstorm. The Capital Weather Gang thinks the probability of getting an inch or more of snow is now in the 30 to 50% range across the area with the higher probabilities being in the western and northern suburbs and the lower ones for those living from the city south and east.

I don’t see this as being a major snowstorm for our area as it will be fast moving with marginal temperatures and will be occurring during the day. And there’s still a chance the storm largely misses us to the south. Still, this system has the makings of the heaviest snowstorm this season providing the surface temperatures cooperate

MODEL DISCUSSION

Almost all the models are now forecasting that a storm will track across the Gulf States to a position off the North Carolina coast by Sunday evening. Most but not all (the new European model being the important exception) spread significant precipitation into our region as temperatures fall into the low 30s. Now the biggest point of contention between model camps is whether the storm will be mostly a snow producer or one that bring significant rain before any changeover to snow.

Below is an array of model solutions from last night (top two panel) and this morning (bottom two). All the models have a storm off the North Carolina coast by 7 p.m. Sunday evening. All also have a high pressure system to the northwest of the storm. Since air moves from higher to lower pressure, all the models also introduce low level northerly flow across our area facilitating the southward movement of marginally cold air into the area.


Top panels: last night’s runs of the Euro and GFS models, valid 1 p.m. Sunday. Bottom panels: This morning’s runs of the NAM and GFS models, valid 7 p.m. Sunday.
Last night’s European model (top left) valid at 1 p.m. Sunday afternoon still has the freezing line well to the west of the city. Even at 7 p.m. Sunday evening, it still keeps the freezing line well west of the city (not shown) suggesting that most of the European model precipitation on last night’s run would be rain with the rain ending as a period of sloppy wet snow. This GFS forecast for 1 p.m. Sunday has the freezing line (blue line) almost in the same place as the European model.

Before precipitation begins, both the GFS and Euro (from last night, top two panels) have the surface temperatures in the low 40s. Then the cooling starts but much of the precipitation falls during daylight hours with late February sun.This morning’s GFS and NAM (bottom two panels) both press the freezing line at the surface to just west of DC by 7 p.m. and suggest most of the area would be snow by that time. Both models suggest a wet snow could start in the region as early as 1 p.m. Sunday.

The snowfall intensity will probably have to be moderate to heavy to get much accumulation especially on the roads. Assuming this system comes far enough north, it should have enough moisture to produce enhanced precipitation rates. But that’s not a given. This morning’s UKMET and Euro forecasts have a suppressed track suggesting that a near miss or graze is still a possibility. Taking together all of these uncertainties makes any deterministic forecast of snowfall for this storm this early no better than a guess.


Left panel: track of low pressure at 850 mb (5,000 feet). Right panel: low pressure and vertical velocities at 700 mb.
On the plus side for snow lovers, the GFS and NAM have nearly perfect tracks to give some snow to the area as it track sufficiently south of us to keep us in the cold air and in any deformation zone (comma head). The 850 mb low (the center near 5000 ft) track across southern Virginia (the magenta arrow, left panel in the above image) is one that helps pull in cold air as the system develops. As the low deepens the temperature lines usually start squeezing together which is one of the reasons our temperatures drop during the day. This squeezing of the temperature lines (frontogenesis) increases the lifting along and just north of the low. The darker green area on the 700 mb forecast essentially shows the area where clouds and precipitation is located. The track of the low and mid-level centers on the GFS and NAM are really good ones for producing accumulating snow.

Now that I’ve discussed how the GFS and NAM are pretty bullish about the storm, it’s time to temper snow lovers’ expectations. Below is a plume diagram showing the precipitation for the various Short Range Ensemble Forecast System (SREFS) members from this morning’s 09Z (4AM) run. The lines show the accumulation of precipitation through the run. Note after today’s light rain, all the green lines (indicating rain) flat line until Saturday night into Sunday when some of the lines start climbing rapidly uphill indicating lots of precipitation is occurring while other members remain flat (no additional rain). Just as discouraging is the fact that, for DC and Baltimore, the 09Z run of the SREFS has more rain members (green) than snow members (blue) due to the warm surface temperatures.


Plume diagram which shows how different members of the SREF model tend to favor rain over snow in D.C., and don’t simulate a lot of precipitation.
The plume diagram certainly points out how much uncertainty there is regarding precipitation amounts with the storm as a number of members remain flat and out to sea. The new European came in and has trended south with its track giving us between .15 and .25 inches or precipitation from the system. However, it also has a very tight gradient (north to south variation) that places the 0.50” amounts pretty close to us.

This tight precipitation gradient is another factor that makes trying to offer accumulation forecasts this early a questionable enterprise. Today’s European model run and the plume diagram above are two reasons for suggesting it’s not yet time to go overboard with our probabilities for accumulations. This storm has the potential to be our heaviest snow producer this winter. It also has the potential to be a bust. Stay tuned.

By  |  01:53 PM ET, 02/16/2012

 
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