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Posted at 12:25 PM ET, 02/17/2012

Sunday snow situation: could be boom or bust


Forecast weather map shows developing low pressure over the Carolinas Sunday morning (NOAA)
UPDATE, 1:05 p.m.: New European MODEL indicates storm will go too far south for significant precipitation in D.C. area Sunday. This adds more weight to LOW snow (BUST) scenario described below...

During the day yesterday and last night, it looked like models were coming to consensus that we would get significant precipitation Sunday, and at least some of it in the form of snow. Now, it’s not as clear. As the impressive southern storm moves from the western Gulf of Mexico to the North Carolina coast, precipitation will have a sharp northern cutoff. Whether that cutoff is in D.C. area or southern Pennsylvania will make all the difference with respect to the kind of weather conditions we experience Sunday.

The bottom line is that weather system presents our best shot of accumulating snow of more than 1” this season, and I believe that’s the most likely scenario. But, it’s almost equally likely we get next to nothing or more than 3 or 4”. If we get a period of heavy, accumulating snow, the most likely time would be from mid-to-late afternoon Sunday into Sunday evening.

Keep reading for more details...

Storm timing

Northern track scenario

Precipitation breaks out mid-Sunday morning into midday from southwest to northeast in the metro region, possibly as rain or a rain/snow mix. As precipitation become heavier, it should become all snow by mid-to-late Sunday afternoon. Snow would end from southwest to east between 7 and 10 p.m. Sunday evening.

Southern track scenario

If the storm ends up tracking farther south and just skirts us, then we’d probably just see a little light rain Sunday afternoon.

Potential snow amounts


BUST! The GFS model has moderate to heavy precipitation just south of D.C. between 1 and 7 p.m. Sunday. Temperatures are above freezing until you get to around Winchester and southward to near Charlottesville.
Low scenario - less than 1”:

If the low tracks south and precipitation shuts off over the D.C. area (placing us on the northern fringe), we could see little or no accumulating snow or even light rain - mainly Sunday afternoon. [SECOND MOST LIKELY SCENARIO]

Middle scenario -1-4”:

In order for us to get signficant snow, we must get into the storm’s heavier precipitation bands to sufficiently cool the air for snow accumulation. In this scenario, the metro region is on the northern edge of the heavier precipitation bands. We’d probably start off as rain or a mix, and change to snow as precipitation picked up in intensity Sunday afternoon and evening. Some accumulation on roads might occur after dark Sunday in this scenario, before precipitation shut off. [MOST LIKELY SCENARIO]


BOOM! NAM model shows heavy precipitation having accumulated between 1 and 7 p.m. Sunday around D.C. and just to the south. The freezing line, in dark blue, is well east of D.C. (StormVistaWxModels.com)
High scenario - 4+”

If the storm tracks far enough north, and the region is solidly in the heavy precipitation bands, there is the potential for more than 4” of snow. Snow would fall heavily for 4-8 hours Sunday afternoon and evening, and travel conditions would deteriorate from late afternoon into the evening. [THIRD MOST LIKELY SCENARIO]

Snowfall map?: I hate to reverse course, but uncertainty is too high to issue a map at this time. Last night, when I said we’d issue a map midday, it appeared like models were coming into agreement. They are not. The gradient between getting significant accumulating snow and very little or no snow is so tight, that it’s premature to take a stab. We’ll try to issue one this evening.

For what it’s worth, the National Weather Service is reserving judgment as well, as they stated in their morning discussion:

IT IS MUCH TOO EARLY TO TELL HOW MUCH SNOW WILL FALL AND WHERE. IT DOES APPEAR THAT AT LEAST A LIGHT TO MODERATE ACCUMULATION COULD BE POSSIBLE. WAITING FOR MORE CONSISTENT GUIDANCE BEFORE MAKING THAT DECISION. STAY TUNED.

Where will the most snow fall?

The best chance of significant snow is southwest of Washington, D.C. from southern West Virginia to around Charlottesville, including Roanoke and Harrisonburg.

Is this another Snowmagedddon? In short, no. The most snow we could see if everything came together perfectly would be less than half of Snowmageddon’s output.

What are the models forecasting? The latest NAM supports the high snow amount scenario, with about 0.75” liquid equivalent precipitation and temperatures near freezing. The GFS is in the low scenario camp, simulating just 0.3” liquid equivalent precipitation with temperatures above freezing at Reagan National. Last night’s Euro (ECMWF) model was also in the low scenario camp, but sometimes has a southern bias.


Probability of getting 1” of snow according to NOAA. It’s 70-80% around D.C. in this map. ( NOAA )
What is the National Weather Service saying about the models?

THE MAIN QUESTION IS HOW FAR SOUTH THE LOW WILL TRACK AND THE 00Z ECMWF CONTINUES A MORE SOUTHERN TRACK WITH THE GFS A TAD FURTHER NORTH AND THE NAM ON THE FURTHEST NORTH SOLUTION. THE NAM IS ALSO THE COLDEST SOLUTION. DID USE A BLEND OF THE THREE SOLUTIONS FOR THIS PACKAGE BUT DID COORDINATE WITH COLLEAGUES AT HPC AND AM LEANING MORE TOWARD THE GFS/ECMWF COMBINATION WITH A BIT MORE EMPHASIS ON THE GFS. FEEL THE NAM MIGHT BE AN OUTLIER ALTHOUGH WE HAVE NOT SEEN THE NEW 06Z GFS AS OF THIS WRITING.

By  |  12:25 PM ET, 02/17/2012

Categories:  Latest, Winter Storms

 
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