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Posted at 10:39 AM ET, 12/07/2011

Accumulation map, timeline, FAQs for minor snow event, significant rainfall


Advisory map from National Weather Service. Purple, winter weather advisory. Dark green, flood watch. Pink winter storm warning. Light green, flood advisory. (National Weather Service)
Let’s get real folks: this storm is more about the rain than it is about the snow. In most of the region, rainfall totals will be significantly higher than any snowfall. Already, around 1.5” of rain has fallen at Reagan National, breaking the daily record. Another 1-2” is possible before any changeover to snow. Ponding of water on roads and some minor stream flooding is possible. The afternoon/evening commute doesn’t look fun.

The changeover to snow, including a period of sleet during the transition, is most likely to occur between 9 p.m. and 1 a.m. beginning first in Loudoun and Frederick counties and reaching the I-95 corridor around midnight. Whether snow even falls east of I-95 is questionable. And even if it does, most will be asleep as the wet flakes melt on the bare pavement (temps above freezing).

Where it snows, it may briefly fall heavily (mainly between 10 p.m . and 1 a.m.), so a sloppy inch in the far north and west suburbs (such as western Montgomery and eastern Loudoun counties) can’t be ruled out, mainly on grassy areas. It’s this area under a winter weather advisory, although we think the 1-2” forecast from the National Weather Service is overdone. Inside the beltway, there’s just a 50/50 there’s enough snow to whiten the ground. The best chance for measurable snow that sticks to the roads is in Frederick and western Loudoun counties, at elevations above 1,000 feet.

The precipitation shuts off entirely between 1 a.m. and 4 a.m. from southwest to northeast. Areas mainly north and west of the beltway may need to be careful of slick spots for Thursday morning’s commute as rain and wet snow could re-freeze, especially on the vulnerable ramps, overpasses, and bridges.

ACCUMULATION MAP AND TIMELINE

6 PM to 9 PM WED
Heavy rain, falling temps from 48-41 (coldest W/NW).
9 PM to 12 AM
Rain to sleet and wet snow W suburbs to I-95. Rain elsewhere. Temps 43-35 (coldest W/NW).
12 AM to 3 AM THURS
Wet snow W of I-95 gradually ending. Rain/snow mix E. Temps 38-32 (coldest W/NW).
3 AM to 6 AM
Precip ends N and E of I-95. Slick spots possible W. Temps 36-31 (coldest W/NW).

Forecast Confidence: Low-Medium

IMPACT FORECASTS

SchoolCast (except 1.5 to 2 apples for Loudoun, Fauquier and Frederick counties)

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STORM FAQ

Where is the precipitation now? Follow the progress of the storm on national radar, regional radar and on our Weather Wall.

How confident are you in your forecast? We’re confident there will be a lot of rain, but confidence in the snow part of the forecast is low. Forecasting transitions from rain to snow, particularly when the amount of cold air coming in is marginal, is tricky business. A slight change in the timing of the arrival of the cold air could mean a bit more snow or no snow at all, with the chance of no snow increasing as you head east.

What does Wes Junker, CWG’s winter weather expert think?

Quote: “Run off from heavy rains will be a much bigger problem than snow for most of the area. The models have incrementally shifted the storm track north on the past few runs and now the 850 low (circulation at around 5000 ft) is predicted to track over or just to the north of DC. That’s not a good track for snow as it puts the D.C. area on the warm side of the storm. And once the storm gets to our north and east the cold air coming in behind the storm usually starts the drying process.

“As for models, the NAM is now pretty much in the GFS camp as to how much snow will fall at Reagan National and Dulles with both models showing only light amounts. Essentially, the models are showing a short period of snow with temperatures above freezing making it difficult to get any accumulations except possibly where there is higher terrain well west and north of the city.”

Where will conditions be worst? In terms of snow, the farther north and west you go towards the mountains and up in elevation. Elevated areas in Frederick and Loudoun counties and to the west above 1,000 feet could get a few inches.

What’s the worst case scenario for snow inside the beltway? We get a thump of heavy snow between 12 and 2 a.m., enough to lay down a quick inch on grassy area. But odds of this happening are less than 50/50. It’s equally likely it won’t snow at all and what little snow does fall has trouble sticking (temperatures were in the 60s just last night) .

Is there a risk of a refreeze and slick roads Thursday morning? Could there be school delays? Mainly in the north and west suburbs, outside the beltway. It’s not inconceivable this might result in a few delayed openings - with the best chance in Loudoun, Fauquier, and Frederick counties. My bet is Fairfax and Montgomery counties open on time. Do your homework...

By  |  10:39 AM ET, 12/07/2011

Categories:  Latest, Winter Storms

 
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