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Posted at 08:40 PM ET, 12/07/2011

PM Update: Periods of heavy flooding rain to change to wet snow before ending overnight

* Flood warning thru 12 a.m. immediate metro area (map) *
* Flood warning thru 1:30 a.m. Calvert, Prince Georges and Anne Arundel counties *

* Winter weather advisory N Fauquier, Loudoun, Frederick, Montgomery & Howard counties 7 p.m. to 3 a.m. (map)*
* Snow accumulation map, timeline & frequent questions *

8:40 p.m. update: Heavy rain has pounded the metro area over the past couple hours, increasing the day’s total at Reagan National so far to 2.70”. Meanwhile, colder air has made good progress toward and into the area from the north and west, with temperatures now down to the mid-30s in Loudoun and Fauquier counties, upper 30s to low 40s in Fairfax and Montgomery counties, and low 40s from D.C. to the east and south. We’ve gotten some reports of the rain mixing with snow in the far western suburbs including an unconfirmed report from Leesburg. But for the time being the main story remains the heavy rain, which has prompted flood warnings for much of the area and led to a water rescue in Beltsville. Be careful out there.

8:00 p.m. update: The earlier flood warning for D.C. and the immediate metro area has been extended until midnight and an additional flood warning to the southeast including Calvert, and Prince Georges coutnies, is up ‘till 1:30 a.m. Numerous reports of standing water and flooded roadways have come in this evening. Turn around if you come across high water. It is impossible to judge its depth particularly at night.

6:30 p.m. update: The rain onslaught continues and radar shows impressive coverage and intensity of rain to the southwest. Rainfall at Reagan National is up to 2.14" today. Temperatures are edging colder, ranging from the low 40s in western Loudoun county, to the mid-40s in central Fairfax and Montgomery counties and then to near 50 in the District. It will take two to four more hours before we see temperatures in the 30s more common and the first snow reports locally.

From 4 p.m.: Waves of rain keep on coming and that will continue through the evening as a low pressure system organizes to our south while rapidly heading to the northeast. Temperatures that were near 60 this morning have fallen back into the 40s well north and west of the city to the low-and-mid 50s elsewhere. More cold air is coming, but it will continue to be a race between it and the quickly departing moisture feed later tonight.

Through Tonight: Periods of rain, some heavy with maybe a rumble of thunder, continue into the late evening. By about 7-9 p.m., rain may begin to mix with sleet and snow over the western portions of the area. This mix and ultimate changeover should shift east toward the city heading into midnight. Places to the south and east may never change over before it ends, or just see a brief period of snow with no accumulation. Precipitation probably comes to an end entirely between midnight and 2 a.m. south and west to north and east – the trend has been faster of late. Winds are intense for a bit from the northwest, around 20 mph with gusts past 30 or 35 mph. Lows reach near 30 to the mid-30s.

For more details on accumulation prospects (generally up to a coating to 1” D.C. and northwest except for 1-3” over farther west and north suburbs), see our timeline (though it might be a little slow now) and map from earlier.

Tomorrow (Thursday): Warmth is a memory as we plunge below normal for a change. There’ll be plenty of sun though, and winds should die off pretty quickly — they may still be quite gusty early — as the storm races away to the northeast. Still, we’re looking at west winds around 10 mph or so during the day, perhaps diminishing more as the afternoon wears on. Highs only make the low-and-mid 40s! Brrrrr.

See Dan Stillman’s forecast through the weekend. And if you haven’t already, join us on Facebook and follow us on Twitter .

Southern snow: The upper-level impulse set to spark the changeover to snow around here later has already dropped some snow down into Mississippi, Alabama and other states in the South. Winter Storm Warnings were issued for parts of northeast Alabama as up to 3” of snow was expected from the system. You can track the progress of the upper-level low on this U.S. radar -- it is presently rotating mainly through eastern Tennessee and western North Carolina. It will rocket toward this area the next few hours.

Snow team: The “D.C. snow team” will be out in force tonight, including 45 plows mainly focused on bridges. We don’t anticipate these guys will have too much to do in the city, but it’s not a bad practice run perhaps?

Rainfall records: Through 4 p.m., Reagan National had record 1.93” of rain, breaking the 1971 record of 1.34”for the date. Another 0.16” fell between 4 and 5 p.m edging this event closer to the December monthly record (dating back to 1871) of 2.81”from December 18, 1977. Update (6:30 p.m.): 2.15” through 6 p.m., raining heavily.

By  |  08:40 PM ET, 12/07/2011

Categories:  Forecasts

 
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