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Posted at 02:18 PM ET, 12/07/2011

Flood warning issued due to relentless rain; some snow likely late tonight as cold air filters in

* Flood warning thru 8 p.m. immediate metro area (map) *
* Winter weather advisory N Fauquier, Loudoun, Frederick, Montgomery & Howard counties 7 p.m. to 3 a.m. (map)*
* Snow accumulation map, timeline & frequent questions *


Doppler estimated rainfall totals through 2 p.m. show generally 0.75-2” has fallen in the region (though somewhat lesser amounts to the NW and SE). (National Weather Service)
A flood warning for urban areas and small streams has been issued through 8 p.m. for the greater Washington D.C. area. Flood watches have been issued for surrounding areas through late tonight. Rainfall amounts have already reached 1-2 inches and another 1 to 1.5 inches is possible.

The National Weather Service states:

A FLOOD WARNING MEANS THAT FLOODING IS IMMINENT OR HAS BEEN REPORTED. ALL INTERESTED PARTIES SHOULD TAKE NECESSARY PRECAUTIONS IMMEDIATELY. RAINFALL ON ALREADY SATURATED GROUNDS WILL CAUSE CREEKS AND STREAMS TO RISE OUT OF THEIR BANKS.

Remember: never try to cross a flooded roadway - turn around, don’t drown. Some of the heaviest rain may fall during rush hour, so leave extra time for your commute.


Satellite image at 3 p.m. (NASA)

The red area over northern Georgia and southern Tennessee represents the upper level low described in the text.
You can see on the satellite image above from around 3 p.m. that, in addition to extensive wall of cloud cover covering the East Coast (extending into the Gulf of Mexico, associated with the cold front), a large swirl is present over the Southeast and Tennessee Valley. This comma shape swirl coincides with an intense area of spin in the atmosphere about 18,000 feet up and is called an upper level low or “vorticity maximum” (see image to the right).

As the upper level low draws closer, the intensity of the precipitation will increase. And until it pivots through the region from southwest to northeast later tonight, a steady stream of moisture will continue to feed into the area .

While the rain continues, with some of the heaviest yet to come, cold air is rushing into the region. The cold front has passed to our east (see image to right). To show an example of the temperature contrasts, at 1 p.m. Richmond, southeast of the front was 70, while Pittsburgh, northwest of the front, was 35.


Cold front as of 1 p.m. (National Weather Service)
In the immediate D.C. metro region, temperatures were still in the the low-to-mid-50s, but should fall back to near 50 by dusk. The sharper decline in temperature will occur between about 5 pm and 11 pm. tonight, when the transition from rain to snow will occur in the colder west and northwest suburbs towards 9-11 p.m. For our forecast of how much snow to expect, click here.

By  |  02:18 PM ET, 12/07/2011

Categories:  Latest, Floods

 
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