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Posted at 10:40 AM ET, 09/08/2011

Washington, D.C. flooding: What’s behind the rain train?


Water vapor image from Wednesday night showing deep moisture streaming up the East Coast from the southern and eastern Gulf of Mexico as well as from the Atlantic ocean.
Relentless and punishing: words used to characterize the heat in June and July can now describe the onslaught of torrential showers and thunderstorms since late Sunday. 6-10 inches of rain has swamped areas immediately east of the District, resulting in widespread flooding. An uncommon combination of moisture streams has converged to make this prolific rain-producer possible.

The National Weather Service Office in Binghamton, New York posted an excellent overview of the situation last night:

Bad combo of ingredients in place to continue very heavy rainfall this evening. Closed upper low continues to spin over the Ohio Valley [associated with the remnants of Lee]...with a deep southerly fetch along the Eastern Seaboard...only adding to the moisture in place from the remains of Tropical Storm Lee. As an additive factor...water vapor loops show [high level] tropopause moisture from distant Hurricane Katia...getting steered all the way into the heavty rain area...from the mid-Atlantic into NY/PA. This synoptic pattern resembles the major flood event from June 2006...


Six hour forecast from last night’s NAM model. It shows 10 meter winds, surface temperature, and precipitation. Note the convergence zone lining up along the red line where temperatures were about 70 degrees , which effectively served as the railroad trackfor training precipitation. See this radar animation.
Indeed, the water vapor image above shows moisture from tropical storm Nate deep in the Gulf of Mexico and Katia several hundred miles offshore the mid-Atlantic converging along the U.S. East Coast with the remnant moisture and circulation from tropical storm Lee providing further enhancement.

As I wrote Tuesday, the persistence of the rain is due to a log jam in the atmosphere with the remnants of tropical storm Lee (and its associated upper level low pressure) at one end of the jam and Hurricane Katia on the opposite end. The mid-Atlantic and Northeast U.S. are caught in the middle with moisture being entrained from both sides.

Radar loop every three hours from 11 p.m Sunday night to 8 a.m. Thursday morning.


Four day rainfall totals. (National Weather Service)
The resulting rainfall totals have been remarkable. From the District and points west, generally 3-6” of rain has fallen, but as mentioned earlier, east, north, and northeast of Washington 6-10” has mounted since late Sunday. Today alone, BWI airport has received 2.31”, a new rainfall record for the date. It has received about 9” of rain in the last week. Reagan National Airport received 2.82” yesterday and has received over 5” this week. Just west of the heaviest rain bands, Dulles has received just over 4” of rain since Sunday.

Here are some sample rainfall totals courtesy NOAA:

Maryland

WALDORF 3.6 SSE 11.66
ELLICOTT CITY 1.7 N 11.36
CROFTON 1.5 NNE 10.21
ANDREWS AFB/CAMP SPRINGS 8.18
ANDREWS AFB 7.37
BALTIMORE 7.04
CRESAPTOWN-BELAIR 0.9 SSE 6.55
ANNAPOLIS - US NAVAL ACADEMY 4.59
HAGERSTOWN RGNL ARPT 3.92
HAVRE DE GRACE 4 WNW 3.04
ANNAPOLIS 3.00

Virginia

ROANOKE MUNI ARPT 6.14
FORT BELVOIR/DAVISON AFB 5.60
CHARLOTTESVILLE ARPT 5.60
QUANTICO MCAF 5.32
RICHMOND/BYRD FIELD 4.78
WASHINGTON/DULLES 4.33
RICHMOND 3.83
MANASSAS MUNI ARPT 3.28
NEWPORT NEWS/WILLIAMSBURG INL AP 3.01

Additional rainfall is possible through the weekend - but the coverage, duration, and intensity should decrease some each day.

In terms of amounts, NOAA’s HPC writes:

AN ADDITIONAL 1 TO 4 INCHES OF RAIN...WITH LOCALLY HIGHER AMOUNTS...ARE POSSIBLE THROUGH SATURDAY ACROSS PORTIONS OF THE MID-ATLANTIC AND NORTHEASTERN STATES.

I’d say we’ll probably be on the low end of that range, but any new rain at this point may pose problems.

By  |  10:40 AM ET, 09/08/2011

Categories:  Latest, Floods, Local Climate

 
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