6:45 p.m. update: Partial clearing has already begun along and west of I-95 where the flooding threat is over. For southern Charles and most of St. Mary’s county, however, heavy showers and storms continue prompting flood warnings that remain in effect through 9:30 p.m. Two to three inches of rain are likely there before the rain diminishes (in the next one to two hours), causing flooding.
Elsewhere, for the remainder of the night, skies will be variably cloudy. Some additional shower activity cannot be entirely ruled out (20-30 percent chance) between 11 p.m. and 3 a.m. After that, we dry out with sunshine and drier air for both Saturday and Sunday. Highs both days should be near 90.
Monitor severe warnings issued by the National Weather Service and radar on our Severe Weather Tracking Station below. Also, comment below with conditions where you are and keep reading for earlier updates and techincal discussion...
5:45 p.m. update: The severe thunderstorm watch has been discontinued along and west of I-95, including D.C. and Baltimore. It remains in effect in Prince George’s, Anne Arundel, Calvert, Charles and St. Mary’s counties. A new severe thunderstorm warning has just been issued for southern Anne Arundel, southeast Prince George’s, northwest Calvert and northeast Charles county until 6:30 p.m. for a storm capable of producing winds to 60 mph.
5:30 p.m. update: A severe thunderstorm warning is in effect for much of Charles county and southern Prince George’s county thru 6:15 p.m. Winds to 60 mph are possible with this storm which will affect Indian Head, La Plata, St. Charles and Waldorf.
5:25 p.m. update: While nasty storms rip through south central Prince George’s and Anne Arundel counties, conditions have improved markedly to the northwest. The Severe Thunderstorm Watch has been discontinued for Frederick, Loudoun and Fauquier counties.
5:15 p.m. update: Flash flood warning for southeast D.C., central Prince George’s & southern Anne Arundel county until 8 p.m. One to two inches of rain is likely in these areas which may causes flooded roads/streams. A new severe thunderstorm warning has also been issued for southeast Fairfax county through south central Prince George’s county until 6 p.m. In path: Camp Springs, Andrews Air Force Base, Upper Marlboro. Torrential rain is the main impact with these storms but isolated wind gusts may reach 60 mph.
5:05 p.m. update: The worst appears to be over in Frederick county and northwest Montgomery county where skies are brightening. The heaviest action now stretches from near Annapolis thorugh central Prince George’s county southwestward toward through Mt Vernon and then to around Dale City and ultimately just west of Fredericksburg. A Severe Thunderstorm Warning is in effect for southern Prince William, southern Fauquier, the western two third of Stafford, and northeast Spotsylvania counties through 5:45 p.m. for storms that may contain wind gusts up to 60 mph.
4:40 p.m. update: Summary - Widespread showers and storms - some heavy - are affecting most of the metro region and should continue for the next couple of hours. The heaviest rain has fallen in the north and northwest suburbs from Frederick county into extreme northeast Loudoun county cutting across central Montgomery county north of the beltway. Many of these areas under Flash Flood Warnings (as described below and after the jump) through 6:30-7:15 p.m.
4:15 p.m. update: A Flash Flood Warning has been issued for central and eastern Montgomery county, and northern Prince George’s and Anne Arundel counties until 7:15 p.m. One to three inches of rain is likely in these areas, and may flood streams and roads. Remember, don’t drive through flooded roads: turn around, don’t drown. To the southeast, a Severe Thunderstorm Warning is in effect for eastern Prince George’s and the southern two thirds of Anne Arundel county until 5 p.m. In possible path: Odenton, Severna Park and Annapolis. Damaging winds to 60 mph may accompany this storm.
4:00 p.m. update: Checkout flooding photos from downtown Frederick, which has now received about 1.5-2” of rain (based on doppler radar estimate).
3:40 p.m. update: So far, the worst weather has occurred in the northwest suburbs in Montgomery and Frederick counties, which have experienced torrential rain. In Frederick county, there’s a Flash Flood Warning through 6:30 p.m. Doppler estimated rainfall has exceeded 1” around Frederick. For areas to the south, a solid line of showers and storms is moving east through west central Virginia and should arrive between 5 and 7 p.m., with some isolated showers prior to that.
Overview (from 2:30 p.m.): The Washington, D.C. metro region is under a severe thunderstorm watch covering a large section of the northern mid-Atlantic into southern New England through 9 p.m. tonight. A cold front is slinking south, interacting with a very humid airmass over the region. Intense thunderstorms are developing, capable of producing torrential rain, dangerous lightning, damaging winds, and small hail.
Because the front is slow moving, storms may remain and/or track over (or “train” through) the same areas for extended periods of time resulting in flash flooding. This occurred last night in Baltimore, resulting in four to five inches of rain near the Inner Harbor over just a few hours. Hence, a flash flood watch is in effect. Remember that you should never attempt to drive across a flooded road. Turn around, don’t drown.
Technical discussion for Severe Thunderstorm Watch
NOAA’s Storm Prediction Center in Norman, Oklahoma gave the following reasoning for the severe thunderstorm watch:
CLUSTERS OF THUNDERSTORMS ARE EXPECTED TO CONTINUE DEVELOPING AND INTENSIFY THIS AFTERNOON IN ADVANCE OF A SHORT WAVE TROUGH MOVING EASTWARD TOWARD THE MID ATLANTIC REGION. AIR MASS IS VERY MOIST WITH SURFACE DEW POINTS IN THE LOW 70S...AND DIABATIC HEATING HAS RESULTED IN DESTABILIZATION WITH MID-LEVEL CAPE OF 1500 J/KG. SOUTHWESTERLY MID-LEVEL WINDS OF 30-40 KT ARE CONTRIBUTING TO SUFFICIENT VERTICAL SHEAR TO ENHANCE THREAT FOR SEVERE STORMS. LOCALIZED DAMAGING WIND GUSTS AND HAIL WILL BE THE PRIMARY THREATS.
Technical discussion for Flash Flood Watch
The National Weather Service Forecast Office in Sterling issued the flash flood watch for the following reasons in its Flash Flood Statement:
A WEAK COLD FRONT WILL DROP INTO A VERY MOIST AND HUMID AIRMASS THIS AFTERNOON AND EVENING. NUMEROUS THUNDERSTORMS ARE EXPECTED TO DEVELOP EARLY THIS AFTERNOON...AND CONTINUE INTO THE EVENING. RAINFALL RATES WITHIN THESE THUNDERSTORMS MAY EXCEED 2 INCHES IN LESS THAN AN HOUR. SEVERAL THUNDERSTORMS POTENTIALLY COULD TRACK ACROSS THE SAME AREA...RESULTING IN FLASH FLOODING.