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Posted at 01:45 PM ET, 05/18/2011

Severe thunderstorm watch through 9 p.m.


Area covered by severe thunderstorm watch (NOAA)

Overview (posted at 1:10 p.m.): A severe thunderstorm watch has been issued for the entire metro region through 9 p.m. this evening. Storms with heavy rain (capable of flash flooding), dangerous lightning, damaging winds to 70 mph and hail are possible. In addition, there is a small risk of tornadoes.

Although showers and thunderstorms this afternoon/evening will likely be more widespread than the past several days, they will still tend to be hit or miss - so many will not experience severe weather.

Keep reading for more on the ingredients in place for possible severe weather and earlier updates...

Several of the necessary ingredients have come together for the development of severe thunderstorms and possibly a few tornadoes this afternoon. The tornado risk exists due to the turning of the winds with height or wind shear. The same set of conditions that supported the tornado development near Hagerstown yesterday evening remain in place today.

Here’s how NOAA’s Storm Prediction Center described the threat (it’s technical but, really, fairly easy to comprehend):

A FEW STORMS HAVE ALREADY SHOWN SIGNS OF LOW LEVEL ROTATION. GIVEN EFFECTIVE BULK SHEAR 40-45 KT ACROSS THIS REGION TODAY... . . . ANALYSES SHOWED AN ENHANCED AREA OF AMBIENT SURFACE VORTICITY [SPIN] OVER NORTHERNN VA/EASTERN WV PANHANDLE TO SOUTHWESTERN PA/SOUTHEASTERN OH WITH 0-3 KM CAPE [A MEASURE OF INSTABILITY AND ENERGY] UP TO 100-125 J/KG. THESE FACTORS SUGGEST AN INCREASED POTENTIAL FOR TORNADO DEVELOPMENT. IN ADDITION TO A TORNADO THREAT...THE STEEP MIDLEVEL LAPSE RATES SUPPORT A THREAT FOR HAIL.

The National Weather Service Office in Sterling provided a nice overview of the volatile conditions in its morning discussion:

ABUNDANT MOISTURE ALONG WITH UNUSUALLY COLD AIR ALOFT DUE TO THE UPPER-LEVEL LOW WILL CONTINUE TO CAUSE AN UNSTABLE ATMOSPHERE. THE INSTABILITY ALONG WITH THE DEEP MOISTURE WILL CAUSE SHOWERS AND T-STORMS TO PRODUCE LOCALLY HEAVY RAINFALL. TRAINING CONVECTION IS ALSO LIKELY WITH THE STORM MOTION BEING FROM SOUTHEAST TO NORTHWEST.

THE HEAVIEST RAINFALL IS EXPECTED LATE THIS MORNING THROUGH THIS AFTERNOON WHEN INSTABILITY WILL BE AT ITS HIGHEST. A FLASH FLOOD WATCH IS IN EFFECT FOR THE ENTIRE FORECAST AREA THROUGH THIS AFTERNOON.

ISOLATED THUNDERSTORMS MAY BECOME SEVERE GIVEN THE INSTABILITY LATE THIS MORNING THROUGH THIS AFTERNOON. THE PRIMARY THREAT WILL BE DAMAGING WINDS...LARGE HAIL AND EVEN AN ISOLATED TORNADO.

HOWEVER ... THE COVERAGE OF SEVERE WEATHER APPEARS THAT IT WILL REMAIN ISOLATED DUE TO EXTENSIVE CLOUD COVER. SHOULD THERE BE MORE BREAKS OF SUNSHINE THIS MORNING INTO THIS AFTERNOON THEN MORE WIDESPREAD SEVERE WEATHER IS POSSIBLE.

Earlier (expired, no longer valid) updates:

3:20 p.m. update: Severe thunderstorm warning in effect for central to northwest Loudoun county until 3:45 p.m. North bound storm may produce quarter-size hail from Purcelville to Charles Town WV

2:50 and 3:00 p.m. update: The atmosphere is spinning up. On the Eastern shore (for western Talbot and southwest Queen Anne’s county through 3:30 p.m.) and north of Baltimore (for NE Baltimore and NW Harford counties through 3:30 p.m.) tornado warnings have been issued. In fact, at 2:35 p.m fire department personnel reported tornado on ground near Oxford or 8 miles SW of Easton, moving north at 25 mph.

2:15 p.m. update: A Flash Flood Warning has been issued for central & northern Prince William county, northwest Fauquier county and southwest Loudoun county through 5 p.m.. Thunderstorms, with torrential rain, are continuously moving over the same areas or “training”. 1-2” of rain are possible in these areas, likely leading to flash flooding. Remember: never try to drive through flooded roadways. Turn around, don’t drown.

1:45 p.m. update: No severe thunderstorms warnings are currently in effect across the region. We have a bit of a D.C. split going on. The heaviest activity is southwest of the District in Prince William and Loudoun counties. A second area of showers and storms stretches from the eastern side of the Chesapeake Bay through Baltimore and into southern Pa.

By  |  01:45 PM ET, 05/18/2011

Categories:  Forecasts, Thunderstorms

 
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