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Posted at 02:24 PM ET, 08/14/2012

Strong to severe storm chance this evening


NOAA says storms may develop between 4 and 6 p.m. in the circled region, but potential is “low” for severe wind gusts and hail. (SPC)
A slow-moving cold front may produce scattered strong thunderstorms, beginning late this afternoon and especially into this evening. Considerable sunshine late this morning and this afternoon are helping to destabilize the atmosphere, increasing the potential for heavy storms. NOAA’s Storm Prediction Center indicates a slight risk of severe thunderstorms but that issuance of a severe thunderstorm watch is unlikely.

Timing: Thunderstorms were starting to develop in eastern West Virginia as of about 1:30 p.m. (link: regional radar). They’ll probably reach the I-81 corridor by mid-to-late afternoon and D.C.’s far western regions (Loudoun and Frederick counties) by late afternoon - although they may track northeast rather than directly east, delaying arrival. In the immediate metro region, storms are most likely between 6 and 10 p.m. A few isolated strong-severe storms are possible before that.

Coverage: While we think storms will be more widespread than last Friday (when they did not materialize as forecast) - thunderstorm forecasting is difficult and there’s a chance some chunk of the area may not see much in the way of storms. Areas north of town (60-70 percent chance) are more likely to see storms than south of town (40-50 percent chance).

Impacts: Downpours and lightning are a given in most storms. Because they may be slow moving, isolated flash flooding reports can’t be ruled out. Damaging winds and hail are also a possibility but reports will probably be isolated to scattered rather than widespread (i.e. no derecho). Spin in the atmosphere *might* be sufficient to spin-up an isolated small tornado.

Technical discussion:

This morning, the National Weather Service in Sterling posted a detailed analysis of the storm potential noting an “isolated tornado threat”. Here’s an excerpt:

SINCE THERE/S LESS CLOUD COVER THAN PREVIOUSLY THOUGHT...AND A 90 TEMP/70 DEWPOINT WOULD YIELD ALMOST 3000 J/KG SBCAPE BECOMING MORE CERTAIN THAT THUNDERSTORMS WILL BE REALIZED...A FEW OF WHICH SHOULD BECOME SEVERE.

PRIMARY THREAT WOULD BE DAMAGING WINDS...BUT AN ISOLATED TORNADO THREAT EXISTS NEAR THE SURFACE BOUNDARY. FORECAST HODOGRAPHS FOR NORTH-CENTRAL AND SOUTHERN INDICATE AN INCREASINGLY FAVORABLE SETUP FOR TORNADOES BY LATE AFTERNOON AS STRENGTHENING BOUNDARY-LAYER WINDS BACK SELY TO THE NORTH AND EAST OF THE MESOLOW.

In its afternoon update, NOAA’s Storm Prediction Center, however, said the tornado threat may not materialize:


NOAA’s SPC indicates a 2-5 percent chance of tornadoes with 25 miles of a given point in the shaded areas. (NOAA Storm Prediction Center)
LATEST NAM-WRF GUIDANCE SHOWS MUCH WEAKER LOW LEVEL SHEAR WITH LITTLE OR NO TORNADO POTENTIAL INDICATED. GIVEN RELATIVELY HIGH CONVECTIVE QUANTITATIVE PRECIPATION FORECAST SIGNAL NEAR A FAVORABLE CYCLOGENESIS REGION IT SEEMS PRUDENT TO MAINTAIN A SMALL RELATIVELY HIGHER 5 PERCENT PROBABILITY TORNADO FORECAST FOR THE TIME BEING FROM NOTHERN DELMARVA INTO SOUTHERN PA/CAPE MAY AREA.

By  |  02:24 PM ET, 08/14/2012

 
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