Thursday Bow Echo Storms

Thursday, thunderstorms developed in the humid air ahead of a cold front. The greatest area of instability was just south of the Baltimore-Washington area and some powerful thunderstorms developed what is called a “bow echo”. Below is the statement from the National Weather Service concerning the storms and radar and satellite imagery of the situation.

PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
1003 PM EDT THU JUL 19 2007

…PUBLIC INFORMATION STATEMENT…

…BOW ECHO ACROSS NORTH CENTRAL VIRGINIA AND LOWER SOUTHERN
MARYLAND…

EARLIER THIS AFTERNOON…A STORM PRODUCED WIND DAMAGE ACROSS PORTIONS OF NORTH CENTRAL VIRGINIA FROM FAUQUIER…STAFFORD AND KING GEORGE COUNTIES EASTWARD ACROSS SOUTHERN MARYLAND.

THIS WAS A WELL ORGANIZED TYPE OF CONVECTIVE STORM CALLED A BOW ECHO. SURFACE WINDS ASSOCIATED WITH THIS STORM WERE MEASURED BETWEEN 60 AND 65 MPH…WITH GUSTS UPWARDS OF 80 MPH.

THE STORM RAPIDLY ORGANIZED INTO A BOW ECHO AS IT CROSSED THE POTOMAC RIVER FROM VIRGINIA INTO MARYLAND. A LARGE AREA OF HIGH WINDS WITH GUSTS MEASURED AT NEARLY 65 MPH SPREAD OUT FROM THE STORM SYSTEM ALONG THE GUST FRONT. AT TIMES DOPPLER RADAR
INDICATED WINDS COULD HAVE BEEN AS HIGH AS 80 MPH AS THE STORM MOVED ACROSS SOUTHERN CHARLES COUNTY AND INTO ST. MARYS COUNTY IN LOWER SOUTHERN MARYLAND.

THIS TYPE OF STORM SYSTEM IS NOT UNCOMMON IN THESE PARTS. THEY CAN OCCUR UNDER CERTAIN COMBINATIONS OF ATMOSPHERIC INSTABILITY AND VERTICAL WIND SHEAR. ALTHOUGH MUCH OF THE AREA HAD CONSIDERABLE CLOUDINESS EARLIER DURING THE DAY…THE AIRMASS ACROSS WEST-CENTRAL AND NORTH-CENTRAL VIRGINIA AND LOWER SOUTHERN MARYLAND EXPERIENCED A GOOD DEAL MORE SUN THAN THE REST OF THE REGION. ALSO HIGHER MOISTURE IN THE LOWER ATMOSPHERE IN THIS SAME AREA HELPED FUEL THE STORM. WINDS JUST ABOVE THE SURFACE WERE AROUND 25 MPH FROM THE SOUTHWEST WHILE ALOFT UP AT 10-15 KFT…THEY WERE MUCH STRONGER…RANGING FROM 50 TO 60 MPH FROM THE WEST AND NORTHWEST. THIS
CREATED A FAVORABLE VERTICAL WIND PROFILE TO HELP MAINTAIN THE STORM FOR SEVERAL HOURS.

Notice on the satellite picture above that the line of storms is moving out over the Atlantic Ocean and there is a band of low clouds running out ahead of the main area of convection. This is an indication of the strong outflow from the mature storms that had earlier moved over southern Maryland.

John Collins

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