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September 2012 obs/discussion


Ian

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Lightning has been spectacular at dulles since about 705pm tonight. Left there and just got home and right in Eastern Ashburn the skies opened up and the vivid lightning continued. Temps went from 84 at Dulles to 68 here in Leesburg. Thunder and lightning still rockin the burg.

Excellent storm

Not the best pic but the storm clouds just to the west of Dulles around 630pm

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MESOSCALE DISCUSSION 2001

NWS STORM PREDICTION CENTER NORMAN OK

0757 PM CDT THU SEP 27 2012

AREAS AFFECTED...NRN VA INTO PARTS OF CENTRAL AND SRN MD INCLUDING

THE WASHINGTON DC METRO

CONCERNING...SEVERE THUNDERSTORM WATCH 654...

VALID 280057Z - 280200Z

THE SEVERE WEATHER THREAT FOR SEVERE THUNDERSTORM WATCH 654

CONTINUES.

SUMMARY...THREAT FOR DAMAGING WINDS AND HAIL REMAIN POSSIBLE ACROSS

MUCH OF WW 654 THIS EVENING. A CLUSTER OF STORMS...CURRENTLY

LOCATED IN FAR NRN VA /GENERALLY IN AN AREA FROM 25 W OF DCA TO 50

AND 65 WSW OF DCA/...PER REGIONAL RADAR IMAGERY WILL PROGRESS EWD

WITH THE LEADING PART OF THIS ACTIVITY MOVING ACROSS THE WASHINGTON

DC AREA BETWEEN 01-02Z. A TORNADO THREAT CANNOT BE RULED OUT GIVEN

VERTICALLY VEERING WINDS COMBINED WITH A STORM MOTION TO THE ESE AT

20-25 KT. COUNTIES IN THE WAKE OF THE MAIN TSTM CLUSTER SHOULD

CONTINUE TO HAVE A DIMINISHED SVR WEATHER THREAT AND CAN BE LOCALLY

CANCELLED FROM THIS WATCH.

PW VALUES OF 1-1.5 INCH AND A SOMEWHAT SLOWER STORM MOTION SUGGEST

HEAVY RAINFALL RATES WILL BE POSSIBLE WITH THE STORMS MOVING FROM

NRN VA INTO THE WASHINGTON DC METRO AND EVENTUALLY PARTS OF

CENTRAL/SRN MD.

DISCUSSION...A PROGRESSIVE CLUSTER OF STORMS LOCATED TO THE WSW OF

DCA IS LOCATED WITHIN A ZONE OF DEEP LAYER ASCENT ASSOCIATED WITH

DIVERGENCE IN THE EXIT REGION OF A 60-65 KT WLY UPPER LEVEL JET

EXTENDING FROM SRN WV INTO VA. LOW LEVEL CONVERGENCE MAY BE

ENHANCED IN NRN VA ATTENDANT TO A MESOLOW WITHIN THE SRN LOBE OF

VORTICITY EXTENDING SWD FROM THE MORE VIGOROUS MIDLEVEL IMPULSE

ADVANCING EWD ACROSS SWRN-SOUTH CENTRAL PA TO NRN WV. DESPITE WEAK

MIDLEVEL LAPSE RATES...A RELATIVELY MOIST AIR MASS WITH PW VALUES

EXCEEDING 1-1.5 INCH CONTINUE TO CONTRIBUTE TO MLCAPE OF 500-1000

J/KG. THIS COMBINED WITH EFFECTIVE BULK SHEAR OF 30-35 KT IS

SUFFICIENT FOR STORM ORGANIZATION.

ALTHOUGH STRONG WIND GUSTS AND HAIL SHOULD BE THE PRIMARY THREATS...

VERTICALLY VEERING WINDS IN THE LOWER TROPOSPHERE AND STORM MOTIONS

TO THE ESE AT 20-25 KT RESULT IN LOW LEVEL SRH VALUES /0-3 KM NEAR

400 M2 PER S2/ SUPPORTING LOW LEVEL STORM ROTATION AND A LOW

PROBABILITY FOR A TORNADO.

..PETERS.. 09/28/2012

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