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Mesoscale Discussion 1643
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   MESOSCALE DISCUSSION 1643
   NWS STORM PREDICTION CENTER NORMAN OK
   1230 PM CDT SUN AUG 31 2014

   AREAS AFFECTED...PORTIONS OF ERN PA...SERN NY...SWRN
   CT...NJ...MD...NRN DE...THE DISTRICT OF COLUMBIA

   CONCERNING...SEVERE POTENTIAL...WATCH POSSIBLE 

   VALID 311730Z - 312000Z

   PROBABILITY OF WATCH ISSUANCE...40 PERCENT

   SUMMARY...THE DC-BALTIMORE-PHILADELPHIA-NYC CORRIDOR AND SURROUNDING
   AREAS ARE BEING MONITORED FOR AN INCREASING POTENTIAL FOR STRONG TO
   LOCALLY SVR TSTM DEVELOPMENT THIS AFTERNOON. WHILE NOT IMMEDIATELY
   LIKELY...THE ISSUANCE OF SEVERE THUNDERSTORM WATCH IS POSSIBLE LATER
   THIS AFTERNOON.

   DISCUSSION...AREAS OF INSOLATION S OF A PERSISTENT MULTI-LAYERED
   CLOUD BAND STRETCHING FROM THE OHIO VALLEY TO NEW ENGLAND IS
   ENCOURAGING CONTINUED DESTABILIZATION OF THE BOUNDARY LAYER ACROSS
   THE AREA. THE COMBINATION OF RICH BOUNDARY-LAYER MOISTURE -- E.G.
   SFC DEWPOINTS IN THE LOWER 70S -- AND TEMPERATURES IN THE MIDDLE 80S
   TO AROUND 90F IS SUPPORTING SBCAPE ON THE ORDER OF 1500-3000 J/KG.
   WITH NEGLIGIBLE INHIBITION REMAINING...VISIBLE SATELLITE IMAGERY
   INDICATES SW/NE-ORIENTED CLOUD STREETS EVOLVING FROM THE
   INSOLATION-HEATED MOIST AIR MASS...FROM WHICH ISOLATED CONVECTION IS
   BEGINNING TO BLOSSOM IN SERN PA AND ADJACENT MD.

   A FURTHER UPTICK IN CONVECTIVE COVERAGE/INTENSITY IS EXPECTED DURING
   THE AFTERNOON AS DIFFERENTIAL HEATING AND PBL CIRCULATIONS ARE
   BOLSTERED...THOUGH DEEP-LAYER FORCING FOR ASCENT WILL REMAIN VERY
   WEAK ACROSS THE REGION. GIVEN 25-40 KT OF EFFECTIVE BULK SHEAR...A
   FEW ORGANIZED MULTICELL CLUSTERS...BOWING LINE SEGMENTS...AND
   PERHAPS A COUPLE OF MARGINAL SUPERCELL STRUCTURES MAY EVOLVE. THE
   PRESENCE OF 25-35 KT OF 1-3-KM SWLYS/WSWLYS SAMPLED BY AREA VWPS AND
   INDICATED BY FORECAST SOUNDINGS SUGGESTS THAT CONVECTIVE MOMENTUM
   TRANSPORT MAY SUPPORT STRONG SFC WIND GUSTS...FURTHER ENHANCED BY
   WATER-LOADING ASSOCIATED WITH PW VALUES OF 1.75 INCHES PER GPS DATA.
   DMGG WIND GUSTS WILL BE POSSIBLE...AND SUFFICIENT LOW-LEVEL SHEAR
   AND MOISTURE EXIST FOR THE RISK OF A WEAK TORNADO. HOWEVER...POOR
   MID-LEVEL LAPSE RATES SAMPLED BY 12Z RAOBS CAST DOUBT ON THE OVERALL
   POTENTIAL FOR ROBUST STORMS AND RELATED SVR POTENTIAL. THIS IS
   PARTICULARLY THE CASE IN THE ABSENCE OF STRONGER DEEP-LAYER
   ASCENT/VERTICAL SHEAR.

   ..COHEN/HART.. 08/31/2014


   ATTN...WFO...OKX...PHI...BGM...CTP...LWX...

   LAT...LON   39747768 40727667 41497476 41277320 40677301 39207491
               38867690 39747768 

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Page last modified: August 31, 2014
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