MESOSCALE DISCUSSION 0518
   NWS STORM PREDICTION CENTER NORMAN OK
   0221 PM CDT WED MAY 06 2015

   AREAS AFFECTED...PORTIONS OF SOUTHERN PA...WESTERN WV...NORTHERN
   VA...WESTERN MD

   CONCERNING...SEVERE POTENTIAL...WATCH UNLIKELY 

   VALID 061921Z - 062115Z

   PROBABILITY OF WATCH ISSUANCE...5 PERCENT

   SUMMARY...THREAT OF SEVERE HAIL AND WIND THROUGH THE AFTERNOON HOURS
   IS EXPECTED TO REMAIN MARGINAL ENOUGH TO PREVENT A WATCH AT THIS
   TIME.

   DISCUSSION...THUNDERSTORMS HAVE DEVELOPED OVER THE HEATED TERRAIN OF
   THE NORTHERN AND CENTRAL APPALACHIAN MOUNTAINS INVOF A WEAKENING
   FRONTAL BOUNDARY. MIXED-LAYER CAPE HAS INCREASED ABOVE 500 J/KG OVER
   MOST OF THE MCD AREA WITH VALUES OF 1000-1500 J/KG IN A SMALL
   CORRIDOR OVER SOUTHWEST PA AND IN A SEPARATE AREA ALONG AND EAST OF
   THE MOUNTAINS IN NORTHERN VA. THE INSTABILITY IS LIMITED ON THE
   EASTERN EDGE BY MARINE BOUNDARY-LAYER AIR THAT IS MOVING WESTWARD
   ACROSS THE CHESAPEAKE BAY. THE PULSE NATURE OF THE CONVECTION IS
   EXPECTED TO CONTINUE IN WEAK LOW-LEVEL WIND FIELDS AND MID-LEVEL
   WEST-NORTHWESTERLY WINDS THAT INCREASE FROM 10-15 KT OVER THE
   SOUTHERN PART OF THE MCD AREA TO ONLY ABOUT 25-30 KT OVER PA. THE
   WEAK WIND FIELD IS EXPECTED TO PRECLUDE MUCH MOVEMENT OF THE
   THUNDERSTORMS OFF THE HIGHER TERRAIN.  ONLY A VERY NARROW AREA OF
   ENHANCED EFFECTIVE SHEAR OF 25-30 KT EXISTS ALONG THE NORTHEASTERN
   EDGE OF THE INSTABILITY OVER CENTRAL MD...BUT THIS AREA WILL SOON BE
   MODIFIED BY THE INCOMING MARINE BOUNDARY LAYER. ELSEWHERE...GIVEN
   MID-LEVEL LAPSE RATES UNDER 7 C/KM AND HIGH RELATIVE HUMIDITY
   THROUGH 700 MB...ONLY IN AREAS WITH STEEP LOW-LEVEL LAPSE RATES
   OWING TO STRONG HEATING AND IN THE STRONGEST DOWNDRAFTS WILL SEVERE
   WIND GUSTS BE SUPPORTED.

   ..CONIGLIO/HART.. 05/06/2015


   ATTN...WFO...AKQ...CTP...LWX...RNK...PBZ...RLX...

   LAT...LON   41267986 41017914 40607812 40097744 39167651 38227690
               37677794 37407911 37688004 39067945 40758045 41267986