ZCZC SPCSWOMCD ALL ACUS11 KWNS 061921 SPC MCD 061921 MDZ000-VAZ000-PAZ000-DCZ000-WVZ000-062115- 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 ...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT... ATTN...WFO...AKQ...CTP...LWX...RNK...PBZ...RLX... LAT...LON 41267986 41017914 40607812 40097744 39167651 38227690 37677794 37407911 37688004 39067945 40758045 41267986 NNNN