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Mesoscale Discussion 404
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   MESOSCALE DISCUSSION 0404
   NWS STORM PREDICTION CENTER NORMAN OK
   0457 PM CDT MON APR 20 2015

   AREAS AFFECTED...PORTIONS OF N-CNTRL PA AND S-CNTRL NY

   CONCERNING...TORNADO WATCH 90...

   VALID 202157Z - 210000Z

   THE SEVERE WEATHER THREAT FOR TORNADO WATCH 90 CONTINUES.

   SUMMARY...ISOLATED TO SCATTERED TSTMS WILL CONTINUE TO DEVELOP NEWD
   ACROSS N-CNTRL PA INTO S-CNTRL NY WITH SOME SVR RISK.

   DISCUSSION...THE CCX VWP IS SAMPLING 35-50 KT OF MID-LEVEL
   FLOW...WHICH IS ENCOURAGING THE ONGOING NEWD SPREAD OF CONVECTION
   FROM CNTRL PA WHILE OFFERING MODERATE DEEP SHEAR AND SOME DMGG
   WIND/BRIEF TORNADO POTENTIAL. THE TORNADO WATCH HAS BEEN LOCALLY
   EXTENDED NWD ACROSS PARTS OF N-CNTRL PA WHERE MARGINAL INSTABILITY
   EXISTS.

   AREAS FARTHER N INTO FAR N-CNTRL PA AND S-CNTRL NY WILL CONTINUE TO
   BE MONITORED FOR POSSIBLE WW ISSUANCE THROUGH THE EVENING. THIS WILL
   OCCUR AS MID-LEVEL HEIGHT FALLS CONTINUE OVERSPREADING THE AREA
   ATTENDANT TO MIGRATORY IMPULSES ON THE ERN RIM OF A CNTRL/ERN CONUS
   CYCLONE AND COLD MID-LEVEL TEMPERATURES CONTINUE TO OVERLIE THE
   REGION. HOWEVER...WITH STABLE WAVE-CLOUD STRUCTURES OBSERVED IN VIS
   IMAGERY AND SFC OBS SUGGESTING ONLY MODEST THETA-E...AND EVENTUAL
   NOCTURNAL STABILIZATION...THE OVERALL MAGNITUDE OF THE SVR POTENTIAL
   IS UNCERTAIN N OF THE EXPANDED OUTLINE OF WW 90.

   ..COHEN.. 04/20/2015


   ATTN...WFO...PHI...BGM...BUF...CTP...

   LAT...LON   40897827 41517870 42227784 42117620 41027539 40667655
               40897827 

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