MESOSCALE DISCUSSION 0535
   NWS STORM PREDICTION CENTER NORMAN OK
   0236 PM CDT FRI APR 19 2013
   
   AREAS AFFECTED...ERN PA
   
   CONCERNING...SEVERE POTENTIAL...WATCH POSSIBLE 
   
   VALID 191936Z - 192030Z
   
   PROBABILITY OF WATCH ISSUANCE...60 PERCENT
   
   SUMMARY...A TORNADO WATCH MAY BE NEEDED ACROSS PARTS OF ERN PA
   DURING THE LATTER HALF OF THE AFTERNOON INTO THE EVENING.
   
   TSTMS SHOULD DEVELOP ACROSS ERN PA AS A NARROW LOW-TOPPED CONVECTIVE
   LINE ADVANCES EWD INTO A DESTABILIZING AND STRONGLY SHEARED
   ENVIRONMENT LOCATED E OF THE HIGHER TERRAIN IN CENTRAL PA.
   
   DISCUSSION...EARLY-MID AFTERNOON SURFACE ANALYSES SHOWED A COLD
   FRONT CONTINUING TO ADVANCE EWD...AND EXTENDED FROM WRN NY SWD
   THROUGH WRN PA TO ERN WV AND SWRN VA...HAVING ALREADY MOVED THROUGH
   KBUF...KPIT AND KEKN. REGIONAL RADAR IMAGERY INDICATED A BETTER
   DEFINED NARROW LOW-TOPPED LINE OF CONVECTION ORIENTED N-S ALONG THE
   COLD FRONT FROM WRN NY TO THE WRN MD PANHANDLE.  OBJECTIVE ANALYSES
   INDICATED THE ENVIRONMENT IN ERN PA ALONG AND E OF THE HIGHER
   TERRAIN WAS GRADUALLY DESTABILIZING FROM S-N AS DIABATIC HEATING AND
   BOUNDARY LAYER MOISTENING SUPPORTED INCREASING INSTABILITY.
   HOWEVER...GENERALLY WEAK MIDLEVEL LAPSE RATES ARE TENDING TO TEMPER
   THE DEGREE OF DESTABILIZATION THUS FAR. VWP DATA PER AREA WSR-88D/S
   SHOWED STRENGTHENING DEEP LAYER WIND FIELDS WITH INCREASING
   EFFECTIVE BULK SHEAR IN EXCESS OF 50 KT. THESE SHEAR VECTORS ARE
   FORECAST TO REMAIN ORIENTED ACROSS THE INITIATING BOUNDARY SUCH THAT
   BOTH SUPERCELLS AND QLCS STRUCTURES WILL BE POSSIBLE AS THE
   CONVECTIVE LINE ENCOUNTERS THE GREATER INSTABILITY. ALTHOUGH THIS
   WEAK DESTABILIZATION MAY CONTINUE TO LIMIT THE DEPTH OF THE
   CONVECTION...STRONG FORCING COMBINED WITH THE STRENGTHENING DEEP
   LAYER AND LOW LEVEL SHEAR COULD PROVE SUFFICIENT TO WARRANT ISSUANCE
   OF A TORNADO WATCH.
   
   ..PETERS/MEAD.. 04/19/2013
   
   
   ATTN...WFO...PHI...BGM...CTP...
   
   LAT...LON   39747812 41237713 41977647 41987532 41527478 40627506
               39877527 39717585 39747812