MESOSCALE DISCUSSION 2277
   NWS STORM PREDICTION CENTER NORMAN OK
   0636 AM CDT SAT OCT 29 2011
   
   AREAS AFFECTED...PORTIONS NERN WV...SERN NY...NWRN MD INCLUDING
   PANHANDLE...CENTRAL/ERN PA...NWRN NJ...EXTREME NRN VA.
   
   CONCERNING...HEAVY SNOW 
   
   VALID 291136Z - 291700Z
   
   PRIND HEAVIEST SNOW RATES THROUGH 18Z...AND MOST SUSTAINED AT 1-2
   INCHES/HOUR...WILL BE OVER AND W OF BLUE RIDGE FROM NERN WV AND VA
   BORDER REGION NEWD ACROSS NWRN MD TO S-CENTRAL PA...SHIFTING NEWD
   ACROSS NERN PA INTO INTERIOR SERN NY BY ABOUT 18Z.  RAIN SHOULD
   BECOME SNOW OVER PORTIONS CENTRAL-ERN PA THROUGH 16Z.  ISOLATED
   CONVECTIVE BURSTS ALSO ARE POSSIBLE ALONG THAT PORTION OF RAIN/SNOW
   MIX ZONE FROM NRN VA TO SERN PA AND EVENTUALLY NWRN NJ.
   
   CONCENTRATION OF HEAVY SNOW POTENTIAL ACROSS DISCUSSION AREA WILL BE
   RELATED TO SEVERAL JUXTAPOSED REGIMES OF LIFT AND INSTABILITY. 
   THESE INCLUDE...FROM TOP TO BOTTOM OF TROPOSPHERE...
   -- STRENGTHENING/NEWD-MOVING UPPER DIVERGENCE AS STG SHORTWAVE
   TROUGH...NOW EVIDENT IN MOISTURE CHANNEL IMAGERY OVER PORTIONS
   IL/INDIANA/WRN KY/TN...APCHS AREA WITH MORE NEGATIVE TILT. 
   ASSOCIATED BAROCLINIC-LEAF SIGNATURE HAS BECOME VERY WELL-DEFINED
   WITH COOLING IR CLOUD TOPS ACROSS NRN WV AND WRN PA DURING PAST 2-3
   HOURS.
   -- MIDLEVEL DCVA PRECEDING THAT TROUGH.
   -- NARROW SW-NE ALIGNED RIBBONS OF MIDLEVEL CONDITIONAL SYMMETRIC
   INSTABILITY SUPPORTING SLANTWISE CAPE UP TO ABOUT 200 J/KG.
   -- LOW-MIDDLE LEVEL FRONTOGENETIC FORCING...ESPECIALLY STRONG 15-18Z
   OVER S-CENTRAL THROUGH NERN PA AND INTERIOR SERN NY PER SREF
   GUIDANCE.
   -- ZONE OF ELEVATED CONVECTIVE INSTABILITY IN LOW-MIDDLE
   LEVELS...INVOF RAIN/SNOW TRANSITON ZONE...SUPPORTING 100-300 J/KG
   ELEVATED MUCAPE OVER ERN FRINGES OF DISCUSSION AREA THAT IN TURN CAN
   ENABLE CONVECTIVELY AIDED SNOW BURSTS WITH OCNL THUNDER.
   
   MODIFIED 6Z RNK/DCA RAOBS AND MODEL SOUNDINGS SUGGEST
   NEAR-SATURATION IN LOW LEVELS...PRECLUDING APPRECIABLE WET-BULB
   COOLING.  HOWEVER...COLUMNAR DYNAMIC COOLING SHOULD OFFSET WAA IN
   ELEVATED CONVEYOR ENOUGH TO YIELD DOMINANT PRECIP TYPE AS SNOW OVER
   APPALACHIANS.
   
   ..EDWARDS.. 10/29/2011
   
   
   ATTN...WFO...OKX...ALY...PHI...BGM...CTP...LWX...PBZ...RLX...
   
   LAT...LON   38997991 41957649 41897395 38947787 38997991