MESOSCALE DISCUSSION 2279
   NWS STORM PREDICTION CENTER NORMAN OK
   0649 PM CDT SAT OCT 29 2011
   
   AREAS AFFECTED...CNTRL/SRN ME...ERN NH...NERN MA
   
   CONCERNING...HEAVY SNOW 
   
   VALID 292349Z - 300445Z
   
   BANDS OF HEAVY SNOW...WITH RATES OF 1 TO LOCALLY 2 IN/HR...WILL
   OVERSPREAD NRN NEW ENGLAND THIS EVENING...AND CONTINUE THROUGH THE
   NIGHT BEFORE TAPERING OFF SUNDAY MORNING.
   
   A LARGE SHIELD OF PRECIPITATION NW OF INTENSIFYING SFC LOW PRESSURE
   OFF THE ATLANTIC SEABOARD WILL SHIFT NEWD...AS THE LOW ALSO TRACKS
   NEWD ALONG A STRONG BAROCLINIC ZONE NEARLY PARALLEL TO THE COAST.
   LOW/MID-LEVEL FRONTOGENESIS WITHIN THIS ZONE...COUPLED WITH THE
   ASCENDING BRANCH OF THE AGEOSTROPHIC CIRCULATION OF A
   CYCLONICALLY-CURVED UPPER-LEVEL JET MAX...WILL YIELD STRONG
   DYNAMICAL FORCING FOR ASCENT AND ENHANCE PRECIP RATES. RUC FORECAST
   SOUNDINGS SUGGEST THAT THE DEPTH OF SUB-FREEZING TEMPERATURES WILL
   BE SUFFICIENTLY LARGE THROUGH THE ATMOSPHERIC COLUMN TO SUPPORT
   PREDOMINANTLY SNOW. ONE EXCEPTION WILL BE INITIALLY ACROSS CNTRL/SRN
   MAINE /PARTICULARLY COASTAL LOCATIONS/...WHERE A SHALLOW LAYER OF
   ABOVE-FREEZING TEMPERATURES NEAR THE SFC MAY INITIALLY SUPPORT RAIN.
   HOWEVER...A RELATIVELY QUICK PHASE CHANGE TO SNOW WILL OCCUR...OWING
   TO /1/ ADIABATIC COOLING DUE TO THE AFOREMENTIONED FORCED
   ASCENT.../2/ NOCTURNAL COOLING...AND /3/ COOLING DUE TO LATENT HEAT
   ABSORPTION ASSOCIATED WITH PHASE CHANGES.
   
   SNOWFALL RATES OF 1 IN/HR WILL BE LIKELY ACROSS THE DISCUSSION
   AREA...AS HAS BEEN OBSERVED UPSTREAM. LOCALLY HIGHER RATES TO 2
   IN/HR WILL BE POSSIBLE WITHIN 20-60 MILES N/NW OF THE COAST...WHERE
   THE STRONGEST ASCENT IS ANTICIPATED. IN THIS AREA...CROSS SECTIONS
   OF THETA-E AND GEOSTROPHIC MOMENTUM SUGGEST INCREASING LOW-LEVEL
   CONDITIONAL SYMMETRIC INSTABILITY THROUGH THE NIGHT...WITH SLANTWISE
   CAPE VALUES REACHING 200-300 J/KG. AS SUCH...ONE OR MORE BANDS OF
   HEAVY SNOW WILL MATERIALIZE...AND AN ISOLATED LIGHTNING STRIKE OR
   TWO CANNOT BE RULED OUT. CLOSER TO THE COAST...RELATIVELY WARMER
   THERMODYNAMIC PROFILES IN THE LOW LEVELS WILL BE ASSOCIATED WITH
   MORE UPRIGHT INSTABILITY...ALONG WITH LOWER SNOW-LIQUID RATIOS AND
   LOWER SNOWFALL RATES. THE THREAT FOR HEAVY SNOW WILL CONTINUE
   THROUGH MUCH OF THE NIGHT BEFORE TAPERING OFF FROM SW TO NE SUNDAY
   MORNING.
   
   ..COHEN.. 10/29/2011
   
   
   ATTN...WFO...CAR...GYX...BOX...
   
   LAT...LON   46036912 46306855 46246792 45946745 45196698 44666707
               44376819 43557006 42857077 42397135 42637196 43467176
               45187024 46036912