The Storm Prediction Center has issued a mesoscale discussion (usually associated with severe weather) for the burst of moderate to heavy snow that is likely to occur in inland Southern New England this evening. The discussion is a bit technical, but the highlights are that 1-2″ per hour rates are possible in the heaviest bands of snow, that thundersnow is a possibility given the strong dynamics of the storm, and that by 10pm (03Z) warmer air will get involved and quickly change the precipitation in many places from snow to sleet, freezing rain, or rain.
MESOSCALE DISCUSSION 2239 NWS STORM PREDICTION CENTER NORMAN OK 0339 PM CST WED DEC 26 2012 AREAS AFFECTED...SRN VT...SRN NH...CNTRL/WRN MA...MUCH OF CT/RI...PARTS OF FAR ERN NY CONCERNING...WINTER MIXED PRECIPITATION VALID 262139Z - 270245Z SUMMARY...SNOW WITH RATES OF 1-2 IN/HR WILL OVERSPREAD THE AREA AFTER 00Z. A TRANSITION TO WINTER MIXED PRECIPITATION IS EXPECTED TO OCCUR FROM S TO N AFTER 03Z. DISCUSSION...INTENSIFYING SFC LOW PRESSURE WILL TRACK NNEWD ALONG THE ATLANTIC COAST FROM THE MID ATLANTIC TO NEW ENGLAND THROUGH TONIGHT. STRONG ISENTROPIC ASCENT PRECEDING THE CYCLONE CENTER WILL ALLOW PRECIPITATION TO SPREAD ACROSS THE AREA LATE THIS AFTERNOON INTO THIS EVENING. CURRENT OBSERVATIONAL TRENDS AND THE BULK OF HIGH-RESOLUTION WRF GUIDANCE SUGGEST THAT HEAVY SNOW WILL OVERSPREAD PARTS OF SRN/WRN NEW ENGLAND AND ADJACENT FAR ERN NY FROM SW TO NE AFTER 00Z. THE AFOREMENTIONED ASCENT...AUGMENTED BY THE ASCENDING BRANCHES OF FRONTOGENETIC CIRCULATIONS...WILL INTERSECT DENDRITIC GROWTH ZONES BETWEEN THE 560- AND 500-MB LEVELS AS EVIDENT IN FORECAST SOUNDINGS. THIS WILL SUPPORT EFFICIENT SNOW GROWTH. FORECAST SOUNDINGS ALSO DEPICT ISOTHERMAL LAYERS FEATURING TEMPERATURES AROUND/JUST BELOW 0C BETWEEN THE 900- AND 700-MB LEVELS...WHICH WILL SUPPORT SNOW AGGREGATION. AS SUCH...SNOWFALL RATES OF 1-2 IN/HR WILL BE POSSIBLE...WITH THE HIGHEST RATES ACCOMPANYING ANY CONVECTIVELY ENHANCED PRECIPITATION OWING TO THE PRESENCE OF NARROW ELEVATED CAPE LAYERS. AN ISOLATED LIGHTNING STRIKE OR TWO MAY OCCUR WITH THE HEAVIEST SNOW. AFTER 03Z...STRONG WARM ADVECTION ACCOMPANYING A NWD-MOVING LOW-LEVEL JET MAX WILL RESULT IN A PRECIPITATION-TYPE TRANSITION FROM S TO N. THIS WILL OCCUR AS WARMER TRAJECTORIES EXTEND INLAND FROM THE MARINE LAYER OVER THE ATLANTIC WATERS. SLEET/FREEZING RAIN WILL BE POSSIBLE AS PARTIAL/COMPLETE MELTING OF FALLING HYDROMETEORS OCCURS ATOP SUB-FREEZING SFC TEMPERATURES. A PHASE CHANGE TO RAIN MAY OCCUR WHERE SFC TEMPERATURES RISE ABOVE FREEZING. WINTER MIXED PRECIPITATION COULD CONTINUE INTO THE NIGHT.
