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Mesoscale Discussion 39
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   MESOSCALE DISCUSSION 0039
   NWS STORM PREDICTION CENTER NORMAN OK
   1048 PM CST FRI JAN 23 2015

   AREAS AFFECTED...PORTIONS OF CNTRL AND ERN PA...CNTRL/NRN NJ...SE NY
   INCLUDING NYC AND LONG ISLAND...PORTIONS OF CT

   CONCERNING...HEAVY SNOW 

   VALID 240448Z - 241045Z

   SUMMARY...HEAVY SNOW WILL BE DEVELOPING OVERNIGHT TONIGHT --
   ESPECIALLY FROM 06Z TO 09Z -- FROM PORTIONS OF THE CNTRL
   APPALACHIANS ENEWD TO SRN NEW ENGLAND AND THE LONG ISLAND
   COAST...INCLUDING THE NEW YORK CITY METROPOLITAN AREA. LOCAL
   SNOWFALL RATES TO 1 INCH PER HOUR WILL BE POSSIBLE.

   DISCUSSION...MOSAIC RADAR IMAGERY INDICATES A SHIELD OF WIDESPREAD
   PRECIPITATION EXTENDING FROM THE MID-ATLANTIC TO THE OHIO
   VALLEY...EMBEDDED WITHIN A BROAD WARM-ADVECTION ZONE ANALYZED AT
   H85. THIS PRECIPITATION WILL CONTINUE TO SPREAD ENEWD THROUGH THE
   NIGHT...AS RELATED ISENTROPIC ASCENT IS MAINTAINED WELL DOWNSTREAM
   OF A POSITIVELY TILTED MIDLEVEL TROUGH...WHICH WATER VAPOR LOOPS
   DEPICT AS EXTENDING FROM THE MIDDLE OH VALLEY TO THE LOWER MS
   VALLEY. RECENT SFC OBSERVATIONS SUGGEST 2-HOURLY PRESSURE FALLS
   AROUND 3.5-4.5 MB FROM SERN VA TO ERN NC...MOST LIKELY A REFLECTION
   OF INCREASING LARGE-SCALE ASCENT PRECEDING THE TROUGH BECOMING
   JUXTAPOSED WITH A NE/SW-ORIENTED BAROCLINIC ZONE/INVERTED TROUGH
   ANALYZED ROUGHLY 40-75 MILES INLAND FROM THE COAST. SFC LOW PRESSURE
   IS FORECAST TO CONSOLIDATE/DEEPEN FROM THIS INTERACTION AND MOVE OFF
   THE COAST...WITH A CORRESPONDING INCREASE IN WAA AND FRONTOGENETIC
   ASCENT TO ITS N. IN TURN...PRESENT PRECIPITATION INTENSITY WILL BE
   MAINTAINED...OR PERHAPS INCREASE...ACROSS THE DISCUSSION
   AREA...PARTICULARLY AS A DEFORMATION BAND BECOMES ESTABLISHED WELL
   N/NW OF THE EVOLVING SFC CYCLONE.

   SUBJECTIVE SPATIAL INTERPOLATION BETWEEN THE 00Z IAD AND OKX RAOBS
   SUGGESTS NEAR- TO BELOW-FREEZING TEMPERATURES EXISTING...AND
   CONTINUING TO EXIST...THROUGHOUT THE TROPOSPHERE ACROSS THE MCD
   AREA. THIS IS ESPECIALLY THE CASE GIVEN CONTINUED NOCTURNAL DIABATIC
   COOLING AND MODEST ADIABATIC COOLING ASSOCIATED WITH DCVA PRECEDING
   THE APPROACHING MIDLEVEL TROUGH. WHERE SFC TEMPERATURES REMAIN
   SLIGHTLY ABOVE 0C FROM SERN PA TO CNTRL NJ...WET-BULB-ZERO HEIGHTS
   WILL GENERALLY REMAIN AOB 500 FT AGL -- AS AFFIRMED BY RAP FORECAST
   SOUNDINGS AND INTERPOLATED OBSERVED SOUNDINGS. AS SUCH...SNOW IS
   FORECAST TO BE THE PREDOMINANT PRECIPITATION TYPE ACROSS THE
   DISCUSSION AREA...WITH DEEP...NEAR-ISOTHERMAL/NEAR-0-C LAYERS
   FOSTERING EFFICIENT GROWTH OF SNOW AGGREGATES. AS SUCH...SNOWFALL
   RATES WILL LOCALLY REACH 1 INCH PER HOUR. SUCH RATES IN HEAVY SNOW
   WILL LIKELY MATERIALIZE BETWEEN 06Z AND 09Z FROM CNTRL PA TO CNTRL
   NJ...WHILE SPREADING ENEWD TOWARD THE NEW YORK CITY AREA AND SRN NEW
   ENGLAND THROUGH THE LATE-NIGHT HOURS.

   VERY LATE TONIGHT...GENERALLY AFTER 10Z...IT IS POSSIBLE THAT
   STRENGTHENING ONSHORE FLOW ALONG THE E-CNTRL NJ COAST -- IN RESPONSE
   TO CYCLOGENESIS FARTHER SOUTH -- MAY SUPPORT SUFFICIENT LOW-LEVEL
   WARMING FOR PRECIPITATION TO MIX WITH AND/OR CHANGE TO RAIN.
   HOWEVER...THIS WILL GENERALLY BE IN A CONFINED AREA NEAR THE COAST
   WHERE OCEANIC HEAT FLUXES ARE MAINTAINED INLAND ONLY OVER A MODEST
   DISPLACEMENT.

   ..COHEN.. 01/24/2015


   ATTN...WFO...BOX...OKX...ALY...PHI...BGM...CTP...

   LAT...LON   41797444 41737248 41067195 40797273 40627320 40507391
               40297437 40067518 40187697 40517822 41007773 41797444 

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