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 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 spc.noaa.gov/products/md/mcd0039.gif
Posted on: Sat, 24 Jan 2015 06:31:45 +0000
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