000
ACUS11 KWNS 240448
SWOMCD
SPC MCD 240448
CTZ000-NYZ000-NJZ000-PAZ000-241045-

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

...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT...

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

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