PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
359 PM EST TUE FEB 09 2010
 
VALID 00Z WED FEB 10 2010 - 00Z SAT FEB 13 2010
 
DAYS 1 THROUGH 3...

GTLKS TO APPALACHIANS AND MID ATLC THROUGH NORTHEAST...
STRONG LOW PRES WILL MOVE FROM THE LWR GTLKS OVERNIGHT TO THE MID
ATLC BY MIDDAY WEDNESDAY.  ENERGETIC/DIVERGENT SW FLOW AHEAD OF
THE SYSTEM WILL PROMOTE BROAD SCALE UPLIFT AS SFC LOW PRES
DEVELOPS/RAPIDLY INTENSIFIES OFF THE SEABOARD.  SIGNIFICANT WINTER
EVENT STILL ON TAP FOR THE NORTHEAST...BUT A NOTABLE SHIFT
NORTHWARD WAS INCLUDED IN THIS ISSUANCE.  CHANGES IN FORECAST
THINKING ARE RELATED TO THE EVOLUTION OF THIS SFC FEATURE OFF THE
COAST. 12Z GUIDANCE SUITE UNANIMOUSLY SHIFTED THE LOW PRES CLOSER
TO THE COAST...BUT THE NAM WAS THE MOST PRONOUNCED IN HUGGING THE
COAST AND HAVING THE DEEPEST CENTER.  A MORE MODEST SHIFT IN THIS
DIRECTION IS PRUDENT...AND THE PRIMARY EFFECT WAS TO WRAP THE MSTR
MORE NWD WHICH LOWERED D.C. METRO AREA TOTALS BY SEVERAL INCHES. 
TIGHT GRADIENT IS ANTICIPATED WITH AROUND 20 INCHES CENTERED ON SE
PA...AND GRADUALLY LOWERING AMTS THROUGH SRN NY AND SOUTHERN NEW
ENGLAND.  P-TYPE WILL BE A CONCERN ALL ALONG THE COAST AS
RELATIVELY WARMER ATLC AIR IS DRAWN WESTWARD...AND THE RAIN LINE
WAS INDEED SHIFTED NWD THROUGH CNTRL/SE VA...BUT SNOW SHOULD BE
DOMINANT ELSEWHERE.                 

MEANWHILE...AN AREA OF MDT TO HVY SNOW IS LIKELY TONIGHT NEAR AND
NORTH OF THE COMPACT BUT DEEP MID/UPPER LOW TRACKING EWD THROUGH
THE OH VALLEY AREA.  12Z GUIDANCE REMAINS IN CLOSE AGREEMENT WITH
THE TRACK AND QPF COVERAGE...FOCUSING THE HVST SNOW OVER SRN LOWER
MI AND NRN OH/IN WHERE LOCALIZED 8 INCH PLUS AMTS ARE POSSIBLE. 
AS THE CIRC PULLS EAST...AN AREA OF ENHANCED SNOW ON THE WRN SIDE
SHOULD WRAP SWD TOWARD ERN KY AND WV BY WEDS MORNING.  


SRN ROCKIES TO LOWER MS VALLEY...
PAC SHRTWV SLIDING SOUTH ALONG THE CA COAST TONIGHT WILL BECOME
ENVELOPED WITHIN THE STRONG SRN JET STREAM AND PUSH EAST ACROSS
THE SWRN STATES AND NRN MEXICO ON WEDS AND WEDS NIGHT.  THE LOW
HEIGHTS ASSOCD WITH THIS FEATURE WILL SUPPORT SNOW THROUGH MUCH OF
THE HIGHER ELEVS OF SRN CA EWD TO THE MTNS OF AZ/NM.  SNOW AMTS
SHOULD BE GENERALLY LIGHT GIVEN THE LIMITED AMOUNT OF PACIFIC
MOISTURE REACHING INTO THIS SYSTEM.  THE SHRTWV WILL PRESS EWD
ACROSS TX ON THURSDAY AND INTO THE SOUTHEAST ON FRIDAY.  AS IT
DOES IT WILL ENCOUNTER AN INCREASING SELY FLOW OF GULF MOISTURE
WHICH WILL OVERRUN THE UNSEASONABLY COLD AIRMASS IN PLACE OVER TX
AND THE LOWER MS VALLEY.  AT THIS TIME...ECMWF IS THE COLDEST AND
EVEN IT SHOWS ONLY SLR OF 8 TO 1 OR LESS.  OTHER CONSENSUS IS
LIGHTER WITH THE SNOW...BUT WE DECIDED TO INCLUDE A 4 INCH RISK
GIVEN THE TREND AND REGIONAL IMPACT.  GENERALLY FOLLOWED THE
CRITICAL TEMPS AND QPF COVERAGE FROM THE 12Z GFS/09Z SREF MODELS
HERE.    


NORTHWEST...
A TANDEM OF PACIFIC SHRTWVS AND ASSOCD FRONTAL BNDRYS ARE FCST TO
SWEEP QUICKLY EWD INTO THE PAC NW THURS AND FRI.  A PERIOD OF
STRONG ONSHORE FLOW WILL ACCOMPANY EACH RESULTING IN LOCALLY HVY
SNOW ACROSS THE CASCADES...REACHING EWD INTO THE NRN ROCKIES EARLY
FRI.  THE FRIDAY SYSTEM WILL BE FARTHER SOUTH AND WILL ACCOUNT FOR
HVY SNOW TO THE HIGHER ELEVATIONS OF THE CASCADES.  THE UKMET/SREF
SOLNS WERE PREFERRED HERE.


SOWKO