Coast-to-Coast Winter: Snow for NY & PA, GFS is aggressive on Surface Low
Posted by JAC on 10/14/2009, 7:44 am
SHORT RANGE FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
405 AM EDT WED OCT 14 2009

VALID 12Z WED OCT 14 2009 - 00Z FRI OCT 16 2009

THE RAPIDLY DEEPENING LOW PRESSURE SYSTEM OFF THE EAST COAST OF
THE U.S. HAS THE POTENTIAL TO BRING SOME UNSEASONAL SNOW TO THE
MIDDLE ATLANTIC REGION. IT WILL CERTAINLY BRING MODERATE TO HEAVY
RAIN TO THE CENTRAL EAST COAST AS WE HEAD INTO THE AFTERNOON ON
THURSDAY. BELOW AVERAGE TEMPERATURES WILL BE FELT ACROSS THE
MIDDLE ATLANTIC STATES AND NEW YORK. THE POTENTIAL FOR SNOW...OR A
RAIN AND SNOW MIX...RESTS ALONG THE NEW YORK AND PENNSYLVANIA STATE LINES FRIDAY EVENING.

IMPORTANT DETAIL ISSUES WITHIN THE DEEP AMPLIFIED TROF INITIALLY
OVER ERN NOAM ARE YET TO BE RESOLVED.  ENSEMBLES HAVE CONSISTENTLY
DISPLAYED ENOUGH SPREAD FOR THEIR MEANS TO MAINTAIN AN OPEN
TROF... AND THE UKMET/CANADIAN ARE GENERALLY OPEN AS WELL.
HOWEVER THE 00Z GFS/ECMWF BOTH DEVELOP EMBEDDED CLOSED LOWS BY DAY
4 SUN.  

THE GFS APPEARS EXCESSIVELY DEEP WITH THE UPPER LOW...
RESULTING IN A CORRESPONDING SFC LOW TRACK THAT IS A NWWD OUTLIER
COMPARED TO OTHER MODEL/ENSEMBLE GUIDANCE.

REGARDLESS OF EXACTLY
HOW THE MID LVL TROF EVOLVES... GEFS MEAN TRENDS APPEAR TO SUPPORT
THE RELATIVE CONSENSUS OF OPERATIONAL SOLNS TOWARD A SLOW MOVING
SFC LOW OFF THE EAST COAST DURING THE WEEKEND INTO EARLY NEXT
WEEK.  THE 12Z ECMWF ENSEMBLE MEAN APPEARS TOO PROGRESSIVE
RELATIVE TO MOST 00Z GUIDANCE.


EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
211 PM EDT TUE OCT 13 2009

VALID 12Z FRI OCT 16 2009 - 12Z TUE OCT 20 2009


PRELIMINARY UPDATE...

THE PRELIMINARY FRONTS AND PRESSURES FOR DAYS 3 THROUGH 7 WERE
UPDATED USING THE 00Z/13 ECMWF FOR MUCH OF THE FORECAST
PERIOD...WITH A SUBSTANTIAL INCORPORATION OF ITS ATTENDANT
ENSEMBLE MEAN BY DAY 7.  WHILE NONE OF THE GLOBAL NUMERICAL MODELS
HAVE SHOWN THE MOST HEARTENING CONTINUITY OVER THE LAST SEVERAL
RUNS...THE ECMWF AT LEAST RESEMBLES ITSELF FROM
YESTERDAY...PARTICULARLY ALONG THE EAST COAST WHERE THE LONGITUDE
OF CYCLOGENESIS HAS GREAT BEARING ON THE CONDITIONS OVER THE MID
ATLANTIC AND NORTHEAST DURING THE FIRST FEW DAYS.  

THE FLOW REGIME
IS PURE WINTER...COAST TO COAST...WITH A SPRAWLING...POLAR HIGH
BANKED ACROSS SOUTHEAST CANADA FEEDING ENOUGH COLD AIR INTO THE
CENTRAL AND NORTHERN APPALACHIANS AND THE ADJACENT HIGHER TERRAIN
TO PRODUCE SOME SNOW ON THE BACKSIDE OF THE PRECIPITATION SHIELD
THERE DAYS 3 THROUGH 5.

WOULD NOT BE SURPRISED IF SOME
MOUNTAINTOP IN THE MID ATLANTIC STATES SEES A HEFTY ACCUMULATION
DURING THIS TIME.

THE POSITIVE TILT OF THE LONGWAVE TROUGH OVER
THE EAST WILL PRECLUDE GULF MOISTURE RETURNING INTO THE CENTRAL
STATES...WITH ANY CLIPPER SYSTEMS HAVING TO WORK WITH LIFT ALONE
TO PRODUCE MUCH PRECIPITATION.  NOW THAT THE PACIFIC FLOOD GATES
ARE OPEN...POINTS FARTHER WEST WILL SEE AN UNSETTLED THIRD WEEK OF
OCTOBER...WITH THE COASTLINE FROM 40N UP NETTING THE MOST
RAINFALL.  THE SOUTHWEST WILL REMAIN REMOVED FROM THE
FRAY...TUCKED SAFELY WITHIN THE SUBTROPICAL RIDGE.


FINAL...

DESPITE THE MORE AMPLIFIED SOLUTION OF THE 12Z/13 GFS...THE NEW
GUIDANCE OVERALL SPELLS LESS OF A THREAT OF PROLONGED
PRECIPITATION OVER THE UPCOMING WEEKEND FOR THE CENTRAL
APPALACHIANS...AND HENCE...LESS OF A CHANCE OF HEAVY SNOW THERE.
THE GEM GLOBAL IS CLOSE TO THE SOLUTION WHICH INFORMED THE UPDATE
PACKAGE...WITH THE UKMET TRENDING STRONGLY TOWARD IT BY DAYS 5 AND
6 WITH ITS MUCH MORE PROGRESSIVE SOLUTION OVER THE WESTERN
ATLANTIC.  STILL FEEL THAT THE WESTERN STATES ARE WELL COVERED BY
THE PRELIMINARY UPDATE.








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A Noreaster for the Weekend? - JAC, 10/13/2009, 7:30 am
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