NAO still negative. GEM keeps most precip off-shore
Posted by JAC on 12/14/2010, 6:07 am
PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
251 AM EST TUE DEC 14 2010

VALID 12Z SAT DEC 18 2010 - 12Z TUE DEC 21 2010

THE NEGATIVE PHASE OF THE NORTH ATLANTIC OSCILLATION WHICH
CURRENTLY MANIFESTS ITSELF AS AN ANOMALOUSLY LARGE BLOCKING REGIME
OVER THE NORTHERN/CENTRAL ATLANTIC WILL REMAIN THE DOMINANT
LONGWAVE FEATURE THROUGH THE MEDIUM RANGE PERIOD. THE LATEST
GUIDANCE INCLUDING ENSEMBLES DEPICT THE SAME GENERAL EVOLUTION
THROUGH DAY 7...WITH AN UPPER RIDGE NEAR GREENLAND EXPANDING AND
RETROGRADING INTO CANADA WHILE MEAN TROUGHS OVER THE GULF OF
ALASKA AND CENTRAL CONUS ON DAY 3 DRIFT SLOWLY EASTWARD THROUGH
DAY 7. HOWEVER...AS IS OFTEN THE CASE THE DAY-TO-DAY DETAILS
DIFFER CONSIDERABLY...WITH SOLUTION SPREAD FROM THE UPPER MIDWEST
INTO NEW ENGLAND ALREADY WELL ABOVE AVERAGE ON DAY 3 AND GROWING
RAPIDLY THEREAFTER. GIVEN THE LARGE SPREAD SO EARLY IN THE
FORECAST PERIOD ALONG WITH STRONG INFLUENCES FROM HIGHER LATITUDES
WHERE DATA INGEST FOR MODEL GUIDANCE IS MORE LIMITED...THE
RECOMMENDATION IS FOR A HEAVY RELIANCE ON ENSEMBLE MEAN DATA
THROUGHOUT THE ENTIRE PERIOD. THE PRIMARY DIFFERENCE CONCERNING
THE ENSEMBLES INVOLVES THE 12Z ECMWF ENSEMBLE MEAN WHICH IS FASTER
AND A LITTLE CLOSER TO THE EAST COAST WITH A DEVELOPING CYCLONE
DAYS 5-7 THAN THE GFS AND CANADIAN ENSEMBLE MEANS...WITH THE
GFS/CANADIAN ENSEMBLE MEAN SOLUTIONS MOST PREFERRED DUE TO
CONSENSUS.

JAMES    






EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
1238 PM EST MON DEC 13 2010

VALID 12Z THU DEC 16 2010 - 12Z MON DEC 20 2010


...PRELIMINARY UPDATE...

USED THE 00Z ECMWF TO UPDATE THE FRONTS AND PRESSURES FOR DAYS 3
AND 4...WITH A GRADUAL TRANSITION TO THE 06Z GEFS MEAN BY DAY 6.
THE BLOCKY REGIME IS INDICATED TO HOLD THROUGH THE PERIOD BY ALL
THE MODELS...WITH A SUPPRESSED POLAR JET ACROSS THE ENTIRE NATION.
HEIGHTS WILL BE THE GREATEST OVER THE SOUTHERN HIGH PLAINS WHERE
THE SUBTROPICAL RIDGE WILL BE ABLE TO HERNIATE NORTHWARD.  THE
DETAILS OF THE ENERGY COMING INTO THE WEST COAST ARE WILDLY
UNCERTAIN...BUT THERE IS ENOUGH AGREEMENT BETWEEN THE ENSEMBLE
MEANS FROM THE VARIOUS MODELING CENTERS TO SUPPORT THE BLEND USED
FOR THIS FORECAST PACKAGE.  THE SITUATION ALONG THE EAST COAST IS
FAR MORE CONTENTIOUS...WITH THE 00Z AND 06Z GFS RUNS BOTH BRINGING
A MAJOR SNOWSTORM TO THE MID ATLANTIC AND NORTHEAST DAYS 6 AND 7.
THE 00Z ECMWF KEEPS THE NORTHERN STREAM DOMINANT OVER THIS
REGION...AS DOES THE GEM GLOBAL.  HOWEVER...THE 12Z/12 AND 12Z/11
RUNS OF THE ECMWF BOTH SHOWED THE SNOWSTORM.  OPTED TO GO WITH THE
06Z GEFS MEAN FOR THIS POTENTIAL EVENT...WHICH AT LEAST BRINGS A
CYCLONE NORTHWARD OVER THE WESTERN ATLANTIC...JUST FARTHER
OFFSHORE AND WITH LESS ISOBARIC COMMITMENT...IF YOU WILL.  THIS
CHOICE LEAVES ROOM FOR TRENDING EITHER WAY.


...FINAL...

THE 12Z GUIDANCE OFFERS A DISCONCERTING ARRAY OF POSSIBLE OUTCOMES
FOR THE SPLIT FLOW OVER THE NATION DURING THIS MEDIUM RANGE
PERIOD.  FORTUNATELY...THE LATEST GEFS MEAN HAS NOT TRENDED MUCH
FROM THE 06Z RUN...WHICH INFORMED MUCH OF THE UPDATE PACKAGE.  THE
DETERMINISTIC GFS HAS GONE FLATTER WITH THE ATLANTIC COAST WAVE
DAY 6...WITH THE GEFS MEAN SHOWING MORE AMPLITUDE...ENOUGH TO
THREATEN PORTIONS OF THE EASTERN SEABOARD WITH SNOW.  THE GEM
GLOBAL HAS TRENDED CONSIDERABLY MORE AMPLIFIED WITH THE DAY 6
SYSTEM...BRINGING SNOW BACK TO INTERSTATE 95 FROM RICHMOND
NORTHWARD.  THE UKMET REMAINS ON ITS OWN WITH SHOWING ENOUGH
AMPLITUDE TO BACK THE FLOW OVER THE ATLANTIC STATES DAYS 5 AND 6
TO BRING RAIN TO MUCH OF THE REGION.  THE LESSON WITH THIS MOST
RECENT COMPLEX AMPLIFICATION OVER THE MIDWEST AND EAST WAS TO GO
BETWEEN THE EXTREME SOLUTIONS...WHICH THE UPDATE BLEND
ACCOMPLISHED.  FOR THIS REASON...NO CHANGES WERE MADE FOR THE
FINAL ISSUANCE.


CISCO
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Models Uncertain on a Major East-Coast Snowstorm around Dec 20 - JAC, 12/13/2010, 5:20 am
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