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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In this thread:
Models Uncertain on a Major East-Coast Snowstorm around Dec 20 -
JAC,
12/13/2010, 5:20 am- High Amplitude Omega Block Sets In - JAC, 12/15/2010, 6:48 am
- Boston NWS Discussion. Nada from NYC - JAC, 12/13/2010, 8:28 am
- NAO Forecast - JAC, 12/13/2010, 6:45 am
- NAO still negative. GEM keeps most precip off-shore - JAC, 12/14/2010, 6:07 am
- 06Z GFS whacks Fred big time - JAC, 12/13/2010, 6:16 am
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