EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
228 PM EST TUE NOV 24 2009
 
VALID 12Z FRI NOV 27 2009 - 12Z TUE DEC 01 2009
 
A LARGE NEGATIVE 500 MB HEIGHT ANOMALY NEAR ALASKA AND A
SUBSEQUENT INTENSE POLAR JET STREAM DEVELOPING TO ITS SOUTH HELP
SET THE STAGE FOR A GRADUAL SPLITTING OF THE SHORTWAVE PATTERN
OVER THE CONUS AND SOUTHERN CANADA BY THE START OF THE MEDIUM
RANGE PERIOD...WITH GUIDANCE ALREADY SHOWING ABOVE AVERAGE
SOLUTION SPREAD BY DAY 3...WHICH IS 1-2 DAYS EARLIER THAN USUAL.
12Z GUIDANCE ALSO SHOWS A LARGE ARRAY OF DIFFERING SCENARIOS
BEYOND DAY 5...ALL WITH VERY DIFFERENT SENSIBLE WEATHER
IMPLICATIONS....PARTICULARLY FROM THE PLAINS EASTWARD. THE 12Z GFS
AND THE MAJORITY OF THE ITS ENSEMBLE SUITE REPRESENT THE FAST HALF
OF THE GUIDANCE WITH A TROUGH REACHING THE PLAINS BY DAY 5...WHICH
IN PART IS DUE TO LESS SPLITTING...WHILE THE 00Z ECMWF (12Z RUN IS
FASTER) AND MOST OF ITS 00Z ENSEMBLE SUITE DISPLAY MORE SPLITTING
AND THUS SLOWER SPEED. THE 00-12Z CANADIAN/UKMET/NOGAPS FALL
BETWEEN THESE TWO CAMPS WITH LARGE SPREAD IN BOTH THE NORTHERN AND
SOUTHERN STREAMS. WITHOUT MUCH MODEL CONSENSUS...
CONFIDENCE IS LOW IN CHOOSING ANY PARTICULAR SOLUTION.
HOWEVER...THE RELATIVE POSITIONS OF THE SHORTWAVE TROUGH NEARING
THE WEST DAY 3 AND THE RAPIDLY APPROACHING PACIFIC JET DO LEND
THEMSELVES THE SPLITTING OF STREAMS...WHICH IS WHAT MOST GUIDANCE
SHOWS ALBEIT WITH INCREASINGLY DIFFERENT OUTCOMES BY DAYS 6/7.
THUS...GIVEN AN ANTICIPATED PATTERN CONDUCIVE FOR
SPLITTING...PREFER TO LEAN TOWARD THE SLOWER/MORE SPLIT HALF OF
THE GUIDANCE...WHICH INCLUDES ABOUT 1/3 EACH OF THE 00-12
ECMWF/06Z GFS IN THE FINAL PRESSURES/FRONTS PROGS...AND LITTLE TO
NONE OF THE EXTREME SOLUTIONS...WHICH SEEMS LIKE A GOOD PLAY TO
LIE UNTIL BETTER MODEL AGREEMENT DEVELOPS. 

IN ANY EVENT...THE CYCLONE INTENSIFYING NEAR THE EASTERN GREAT
LAKES DAY 3 IS EXPECTED TO BE A FORMIDABLE ONE BEFORE
STRENGTHENING FURTHER OVER NEW ENGLAND/CANADIAN MARITIMES DAY
4...WITH COLD/WINDY CONDITIONS FORMING ON THE BACK SIDE OF THE
CYCLONE CENTER ALONG WITH THE POTENTIAL FOR HEAVY RAIN/POSSIBLY
SNOW FOR PART OF THE INTERIOR NORTHEAST/NEW ENGLAND BEFORE THE
SYSTEM EXITS THE REGION. MEANWHILE...A QUICK ROUND OF HEAVY
PRECIPITATION...PARTICULAR FOR THE HIGHER ELEVATIONS...SHOULD
IMPACT THE PACIFIC NORTHWEST WITH ARRIVAL OF THE ANTICIPATED
SHORTWAVE TROUGH DAY 3 BEFORE THE TROUGH EVENTUALLY SPLITS OVER
THE ROCKIES. WHILE MODELS SHOW CONSIDERABLE DETAIL
DIFFERENCES...THE POTENTIAL EXISTS FOR A SIGNIFICANT RAIN EVENT
FROM THE CENTRAL/SOUTHERN MISSISSIPPI/TENNESSEE RIVER VALLEYS
EASTWARD DAYS 6/7 AS THE TROUGH AND ITS ACCOMPANYING COLD FRONT
ENCOUNTER INCREASING GULF OF MEXICO MOISTURE AND AT LEAST LIMITED
INSTABILITY ALONG THE GULF COAST...WHILE A NARROW SWATH OF SNOW IS
DEPICTED BY AT LEAST SOME GUIDANCE ON THE NORTHERN PERIPHERY OF
THE LOW/MID CYCLONE CENTER.

JAMES
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