PROGNOSTIC METEOROLOGICAL DISCUSSION

 
 FXUS02 KWBC 241929
 PMDEPD
  
 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