729
FXUS02 KWBC 201930
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
329 PM EDT SUN SEP 20 2009
VALID 12Z WED SEP 23 2009 - 12Z SUN SEP 27 2009
A STUBBORN UPPER LOW IN THE PLAINS STATES...AND NRN STREAM PACIFIC
FLOW CRASHING THROUGH AN INITIALLY STRONG WRN NOAM RIDGE...WILL BE
THE MAIN CHALLENGES OF THIS MEDIUM RANGE FORECAST. CONSENSUS OF
THE GEFS/ECMWF/CANADIAN ENSEMBLE MEANS INDICATES A REBOUND OF THE
W COAST OF NOAM RIDGE AND A FULL LATITUDE TROF OVER ERN NOAM BY
NEXT WEEKEND.
12Z/29 MODELS: CONCERNING THE PLAINS CLOSED LOW...THE LATEST
12Z/20 GFS AND ITS ENSEMBLE MEAN ARE ON THE W SIDE OF THE ENVELOPE
OF GUIDANCE WHILE THE UKMET IS ON THE E SIDE...BECOMING THE MOST
PROGRESSIVE BY NEXT SAT. THE CANADIAN IS FAIRLY CLOSE TO THE UKMET
THRU EARLY FRI DAY 5 THEN BECOMES FASTER BY DAY 6. THE THEME OF A
NRN STREAM TROF OPENING UP THE PLAINS CLOSED LOW STILL HOLDS. THE
UKMET IS FASTEST TO OPEN THE SYS UP...FOLLOWED BY THE
CANADIAN...ECMWF...AND GFS...IN THAT ORDER. THE NEW 12Z/20 ECMWF
IS FAIRLY CONSISTENT WITH ITS 00Z RUN BUT IS MORE AMPLIFIED/CLOSED
OFF NEXT WEEKEND THAN ITS PREVIOUS RUN OVER THE MS VLY.
FINAL PROGS SHOWED A SLIGHTLY DEEPER SURFACE REFLECTION OF THE
UPPER LOW IN THE PLAINS DAYS 3-4 BASED ON THE NEW CANADIAN/UKMET.
OTHERWISE NO CHANGES FROM THE UPDATED PRELIM. RECENT DETERMINISTIC
GFS RUNS...INCLUDING THE NEW 12Z/20...CONTINUE TO BE OUTLIERS IN
THE SENSE THAT THEY MOVE THE UPPER LOW FORMING MON EVE IN THE
CENTRAL HIGH PLAINS TOWARDS THE 4 CORNERS REGION THEREAFTER.
HOWEVER...THE 00Z/20 GEFS AND ECMWF ENSEMBLE MEANS ARE IN PRETTY
GOOD AGREEMENT IN SHOWING THE PLAINS CLOSED LOW OPENING UP AND
MERGING INTO A MORE FULL LATITUDE PROGRESSIVE TROF BY SAT-SUN.
DETERMINISTIC 12Z MODELS ALSO SUPPORT THE FULL LATITUDE TROF IDEA
BY NEXT SAT.
12Z MODELS IN A BIT BETTER AGREEMENT ON THE FIRST MAJOR SHORTWAVE
TO MOVE THRU THE WRN NOAM RIDGE THU DAY 4. STILL SUBSTANTIAL MODEL
DIFFERENCES OVER CANADA THEREAFTER. THE GEFS AND ECMWF ENSEMBLE
MEANS ARE ALSO PREFERRED IN TONING DOWN A STRONG DEVELOPMENT DAY 6
DEPICTED BY THE GFS/ECMWF OVER THE CANADIAN MARITIMES. THE HUGE
DIFFERENCES BETWEEN THE 12Z/20 GFS/ECMWF AND THE CORRESPONDING
UKMET/CANADIAN ALSO SUPPORT THIS TONING DOWN.
THE CLOSED LOW FORMING OVER THE CENTRAL HIGH PLAINS WED/THU MAY BE
JUST BARELY COLD ENOUGH FOR SN AT THE HIGHEST ELEVATIONS ALONG THE
FRONT RANGE OF WY/CO VCNTY OF THE COMMA HEAD. OTHERWISE CONVECTION
E AND SE OF THE UPPER LOW WILL ASSUME A MORE PROGRESSIVE CHARACTER
BY NEXT WEEKEND AS THE CLOSED UPPER LOW OPENS UP INTO A FULL
LATITUDE FEATURE.
FLOOD
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