648
FXUS02 KWBC 221932
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
331 PM EDT FRI AUG 22 2008
VALID 12Z MON AUG 25 2008 - 12Z FRI AUG 29 2008
THE LONGWAVE PATTERN DURING THE MEDIUM RANGE PERIOD WILL BEGIN
WITH TROUGHS OVER THE EAST AND WEST COASTS...AND A RIDGE IN
BETWEEN. THIS PATTERN WILL BE PROGRESSIVE AT MID LATITUDES. FAY
REMNANTS SHOULD MEANDER WWD INLAND FROM THE GULF COAST....THEN
BEGIN A SLOW NNE DRIFT DAYS 6-7. ONE OF THE CHALLENGES IN THE
MEDIUM RANGE PERIOD WILL BE DETERMINING THE EWD SPEED OF THE MEAN
500MB WEST COAST TROUGH...AND THE DEGREE OF RIDGING BEHIND IT IN
THE ERN PACIFIC FROM DAY 4 ONWARD. THE 00Z/22 GEFS/ECMWF ENSEMBLE
MEANS WERE SHOWING A MAXIMUM OF SPREAD OVER THE NWRN STATES DAYS
5-6 WHERE IS A POTENTIAL FOR MORE DIGGING OF HGT FALLS SE INTO THE
CENTRAL GRT BASIN AND POINTS E. FINAL GRAPHICS UNCHANGED FROM THIS
MORNING.
12Z MODELS: THE GFS...ECMWF...AND CANADIAN ARE ALL SHOWING SIGNS
OF SLOWING DOWN THE TRAILING END OF THE UPPER TROF MOVING INTO THE
CANADIAN MARITIMES TUE DAY 4. MODEL CHANGES FROM CONTINUITY HERE
SEEM TOO FAR N FOR FURTHER SLOWING OF THE NEWD SPREAD OF MOISTURE
FROM FAY. THEY ARE IN GOOD AGREEMENT ON PROGRESSING THE UPSTREAM
RIDGE ACROSS ERN CANADA. THE CANADIAN IS THE FARTHEST SW OUTLIER
BOTH WITH THE CANADIAN MARITIMES TROF AND THE NEXT TROF DIGGING
INTO THE NW STATES AS IT CLOSES OFF AN UPPER LOW OVER CO/UT DAY 6.
THE GFS AND ECMWF MAINTAIN DECENT CONTINUITY WITH THE NW
CONUS/PLAINS TROF DAYS 5-6 EXCEPT THAT THE GFS SEPARATES THE SRN
END OF THE TROF INTO A CLOSED SYS OVER OK ON FRI DAY 7. AS WE DID
THIS MORNING...WE WILL WILL BE STAYING IN THE CENTER OF THE
ENVELOPE OF ENSEMBLE AND MODEL 500MB SOLUTIONS ACROSS
NOAM...DISCARDING THOSE UPPER TROUGHS THAT ARE TOO SLOW OR TOO CUT
OFF. PREFER A BLEND OF THE GFS AND ECMWF IN WITH THE TIMING AND
AMPLITUDE OF THE CANADIAN MARITIMES TROF. BELIEVE THAT THE ECMWF
HAS THE BEST CONTINUITY WITH THE TROF MOVING ACROSS THE NWRN
STATES INTO THE NRN PLAINS DAYS 5-6.
..REGIONAL HIGHLIGHTS
..SOUTHEAST
THE SPREAD OF RESIDUAL MOISTURE FROM FAY NWD AND EWD WILL BE
LIMITED BY THE SLOW EWD PROGRESSION OF THE RIDGE/TROF PATTERN IN
CENTRAL AND ERN CANADA DAYS 5-7. THE ENSEMBLE MEANS OF THE GEFS
AND ECMWF ARE A LITTLE SLOWER MOVING THE 500MB TROF/RIDGE PATTERN
EWD ACROSS ERN CANADA. THIS HOLDS IN DRY HIGH SURFACE PRES LONGER
OVER THE ERN GRT LAKES AND OH VLY DAYS 5-6 INSTEAD OF YIELDING TO
MOIST SE FLOW. OUR BLEND WITH THE GEFS MEAN TAKES THIS INTO
ACCOUNT. EVENTUALLY...HOWEVER..SIGNIFICANT MOISTURE FROM FAYS
REMNANTS MAY EVENTUALLY WORK ITS WAY INTO THE TN VLY AND SRN
APPALACHIANS. PLEASE REFER TO TPC BULLETINS CINCERNING FAY.
..NORTHWEST
MODELS AND MODEL MEANS AGREE ON BRINGING A STRONG SHORTWAVE AND
ASSOCIATED FRONT INTO THE PACIFIC NW ON MON DAY 3. RIDGING ALOFT
SHOULD BRING MORE SETTLED WEATHER TO THIS AREA DAYS 5-7.
...PLAINS..
UPPER TROF EXITING THE NRN ROCKIES EARLY DAY 5 SHOULD BE
DYNAMICALLY ACTIVE ENOUGH TO IGNITE AN AREA OF ORGANIZED
CONVECTION WITH AN ASSOCIATED COLD FRONT. CONVECTION SHOULD REMAIN
ACTIVE ALONG THE FRONT DAYS 6-7.
FLOOD
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