FXUS02 KWBC 211930
PMDEPD
 
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
329 PM EDT THU AUG 21 2008
 
VALID 12Z SUN AUG 24 2008 - 12Z THU AUG 28 2008
 
THE LONGWAVE PATTERN SHOWS PROGRESSION OF TWO MAJOR TROUGHS IN THE
POLAR WESTERLIES. THE FIRST OF THESE WILL MOVE FROM THE GULF OF
ALASKA INTO THE NWRN CONUS AND ADJACENT CANADA. THE SECOND ONE
WILL MOVE ACROSS ERN CANADA INTO THE N ATLANTIC...A BIG RIDGE WILL
PROGRESS EWD BETWEEN THE TWO TROUGHS. AT LOWER LATITUDES...THE
REMNANTS OF TS FAY SHOULD  EITHER REMAIN STNRY OR DRIFT EVER SO
SLOWLY NNE UNDERNEATH A STRONG POS HEIGHT ANOMALY OVER ERN
CANADA/THE GRT LAKES THE LATTER HALF OF THE MEDIUM RANGE PERIOD.  
THE NEW 12Z GFS HAS TAKEN A DECIDED TURN TOWARDS THE 00Z
ECMWF....MORE SO THAN THE 06Z/21 GFS RUN. THE NEW 12Z ECMWF
FOLLOWS ITS OWN CONTINUITY FAIRLY WELL THRU DAY 5 THEN DRIVES
HEIGHT FALLS ACROSS THE NWRN STATES INTO THE N CENTRAL STATES EVEN
FASTER THAN IT DID IN ITS PREVIOUS RUN. IT ALSO EMPHASIZES A
DIFFERENT SHORT WAVE BY DAY 7 OVER THE NRN HIGH PLAINS. WE LEFT
FINAL GRAPHICS MOSTLY UNCHANGED FROM THE PRELIM UPDATE.
  
THIS MORNINGS NAEFS/ECMWF ENSEMBLE PLOT (A COMBINATION OF THE
GFS/CANADIAN/ECMWF ENSEMBLE MEMBERS)  HAD SUGGESTED THAT THE
CENTER OF THE ENVELOPE OF SOLUTIONS WAS AS GOOD A PLACE TO BE AS
ANY. THRU MON DAY 4...THE ENSEMBLE MEMBERS AGREED RATHER WELL ON A
PROGRESSIVE TROF/RIDGE/TROF PATTERN ACROSS THE CONUS...AND ON THE
MEANDERING REMNANTS OF FAY OVER THE WRN GULF STATES. BY DAY 5 THE
ENSEMBLE MEMBER SPREAD INCREASED SUBSTANTIALLY OVER THE WRN CONUS.
AN AVERAGE OF THE 00Z/21 ENSEMBLE MEANS OF THE GEFS/ECMWF/CANADIAN
STILL OFFERS A REASONABLE ALTERNATIVE TO THE VARIOUS MODEL
SOLUTIONS INCLUDING THOSE OF THE NEW 12Z/21 MODELS. THE FATE OF
FAY DAY 5 AND BEYOND BECOMES A BIT MURKY...SO DAY 6-7 MANUAL
GRAPHICS JUST SHOWED A SLOW NNE DRIFT OF THAT FEATURE WITHOUT A
SYS FROM THE N TO CLEARLY PICK IT UP. PLEASE REFER TO TPC
DISCUSSIONS CONCERNING FAY.  
...REGIONAL HIGHLIGHTS...
...GULF COAST/S AND MID ATLANTIC  STATES...
THE REMNANTS OF SLOW MOVING BUT WET TS FAY WILL BRING HEAVY
RAINFALL ESPECIALLY TO THE FL PANHANDLE IN THE MEDIUM RANGE. IT
APPEARS THAT THE NRN EXTENT OF THE HEAVY RAINFALL WILL BE SLOW TO
WORK NWD INTO THE DROUGHT STRICKEN AREAS OF AL AND GA....BUT SOME
SHOULD REACH THOSE AREAS BY DAY 6 OR 7. THE 12Z GFS APPEARS TO
REPLACE WEAK STEERING FLOW OVER THE SERN STATES WITH 500MB SWLY
FLOW OVER THE SERN STATES DAYS 5-6...PULLING FAYS MOISTURE TOO
QUICKLY NEWD INTO THE CAROLINAS DAY 5...AND ON INTO THE
MID-ATLANTIC DAY 6. THE NEW 12Z/21 CANADIAN/ECMWF FOLLOW OUR
CONTINUITY BETTER BY MAINTAINING A BLOCKIER 500MB PATTERN TO THE N
THRU E OF THE REMNANTS OF FAY...ALLOWING ONLY A SLOW NNE DRIFT
DAYS WED/THU DAYS 6-7 FROM MON DAY 5 TPC POSITION OF 32N/90W. 
...COLD FRONT MOVING INTO THE ERN PORTION OF THE CONUS THEN
STALLING OUT...
ONLY MODERATE AMOUNTS ARE EXPECTED WITH THIS FRONT SINCE THE DEEP
MOISTURE FROM THE REMNANTS OF FAY IS EXPECTED TO BE SLOW TO
WORKING NWD. LITTLE INTERACTION IS EXPECTED WITH THIS FRONT THRU
TUE DAY 5.
...PACIFIC NW COAST...
A DECENT FRONT FOR THIS TIME OF YEAR WILL BRING SIGNIFICANT PCPN
TO THE HIGHER ELEVATIONS OF WA/OR SUN/MON DAYS 3-4. FAST FLOW ATOP
AN ERN PACIFIC RIDGE OFFERS PERHAPS ANOTHER QUICK SHOT AT PRECIP
FOR WA DAY 6  OR 7 BUT LOW CONFIDENCE IN THE TIMING.   
FLOOD
$$