499
FXUS02 KWBC 181848
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
247 PM EDT MON AUG 18 2008
VALID 12Z THU AUG 21 2008 - 12Z MON AUG 25 2008
PRELIMINARY UPDATE...
GUIDANCE SHOWS THE EVOLUTION OF A BROADENING TROF OVER THE NERN
PACIFIC... WITH AN ASSOC CORE OF NEGATIVE HGT ANOMALIES TO THE S
OR SW OF THE ALASKA MAINLAND BY D+8. THIS FEATURE TELECONNECTS
WELL TO THE WEAK MEAN TROF EXPECTED OVER ERN NORTH AMERICA BY LATE
IN THE PERIOD. IN TERMS OF THE DAILY FCSTS... THE 00Z GFS IS
SOMEWHAT FARTHER SWD THAN OTHER MODELS WITH THE MID LVL CLOSED LOW
THAT TRACKS ALONG OR N OF THE US-CANADIAN BORDER.
CONTINUITY/TRENDS OVER RECENT DAYS FAVOR LEANING AWAY FROM THE GFS
FCST IN THAT RESPECT. MODELS/ENSEMBLES SHOW REASONABLE CLUSTERING
WITH THE EXPECTED PATTERN OVER THE NERN PACIFIC THRU DAY 5 SAT BUT
THEN DIVERGE WITH DETAILS OF THE MEAN TROF. BY DAY 7 MON... A
BLEND OF THE 00Z ECMWF AND ECMWF ENSEMBLE MEAN PROVIDE A
REASONABLE COMPROMISE FOR FLOW FROM THE NERN PAC INTO CNTRL
CANADA.
FARTHER EWD... IT REMAINS TO BE SEEN WHETHER THE RIDGE THAT BUILDS
INTO THE NORTHEAST BY FRI-SAT WILL BE QUITE AS STRONG AS DEPICTED
IN THE 00Z GFS/ECMWF. THE 00Z GFS IS ON THE EXTREME EDGE OF GEFS
MEMBERS BUT THE PAST COUPLE ECMWF RUNS ARE SOMEWHAT BETTER
CONTAINED WITHIN THEIR RESPECTIVE ENSEMBLE ENVELOPES. THIS
FEATURE MAY HAVE SOME INFLUENCE ON TROPICAL STORM FAY... WHOSE
TRACK OVER OR NEAR THE SERN CONUS IS STILL FAIRLY UNCERTAIN.
CONSULT THE LATEST TPC ADVISORY FOR FURTHER INFO REGARDING T.S.
FAY. UPDATED PRELIM FCST DEPICTS INTERPOLATION OF THE 09Z
ADVISORY.
BASED ON THE PREFERENCE TO LEAN AWAY FROM THE GFS WITH THE CLOSED
LOW TRACKING ALONG/N OF THE US-CANADIAN BORDER THE UPDATED PRELIM
FCST IS WEIGHTED TOWARD THE 00Z ECMWF... AND FOR DAYS 5-7
FRI-MON...INCLUDES A BLEND WITH THE 00Z ECMWF MEAN TO ACCOUNT FOR
LOW CONFIDENCE DETAIL UNCERTAINTIES.
FINAL...
SAW NO COMPELLING REASON SORTING THROUGH THE 12Z MODEL CYCLE TO
ALTER THE FRONTS AND PRESSURES FROM THE UPDATE PACKAGE FOR THE
FINAL ISSUANCE OVER MOST OF THE NATION. THE ONE EXCEPTION WAS
OVER THE SOUTHEASTERN STATES...WHERE THE DYNAMICAL MODELS TURN FAY
WESTWARD ACROSS THE FLORIDA PANHANDLE. LEFT A BAGGINESS IN THE
ISOBARS THERE TO ADDRESS THE UNCERTAINTY...WITH QPF FOR DAYS 4 AND
5 STRETCHED SOUTHWESTWARD INTO THE REGION FROM THE PRELIMINARY
CHARTS FOR THE FINAL PROG.
CISCO/RAUSCH
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