857
FXUS02 KWBC 101801
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
150 PM EDT THU SEP 10 2009
VALID 12Z SUN SEP 13 2009 - 12Z THU SEP 17 2009
THE GENERAL FLOW PATTERN THIS PERIOD FEATURES TROUGHING OFFSHORE
THE NORTHWEST AS WELL AS ACROSS THE NORTHEAST...WITH BROAD RIDGING
ACROSS SOUTHWEST/SOUTH CENTRAL CANADA. THIS BROAD RIDGE HAS BEEN
THE FORECAST CHALLENGE...ALTERING THE BEHAVIOR OF THE CLOSED
CYCLONE WHICH IS NOW EXPECTED TO ENCIRCLE ITS CENTER ALONG THE
CENTRAL US/CANADIAN BORDER OVER THE NEXT FIVE DAYS. TO THE
SOUTHWEST OF THE UPPER LOW UNDER THE STRONG RIDGING IN WESTERN
CANADA...A CLOSED CYCLONE IS NOW EXPECTED TO MOVE QUICKER INTO THE
WEST THAN SEEN PREVIOUSLY AS THE WANDERING US/CANADIAN BORDER
UPPER CYCLONE HELPS LURE IT MORE QUICKLY UNDER THE RIDGE. WITH
THE PRESSURES THIS PERIOD...EARLY PRELIMS START WITH A 00Z
ECMWF/00Z GEFS MEAN SOLUTION BEFORE MAKING ADJUSTMENTS ACROSS THE
SOUTH AND MIDWEST AS DETAILED BELOW...WHICH ALLOWS US TO SPEED UP
THE PROGRESSION OF THE SYSTEM INTO THE WEST WHILE STILL
MAINTAINING REASONABLE CONTINUITY.
TO THE SOUTHEAST OF US/CANADIAN BORDER SYSTEM...A NEW CLOSED
CYCLONE ACROSS TEXAS IS LURED NORTHEAST...WHICH CAUSES
CYCLOGENESIS IN THE WESTERN GULF OF MEXICO IN 1-2 DAYS. THIS LOW
MOVES NORTHEAST AND BECOMES A FRONTAL WAVE AS IT MOVES INTO THE
MIDWEST/OHIO VALLEY BEFORE TROUGHING ACROSS THE NORTHEAST
AMPLIFIES AND SHUNTS THE SYSTEM SOUTHEAST/SHEARS IT OUT
THEREAFTER. EARLY WENT WITH A SOLUTION IN THE MIDDLE OF THE 12Z
MULTI-MODEL ENSEMBLE CLUSTERING WITH THIS LOW...WHICH IS NOW THE
DOMINANT CYCLONE SINCE THE SYSTEM MOVING AROUND THE CENTRAL
US/CANADIAN BORDER LOSES MOST OF ITS SURFACE REFLECTION.
UPDATED MORNING PRELIMS INCORPORATE MORE 00Z ECMWF ENS MEAN WITH
AN EVEN BALANCE OF THE OP 00Z ECMWF AND ITS ENS MEAN. THIS FORCES
THE EARLIER PRELIM IDEA MENTIOND ABOVE OF A TN VALLEY LOW SWD
KEEPING IT ALONG THE GLFMEX THIS PERIOD. USE OF ECMWF ENS MEANS
KEEPS ENOUGH POP SPREAD TO ACCOUNT FOR SFC POSITIONAL UNCERTAINTY.
12Z GFS HAS COME AROUND STRONGLY TOWARDS THE ECMWF WITH SHORTWAVE
COMING INTO CA LATE MON DRIFTING EWD INTO THE SRN ROCKIES AND IS
NOW MORE SIMILAR TO THE 00Z ECMWF AND BOTH ENS MEANS OF GFS AND
ECMWF WITH THE PAC TROF MID NEXT WEEK AND RIDGING OVER THE NRN
U.S. AND CANADIAN ROCKIES. THERE REMAINS A LARGE AMOUNT OF
UNCERTAINTY IN THE MID LEVEL WEAKNESS AREA
FROM THE ROCKIES TO THE APPLCHNS BUT AN OVERALL PATTERN
IMPROVEMENT AND ADDED CONFIDENCE.
MINOR SFC CHANGES MADE TO PROGS FOR AFTN FINALS.
ROTH/ROSENSTEIN
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