652
FXUS02 KWBC 121757
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
156 PM EDT MON OCT 12 2009
VALID 12Z THU OCT 15 2009 - 12Z MON OCT 19 2009
PRELIMINARY UPDATE...
THE FRONTS AND PRESSURES FOR DAYS 3 THROUGH 7 WERE UPDATED USING A
PROGRESSIVE BLEND OF THE 00Z/12 ECMWF TOWARD THE 00Z/12 EC
ENSEMBLE MEAN. NONE OF THE DETERMINISTIC MODELS HAVE BEEN
PARTICULARLY STABLE OVER THE PAST SEVERAL RUNS...THOUGH THE ECMWF
HAS BEEN THE LEAST HERKY JERKY. THE PATTERN IS DECIDEDLY WINTRY
OVER THE EAST...WITH VERY COLD HIGH PRESSURE DRAPED ACROSS
SOUTHEASTERN CANADA...AND AT LEAST TWO WAVES OF LOW PRESSURE
TAKING SWIPES AT THE MID ATLANTIC STATES AND NORTHEAST. THE
NORTHERN EDGE OF THE PRECIPITATION SHIELD WITH THE FIRST WAVE IS
LIKELY TO BE SNOW...WITH THE GFS ALGORITHMS SUGGESTING THAT EVEN
AREAS AWAY FROM THE APPALACHIANS FROM SOUTH CENTRAL PENNSYLVANIA
TO THE HILL COUNTRY OF CONNECTICUT AND MASSACHUSETTS MAY SEE SOME
WET SNOW. THE SECOND WAVE...ON THE HEELS OF THE FIRST...IS
DICIER...WITH THE MEAN PROGS INDICATING THAT IT WILL FORM WELL OUT
TO SEA...IF AT ALL. THESE POSITIVE TILT TROUGH DEVELOPMENTS ARE
NOTORIOUSLY TRICKY...WITH A SMALL SHIFT IN LONGITUDE OF THE WAVE
AMPLIFICATION MAKING THE DIFFERENCE BETWEEN A RAGING STORM OVER
THE WEST WALL OF THE GULF STREAM...OR A DIFFUSELY BAROCLINIC LOW
CUTTING OFF NEAR BERMUDA. THE ROCKIES WILL GET A CHINOOK BREAK
THIS PERIOD...WHILE THE WEST COAST WELCOMES THE RAINY SEASON WITH
EXTENSIVE ONSHORE FLOW FROM EUREKA NORTHWARD.
FINAL...
THE 12Z/12 UKMET IS IN SYNC WITH THE BLEND USED FOR THE UPDATE
PACKAGE THROUGH DAY 6...EXCEPT ON DAY 3...WHEN IT IS SLOWER TO
SEND THE INITIAL WAVE OFF THE CAROLINA COAST INTO THE ATLANTIC.
THE LATEST GEFS MEAN SUPPORTS THE FASTER DAY 3 EMERGENCE
OFFSHORE...AND CONSIDERING THE ROBUSTNESS OF A MEAN AT THAT
RELATIVELY SHORT TIME RANGE...DID NOT MAKE ANY CHANGES THERE FOR
THE FINAL ISSUANCE. THE 12Z/12 GFS GETS QUITE OUT OF PHASE WITH
THE OTHER GUIDANCE ALONG THE EAST COAST EARLY IN THE PERIOD...WITH
MAJOR TIMING AND PLACEMENT DIFFERENCES WITH THE COMPLEX EVOLUTION
OF THE WAVES ROLLING THROUGH THE UNSEASONABLY SUPPRESSED FLOW.
CISCO
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