A Noreaster for the Weekend?
Posted by JAC on 10/13/2009, 7:30 am








PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
459 AM EDT TUE OCT 13 2009

WITHIN THE ERN CONUS MEAN TROF... THE DEBATE CONTINUES AS TO
WHETHER SRN CANADA/NRN PLAINS FLOW AS OF EARLY DAY 3 FRI WILL
ULTIMATELY FORM A CLOSED LOW NEAR THE EAST COAST BY SUN.
OPERATIONAL MODELS ARE PROVIDING MIXED SIGNALS WITH THE LAST
COUPLE ECMWF RUNS KEEPING THE TROF MORE OPEN THAN SOME EARLIER
RUNS... WHILE THE 00Z GFS/UKMET/CMC ALL DEVELOP A CLOSED LOW.  THE
EXTREMELY FAR SWWD CANADIAN SOLN BY DAY 7 TUE APPEARS UNLIKELY BUT
OTHER SOLNS CANNOT BE FULLY DISCOUNTED YET SINCE THEY FIT WITHIN
THE MEAN FLOW.  A COMPROMISE AMONG MODEL AND SOMEWHAT WEAKER
ENSEMBLE MEAN GUIDANCE PROVIDES A REASONABLE STARTING POINT TO
ACCOUNT FCST UNCERTAINTY NEAR THE EAST COAST.  BEHIND THIS
FEATURE... THE GFS IS LIKELY TOO AMPLIFIED WITH FLOW REACHING THE
NERN CONUS BY NEXT TUE DUE TO ITS STRONG SOLN WITH THE TROF
CROSSING THE WEST.


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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A Noreaster for the Weekend? - JAC, 10/13/2009, 7:30 am
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