512
FXUS02 KWBC 071915
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
315 PM EDT MON SEP 07 2009
VALID 12Z THU SEP 10 2009 - 12Z MON SEP 14 2009
NUMERICAL GUIDANCE CONTINUES TO SHOW GREATER LONGWAVE FLOW
AMPLIFICATION DEVELOPING IN THE MEDIUM RANGE PERIOD...BUT WITH
SUFFICIENT MODEL DIFFERENCES AND RUN-TO-RUN VARIABILITY WITH THE
SHORT WAVELENGTH DETAILS...THAT CONFIDENCE IS LOW IN MAKING
DRAMATIC CHANGES FROM THE EARLIER PACKAGE. THE MAIN ISSUE INVOLVES
HOW THE MODELS HANDLE THE SPLIT-FLOW REGIME FORMING OVER SOUTHERN
CANADA AND THE SUBSEQUENT REX-BLOCK...IF THE GFS/ECMWF ARE TO
BELIEVED...NEAR THE U.S./CANADA BORDER BEGINNING DAYS 4/5.
GENERALLY PREFER THE HIGHEST SPATIALLY RESOLVING GUIDANCE FOR SUCH
REGIMES WHICH ARGUES FOR THE SLOWER/DEEPER SCENARIO OF THE ECMWF
VERSUS THE MORE PROGRESSIVE UKMET/CANADIAN. HOWEVER...THE NEW
ECMWF'S SOLUTION FOR ITS SURFACE LOW ALONG THE EAST COAST DAYS 3/4
APPEARS TO BE A CULMINATION OF A LARGE NUMBER OF SMALL-SCALE
INTERACTIONS AND PROCESSES WHICH HAVE AN UNUSUALLY LARGE INFLUENCE
ON ITS POSITION. THUS...
ITS MORE WESTWARD SOLUTION IS CONSIDERED QUESTIONABLE.
RATHER...PREFER TO STAY THE COURSE OF
THE PREVIOUSLY-PREFERRED MODEL BLEND WHICH IS FOR A COMBINATION OF
THE OLD OPERATIONAL ECMWF AND ITS ACCOMPANYING ENSEMBLE
MEAN...WITH GREATER EMPHASIS TOWARD THE ECMWF ENSEMBLE MEAN BY
DAYS 5-7.
THE RESULT IS A SLOW MOVING OCCLUDED AND EVENTUALLY WEAKENING
FRONTAL SYSTEM THROUGH THE CENTRAL CONUS DAYS 4-7...AND AN
ACCOMPANYING AXIS OF RAINFALL NEAR THE LEADING EDGE OF THE FRONTAL
BOUNDARY. WHILE THE AIRMASS BEHIND THE FRONT SHOULD BRING COOLER
CONDITIONS TO THE NORTHERN/CENTRAL PLAINS...THE DEVELOPING
BLOCKING PATTERN SHOULD PREVENT MOST OF THE UNMODIFIED CANADIAN
AIR FROM MOVING TOO FAR SOUTH. ELSEWHERE...THE PREFERRED SLOWER
DEPARTURE FOR THE EAST COAST SURFACE LOW SHOULD PROVIDE MODERATELY
TO STRONG ONSHORE FLOW NEAR AND NORTH OF THE LOW'S TRACK ALONG
WITH AN ACCOMPANYING AXIS OF MODERATE TO POSSIBLY HEAVY RAINFALL
ESPECIALLY ALONG THE IMMEDIATE COAST.
JAMES
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