959
FXUS02 KWBC 141909
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
308 PM EDT MON SEP 14 2009
VALID 12Z THU SEP 17 2009 - 12Z MON SEP 21 2009
PRELIMINARY UPDATE...
USED A BLEND OF THE 12Z/13 DETERMINISTIC ECMWF AND THE 00Z/14 EC
ENSEMBLE MEAN TO UPDATE THE PRELIMINARY FRONTS AND PRESSURES FOR
DAYS 3 THROUGH 7. THE LAST THREE RUNS OF THE ECMWF HAVE SLOWLY
BACKED OFF THE DEEP TROUGH OVER THE EAST DAYS 5 AND 6...WITH THE
LATEST RUN QUITE FLAT AND PROGRESSIVE WITH THE ENERGY ASSOCIATED
WITH THIS SYSTEM. THE TROUBLE IS...THE 00Z/14 EC MEAN STILL SHOWS
LOW HEIGHTS OVER THE EASTERN GREAT LAKES AND NORTHEAST AT THAT
TIME RANGE...AND THE 00Z/14 GEM GLOBAL SENDS A ROBUST TROUGH
ACROSS THE REGION. TO KEEP THE PHYSICS IN THE SAME FAMILY...OPTED
TO INCORPORATE THE OLDER ECMWF RUN...WHICH CLUSTERS WELL WITH THE
MOST RECENT EC MEAN...AND AFFORDS SYNOPTIC FIBER...SO TO
SPEAK...TO THE MANUAL BLEND (ANY INCORPORATION OF MEAN GUIDANCE
TENDS TO WASH OUT THE MASS FIELDS). THE 00Z/14 GFS IS
SUFFICIENTLY OUT OF SYNC WITH THE OTHER MODELS WITH THE EVOLUTION
OF THE ROSSBY WAVES IN THE MAIN BAND OF WESTERLIES TO BE OF
DUBIOUS USE THIS FORECAST.
FINAL...
NO CONSENSUS HAS EMERGED WITH THE ADVENT OF THE 12Z/14
GUIDANCE...PARTICULARLY FROM THE EASTERN UNITED STATES INTO THE
WESTERN ATLANTIC. THE NEW GFS IS OUT OF STEP WITH THE GEFS
MEAN...BOTH WITH THE TRACK OF THE CYCLONE OVER THE OPEN
OCEAN...AND THE COLD FRONT SWEEPING DOWN FROM EASTERN CANADA. THE
GEM GLOBAL...UKMET...AND NEW ECMWF ARE ALSO QUITE DIFFERENT WITH
THIS SYSTEM...THOUGH THE GEM GLOBAL AND ECMWF AT LEAST TRACK THE
LOW OVER ATLANTIC COMPARABLY THROUGH DAY 4...DESPITE AN ENORMOUS
DIFFERENCE IN DEPTH. MADE A COMPROMISE TOWARD THE 12Z/14 ECMWF
WITH THE PATTERN OVER THE EAST FOR THE FINAL ISSUANCE...BRINGING
HIGH PRESSURE MORE VIGOROUSLY DOWN THE EASTERN SLOPES OF THE
APPALACHIANS...AND SLOWING THE ATLANTIC LOW SOME. THE FORECAST
HAS LOW CONFIDENCE OVERALL.
CISCO
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