978
FXUS02 KWBC 181746
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
141 PM EDT FRI SEP 18 2009
VALID 12Z MON SEP 21 2009 - 12Z FRI SEP 25 2009
ENERGY DIVING THROUGH THE NORTHERN PLAINS ON DAY 3 IS ON TRACK TO
DEVELOP INTO A LARGE CLOSED LOW OVER THE CENTER OF THE NATION.
OPERATIONAL MEDIUM RANGE GUIDANCE IS IN REASONABLE AGREEMENT THAT
THIS CIRCULATION WILL REMAIN NEARLY STATIONARY FOR SEVERAL DAYS AS
IT WILL BE CUT OFF FROM THE WESTERLIES WELL TO THE NORTH ACROSS
CANADA. MEANWHILE...AN AMPLIFIED RIDGE WILL PERSIST ACROSS THE
WESTERN U.S. AND ANOTHER RIDGE WILL DEVELOP OVER THE WESTERN
ATLANTIC.
LARGELY FOLLOWED THE SFC PATTERN OF THE 00Z/ECMWF AND GLOBAL ENS
MEAN ACROSS CANADA...WHICH FEATURE WEAKER SFC SYSTEMS THAN THE GFS
AND EARLIER ECMWF RUNS. THIS WOULD KEEP MOST OF THE SFC
BOUNDARIES NORTH OF THE BORDER UNTIL DAYS 6-7 WHEN A COLD FRONT
SLIDES INTO THE NORTHERN PLAINS...AND A SEPARATE FRONT CROSSES THE
NORTHEAST.
MODELS HAVE SETTLED DOWN CONSIDERABLY WITH A REASONABLY GOOD
CONSENSUS ON A SIGNIFICANT PLAINS CUTOFF LOW. HPC UPDATED MORNING
PRELIM PREFERENCE IS TOWARDS A HEAVIER WEIGHTING TOWARDS OP MODEL
CONSENSUS INSTEAD OF ENSEMBLES WHICH GIVES A GREATER EMPHASIS ON
THE INTENSITY OF THE MID LEVEL CUTOFF LOW. A 50/50 BLEND OF 00Z OP
MODEL RUNS OF GFS AND ECMWF SETTLES AMPLITUDE DIFFERENCES IN THE
MID LEVEL WESTERLY FLOW OVER CANADA ND THE NRN TIER AND LOCATIONAL
DIFFERENCES OF THE PLAINS CUTOFF LOW.
DIFFERENCES REMAIN IN THE WESTERLY FLOW ACROSS CANADA AS 12Z GFS
TAKES A STRONG SHORTWAVE INTO B.C. RERACHING ALBERTA DAY 7 FASTER
THAN ITS PRIOR RUN WITH NO OTHER MODEL CONSENSUS. THESE
DIFFERENCES WERE PARTIALLY RESOLVED USING A BLEND OF MODELS AS AN
ENSEMBLE FOR MORNING PRELIMS AND WILL REMAIN THE SAME FOR AFTN
FINALS.
DEVELOPING STRONG MID LEVEL CUTOFF LOW IN THE PLAINS WILL BRING A
WIDESPREAD AREA OF CLOUDY COOL CONDITIONS WITH TEMPS RUNNING 8 TO
16 DEGREES BELOW NORMAL ESPECIALLY UNDER THE MID LEVE CENTER THIS
PERIOD WHILE WESTWARD UNDER THE MID LEVEL RIDGE OVER THE WEST
TEMPS WILL BE WELL ABOVE NORMAL WITH SOME PLUS 20 DEGREES
ANOMALIES IN THE NRN INTERMTN REGION. THE EASTERN U.S WILL BE
UNDER THE OTHER END OF THE RIDGING WITH ABOVE AVG TEMPS DEVELOPING
AFTER MID WEEK. WET CONDS WILL CONT IN THE PLAINS ASSOCIATED WITH
THE MID LEVEL CENTER OF THE CUTOFF LOW AND ESPECIALLY EASTWARD
FROM THE GULF COASTAL STATES NORTH THRU THE TN VALLEY AND INTO THE
OH VALLEY.
HEDGE/ROSENSTEIN
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