684
FXUS02 KWBC 061933
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
332 PM EDT TUE OCT 06 2009
VALID 12Z FRI OCT 09 2009 - 12Z TUE OCT 13 2009
12Z DETERMINISTIC GUIDANCE AND THE GEFS PROVIDE REASONABLY GOOD
SUPPORT FOR THE PRIOR MEDIUM RANGE FORECAST THROUGH ABOUT DAY 5
(SUN) BEFORE MODEL SPREAD INCREASES SUFFICIENTLY TO DISCOURAGE
MAKING LARGE CHANGES AFTER DAY 5. THUS...THE PRIOR FORECAST WAS
ISSUED AS IS...WHICH EMPHASIZES A LARGE AMPLIFIED LONGWAVE TROUGH
OVER THE CENTRAL CONUS DAYS 3/4...WEIGHTED MOSTLY TOWARD THE 00Z
CANADIAN...WHICH THEN DEAMPLIFIES AND SHIFTS SLOWLY EASTWARD
TOWARD THE END OF THE PERIOD...WITH INCREASING WEIGHT GIVEN TO THE
00Z NAEFS BIAS CORRECTED MEAN BY DAYS 6/7 TO ACCOUNT FOR
INCREASING SOLUTION SPREAD. THE MAIN UNCERTAINTY INVOLVES THE
LARGE UPPER CYCLONE NEAR HUDSON BAY WHICH CREATES THE POTENTIAL
FOR PERTURBATIONS TO DEVELOP AND/OR ACCELERATE FROM THE DATA
SPARSE REGION OF NORTHERN CANADA. THE MODELS TYPICALLY HAVE A
DIFFICULT TIME ACCURATELY PREDICTING THE SHORTWAVE PATTERN
ACCOMPANYING THIS REGIME. THUS...OUR RATIONALE IS ONE OF
PERSISTENCE UNTIL BETTER MODEL AGREEMENT DEVELOPS. MOST OF THE
SIGNIFICANT PRECIPITATION THAT FALLS DAYS 3-7 WILL FALL IN
VICINITY OF THE SLOWING OR POSSIBLY STALLING FRONTAL BOUNDARY
EXTENDING FROM NEAR THE TEXAS COAST ACROSS NORTHERN
FLORIDA...WHILE ANOTHER AXIS OF LIGHTER AMOUNTS DEVELOPS MAINLY
FROM UPSLOPE FLOW OVER THE NORTHERN/CENTRAL ROCKIES. UNUSUALLY
COLD TEMPERATURES WILL LIKELY INFLUENCE PARTS OF THE NORTHERN
ROCKIES/NORTHERN PLAINS/WESTERN GREAT LAKES AS WELL DUE TO
PASSAGE OF ONE OR MORE REINFORCING COLD FRONTS OR SURFACE
TROUGHS...WITH TEMPERATURES AS MUCH AS 25 TO 35 DEGREES BELOW
NORMAL AT TIMES.
JAMES
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