081
FXUS02 KWBC 101930
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
330 PM EDT MON AUG 10 2009
VALID 12Z THU AUG 13 2009 - 12Z MON AUG 17 2009
GENERALLY GOOD AGREEMENT EXISTS AMONG GLOBAL ENSEMBLE MEANS AND
LAGGED-AVERAGE FORECASTS OF THE GFS/ECMWF IN THE MEDIUM RANGE
PERIOD REGARDING A SLOWLY AMPLIFYING LONGWAVE TROUGH IN THE
WEST...A COMPENSATING UPPER RIDGE EXPANDING BETWEEN WEST TEXAS AND
THE UPPER MIDWEST...
AND A LINGERING BUT SLOWLY WEAKENING TROUGH FROM LOUISIANA INTO
THE GREAT LAKES. HOWEVER...
THE FLOW REMAINS QUITE PROGRESSIVE ACROSS CANADA AND THUS IS THE
MAIN SOURCE OF UNCERTAINTY IN TODAY'S FORECAST. AS MIGHT BE
EXPECTED WITH A SPLIT-FLOW REGIME...DETERMINISTIC/ENSEMBLE SPREAD
IS ABOVE AVERAGE...WITH CAUTIOUS USE OF DETERMINISTIC GUIDANCE
RECOMMENDED ESPECIALLY BEYOND DAY 5.
REGARDING THE DAY-TO-DAY DETAILS...SIGNIFICANT DIFFERENCES EXIST
IN THE EAST DAYS 3-5...WITH SEVERAL MODELS INCLUDING THE 06-12Z
GFS AND 12Z ECMWF/CANADIAN TRENDING SLOWER/STRONGER WITH A SURFACE
LOW OFF THE NORTHEAST COAST DAY 3 BEFORE DEEPENING FURTHER OVER
THE NORTHEAST ATLANTIC. SREF/GEFS/ECMWF ENSEMBLE MEMBERS AND OTHER
DETERMINISTIC GUIDANCE SHOW CONSIDERABLE SPREAD WITH THE
SPEED/INTENSITY OF THIS SYSTEM WITH IT TOO EARLY TO CALL REGARDING
THE EXACT SOLUTION. THUS...PREFER A CONSENSUS APPROACH INVOLVING A
BLEND OF NUMEROUS SOLUTIONS UNTIL THE SOLUTION ENVELOPE SHRINKS.
THE NEXT REGION OF UNCERTAINTY IS WESTERN CANADA...WITH THE 12Z
GFS SIMILAR TO ITS 06Z RUN IN MOVING TOWARD THE FAST SIDE OF THE
GUIDANCE ENVELOPE WITH EASTWARD EJECTION OF AN UPPER LOW OVER
ALASKA DAYS 3/4. WHILE IT MAY SEEM TOO FAR NORTH TO HAVE A DIRECT
BEARING ON THE CONUS WEATHER...THE FASTER GFS SOLUTION ACTUALLY
ALLOWS FOR A SIGNIFICANT PORTION OF THE AMPLIFYING TROUGH ALONG
THE WEST COAST TO ALSO EJECT EASTWARD RESULTING IN A VERY
DIFFERENT SURFACE PRESSURE PATTERN ACROSS PARTS OF THE NORTHERN
PLAINS/UPPER MIDWEST DAYS 5/6 THAN DEPICTED BY THE MAJORITY OF
OTHER GUIDANCE...ESPECIALLY GLOBAL MODEL ENSEMBLE MEANS WHICH
AVERAGE 12-24 HRS SLOWER AND WEAKER.
THE PREFERRED LONGWAVE/SHORTWAVE PATTERN...WHICH IS CLOSEST TO THE
00Z ECMWF DAYS 3/4...AND A 50/50 BLEND OF THE 00Z ECMWF/ECMWF
ENSEMBLE MEAN DAYS 5-7...SHOULD FAVOR TWO MAIN AXES OF
RAINFALL...WITH ONE AXIS STRETCHING ALONG A WEAKENING FRONTAL
BOUNDARY ACROSS THE SOUTHEAST WHERE THE DEEPEST AND MOIST ABUNDANT
WATER VAPOR WILL RESIDE...AND ANOTHER LIGHTER AXIS DEVELOPING NEAR
A SLOW MOVING FRONTAL BOUNDARY ACROSS THE NORTHERN PLAINS. THE
NEXT MOST NOTICEABLE IMPACT WILL BE THE WARMING TREND EXPANDING
FROM WEST TEXAS INTO THE UPPER MIDWEST BENEATH THE BUILDING UPPER
RIDGE AND JUST AHEAD OF THE SLOWLY-SINKING COLD FRONT TO THE
NORTH.
JAMES
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