072
FXUS02 KWBC 141942
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
341 PM EDT FRI AUG 14 2009
VALID 12Z MON AUG 17 2009 - 12Z FRI AUG 21 2009
MEDIUM RANGE GUIDANCE IS FAIRLY AGREEABLE UPON MODERATE
PROGRESSION OF A MEAN TROF ACROSS THE NRN/NERN PAC DURING THE
PERIOD... WITH SOME SHARPENING OF A DOWNSTREAM RIDGE FCST TO BUILD
INTO AND GRADUALLY MOVE ACROSS WRN NOAM. THIS NERN PAC/WRN NOAM
EVOLUTION IS EXPECTED TO SUPPORT RELOADING OF A CNTRL/E-CNTRL NOAM
MEAN TROF WHOSE AXIS SHOULD EDGE A LITTLE EWD WITH TIME...
EJECTING INITIAL LEADING ENERGY FCST TO BE OVER SRN CANADA/NRN
TIER CONUS AS OF EARLY MON. THE GFS MAY BE A LITTLE DEEP AND/OR
FAST WITH THE LEADING SYSTEM AT THAT TIME BASED ON THE ARRAY OF
OTHER MODEL/ENSEMBLE SOLNS... WHILE UPSTREAM THE GFS BECOMES
FLATTER/FASTER THAN CONSENSUS THAT INCLUDES 00Z GEFS ENSEMBLES
WITH NERN PAC/WRN CANADA FLOW BY TUE-WED. BY THE LATTER HALF OF
THE PERIOD OVER THE CONUS THE CONSEQUENCES OF THIS DIFFERENCE IN
THE GFS ARE GREATER SEPARATION OF PLAINS ENERGY FROM NRN STREAM
FLOW AND FASTER PROGRESSION ACROSS THE NERN CONUS/ERN CANADA. GFS
RUNS TEND TO ADJUST SLOWER WHEN LEADING THE SOLN SPREAD IN A
REASONABLY AMPLIFIED PATTERN... SO WILL GENERALLY DISCOUNT THE 00Z
GFS FOR THE FCST TODAY. AMONG REMAINING MODELS THERE ARE STILL
MEANINGFUL DETAIL DIFFS AND RUN TO RUN CONTINUITY CHANGES WITH THE
ENERGY THAT IS FCST TO RELOAD THE E-CNTRL NOAM MEAN TROF...
FAVORING INCREASED WEIGHTING TOWARD THE SIMILAR ENSEMBLE MEANS BY
DAYS 5-7 WED-FRI. THE 00Z GEFS MEAN IS A VIABLE COMPONENT OF THE
PREFERRED ECMWF AND ENSEMBLE MEAN BLEND BUT WAS NOT ACCESSIBLE FOR
GENERATION OF MANUAL PROGS. THEREFORE THE EARLY PRELIM FCST USES
A BLEND OF THE 00Z ECMWF AND 12Z/13 ECMWF ENSEMBLE MEAN... WITH A
70 PERCENT OPERATIONAL WEIGHTING FOR DAYS 3-4 MON-TUE FOLLOWED BY
A 60 PERCENT ENSEMBLE WEIGHTING FOR DAYS 5-7 WED-FRI.
..UPDATE
CONTINUED WITH AN IDENTICAL MODEL BLEND TO THAT USED FOR THE EARLY
PRELIM PACKAGE. THEREFORE...NO CHANGES ARE PLANNED FOR THIS
UPDATE. THE 06Z GFS SHOWS SIMILAR LONGWAVE/SHORTWAVE TROUGH
EVOLUTION OVER CENTRAL NOAM THROUGH ABOUT DAY 5 COMPARED TO THE
00Z GFS... WHEN THE 06Z RUN BEGINS TO DEVELOP A DEEPER CENTRAL
CONUS TROUGH BUT WITH SIMILAR POSITION THROUGH DAY 7.
HOWEVER...GFS/ECMWF/CANADIAN ENSEMBLE SPAGHETTI DIAGRAMS SHOW
CONSIDERABLE SPREAD IN BOTH POSITION AND AMPLITUDE OF THE
TROUGH...WHILE THEIR RESPECTIVE ENSEMBLE MEANS ARE MUCH CLOSER TO
EACH OTHER AND HAVE DISPLAYED LESS RUN-TO-RUN VARIABILITY THAN THE
OPERATIONAL MEMBERS. THEREFORE...UNTIL MODEL CONSENSUS
IMPROVES...RECOMMEND A SOLUTION CLOSEST TO AN ENSEMBLE MEAN FROM
DAY 5 AND BEYOND...WITH A SLIGHT PREFERENCE FOR THE ECMWF ENSEMBLE
MEAN DUE TO BETTER GRID-SCALE RESOLUTION AND GREATER NUMBER OF
ENSEMBLE MEMBERS. THIS APPROACH RESULTS IN A CENTRAL CONUS TROUGH
THAT IS ABOUT 24 HRS SLOWER AND LESS AMPLIFIED THAN THE
GFS/CANADIAN ENSEMBLE MEANS.
..FINAL
THE FINAL MEDIUM RANGE PROGS WERE NUDGED ONE THIRD TOWARD THE 12Z
CANADIAN...WHICH BEST REPRESENTS THE 12Z DETERMINISTIC MEAN...AND
RESULTS IN A MODEST ADJUSTMENT TOWARD A MORE AMPLIFIED TROUGH AND
STRONGER COLD FRONT MOVING THROUGH THE NORTHERN PLAINS/UPPER
MIDWEST/GREAT LAKES DAYS 6/7...WHICH IS PREFERRED UNTIL THE MODELS
SHOW BETTER CONVERGENCE ON THE TROUGH'S SOLUTION. OTHERWISE...THE
MODEL PREFERENCE REMAINS THE SAME...A MIXTURE OF THE LAST 2 RUNS
OF THE ECMWF AND 12Z CANADIAN DAYS 3/4...AND 2/3RDS 00Z ECMWF
ENSEMBLE MEAN TO 1/3 12Z CANADIAN DAYS 5-7. WHILE THE COLD FRONT
ACCOMPANYING THE SLOWLY EVOLVING CENTRAL CONUS TROUGH WILL BE THE
PRIMARY FOCUS FOR RAINFALL THROUGH THE PERIOD...WITH SOME RAIN
HEAVY...THE TROPICS ARE ALSO EXPECTED TO REMAIN QUITE ACTIVE.
EARLIER COORDINATION WITH THE TPC RESULTED IN DEPICTING THE
REMNANTS OF T.D NUMBER TWO REACHING FLORIDA DAYS 6/7...WHICH IS
FAIRLY CLOSE IN POSITION AND INTENSITY TO THE 00Z OPERATIONAL
ECMWF AND ECMWF ENSEMBLE MEAN. HOWEVER THE MODELS REMAIN STRONGER
DEPICTING THE TROPICAL WAVE JUST OFF THE COAST OF AFRICA...WITH A
TRACK FORECAST CLOSEST TO THE 12Z CANADIAN PREFERRED BY DAYS 6/7.
IT SHOULD BE NOTED THAT DETERMINISTIC/ENSEMBLE SPREAD REMAINS VERY
LARGE...WITH EVOLUTION OF THE CENTRAL CONUS TROUGH HAVING A DIRECT
INFLUENCE ON ANY TROPICAL WAVES THAT MOVES INTO THE WESTERN
ATLANTIC TOWARD THE END OF THE MEDIUM RANGE PERIOD. SEE TPC
DISCUSSIONS/ADVISORIES FOR MORE INFORMATION ON THIS TROPICAL
SYSTEMS.
RAUSCH/JAMES
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