393
FXUS02 KWBC 151844
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
243 PM EDT SAT AUG 15 2009
VALID 12Z TUE AUG 18 2009 - 12Z SAT AUG 22 2009
THE FINAL HPC PRELIM MEDIUM RANGE SURFACE FRONTS/PRESSURES AND 500
MB PROGS REMAIN PRIMARILY DERIVED FROM A 50-50 BLEND OF THE 00 UTC
GFS AND THE GENERALLY SUPPORTIVE ALBEIT LESS AMPLIFIED/DEFINED 00
UTC ECMWF ENSEMBLE MEAN. THIS BLENDED SOLUTION ADJUSTS OVERNIGHT
HPC NOAM NRN STREAM FLOW PREFFERENCE TO A SOLUTION THAT IS
SLIGHTLY LESS PROGRESSIVE DAYS 4-7. THIS CHANGE WAS BASED IN
CONSIDERATION OF POTENTIAL WRN NOAM RIDGE AMPLITUDE AND NEW TRENDS
FROM THE 00 UTC ECMWF ENSEMBLES THAT ALONG WITH MOST OTHER HIGHER
RESOLUTION 00 UTC DETERMINISTIC MODEL GUIDANCE THAT ARE NOW LESS
PROGRESSIVE WITH MID-CONTINENT MID-UPPER LEVEL TROFFING DAYS 4-7
THAN LOWER RESOLUTION GUIDANCE LIKE GFS ENSEMBLES OR TO A LESSER
EXTENT CANADIAN/NAEFS ENSEMBLES. WITH HPC CONTINUITY IN MIND
HOWEVER...OUR FINAL BLENDED SOLUTION DOES NOT EMBRACE AN EVEN
SLOWER SOLUTION PROVIDED BY A RUN-RUN INCONSISTENT 00 UTC
ECMWF...A SOLUTION NOT SUPPORTED BY TODAYS 12 UTC
ECMWF/UKMET/CANADIAN AND GFS THAT ARE ALL SLIGHTLY MORE
PROGRESSIVE THAN HPC GUIDANCE...BUT NOT AS FAST AS 00 UTC GFS
ENSEMBLES.
IN MORE DETAIL IN THIS PATTERN...THE 00 UTC ECMWF ENSEMBLES ARE
TRENDING LESS PROGRESSIVE ALONG WITH MOST 00 UTC DETERMINISTIC
MODELS IN THE NRN STREAM. HOWEVER..00 UTC GEFS/CANADIAN ENSEMBLES
AND YESTERDAYS 12 UTC ECMWF ENSEMBLE MEANS IN CONTRAST DID OFFER
BETTER THAN AVERAGE AGREEMENT WITH LARGE SCALE FLOW EVOLUTION
DURING THE MEDIUM RANGE PERIOD. THESE GUIDANCE SOURCES DISPLAY
STEADY EWD PROGRESSION OF A DAY 3 TUE NRN PAC TROF AND NERN PAC
RIDGE INTO CANADA...WITH RIDGING ALSO BUILDING INTO AND MOVING
ACROSS THE WRN CONUS. FLOW DOWNSTREAM FROM THE RIDGE IS FCST TO
AMPLIFY INTO THE EAST-CENTRAL CONUS WITH THE AXIS OF A MEAN TROF
REACHING THE ERN STATES BY DAY 7 SAT.
HOWEVER...COMPARED TO THOSE ENSEMBLE MEANS AND EVEN OTHER 00Z
MODELS THE 00 UTC GFS BECOMES SHARPER/STRONGER WITH ITS NERN
PAC/WRN CANADA FLOW BY DAY 4 WED... MAKING ITS DOWNSTREAM
AMPLITUDE EVOLUTION SOMEWHAT QUESTIONABLE. HOWEVER OTHER MODELS
STILL INDICATE THAT FLOW AMPLIFYING INTO THE PLAINS BY DAY 5 THU
SHOULD BE SLOWER AND MORE SEPARATED THAN INDICATED BY THE ENSEMBLE
MEANS. THE STRONG CLUSTERING OF 00Z MODELS IN PRINCIPLE ALONG
WITH GFS CONTINUITY RECOMMEND SOME WEIGHTING OF A MORE SEPARATED
SOLUTION. HOWEVER CONFIDENCE IS STILL MUTED BY CONSIDERABLE RUN
TO RUN ADJUSTMENTS IN THE ECMWF...WITH THE 00Z RUN BECOMING A SLOW
OUTLIER IN CONTRAST TO THE 12Z/14 RUN THAT KEPT THE CORE OF ITS
ENERGY FARTHER NWD AND MORE CLOSELY RESEMBLED THE ENSEMBLE MEANS.
AGAIN...AMID UNCERTAINTY BUT IN CONSIDERATION OF GUIDANCE AND FLOW
TRENDS HAVE LED TO THE MORE COMPROMISE AND SUPPORTIVE 50-50 HPC
BLEND OF THE 00 UTC GFS AND 00 UTC ECMWF ENSEMBLE MEAN. MEANWHILE
THOUGH...ONE SIGNIFCIANT MANUAL ADJUSTMENT TO THIS BLEND RESULTS
FROM INCREASED TROPICAL ACTIVITY OVER THE ATLANTIC BASIN. TO THAT
END..TS ANA AS PER TPC HAS BEEN INCORPORATED INTO THE HPC PROGS
OVER FLA AND THE SERN US DAYS 5-7.
RAUSCH/SCHICHTEL
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