107
FXUS02 KWBC 271844
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
243 PM EDT SUN JUL 27 2008
VALID 12Z WED JUL 30 2008 - 12Z SUN AUG 03 2008
MEDIUM RANGE GUIDANCE IS IN GOOD AGREEMENT AT LARGER SCALES AND
DEPICTS AN ERN NORTH AMERICA TROUGH...ALONG WITH A DEEP VORTEX
OVER THE NE PACIFIC THAT PROGRESSES EWD INTO WESTERN CANADA WHILE
HANGING BACK ENERGY ACROSS/OFFSHORE THE PAC NW. IN ADDITION...A
RIDGE OVER THE SOUTHWEST AND PLAINS THIS WEEK EXPANDS EAST TO THE
MS VALLEY BY NEXT WEEKEND.
PREFER TO MAINTAIN GOOD CONTINUITY AND THE FINAL HPC PRELIM
GUIDANCE REMAINS PRIMARILY DERIVED FROM A BLEND OF THE 00 UTC
ECMWF AND 12 UTC GFS ENSEMBLE MEAN...WITH THE ECMWF COMPONENT
GRADUALLY DECREASING FROM 70% DAY 3/WED TO 30% DAY 7/NEXT SUN
CONSISTENT WITH LOWERING PREDICTABILITY OF SMALLER SCALE DETAILS.
HPC PRELIM PROGS INSTEAD USED THE 00 UTC GFS ENSEMBLE MEAN THAT
WAS IN GENERALLY LESS AMPLIFIED AT MID-HIGHER LATITUDES THAN THE
NEWER 12 UTC GFS ENSEMBLE MEAN. WE THINK IT MAY BE PRUDENT TO
TREND TOWARD AT LEAST A SLIGHTLY MORE AMPLIFIED SOLUTION LATE WEEK
INTO NEXT WEEKEND CONSIDERING FAVORABLE WAVELENGTH SPACING FROM
THE PACIFIC THROUGH NOAM AND STRONG TRENDS FROM THE 12 UTC GFS AND
CANADIAN...BUT NOT SO MUCH FROM THE 12 UTC UKMET. IT SEEMED
PREMATURE TO MAKE A RADICAL CONTINUITY CHANGE ON THE ORDER OF THE
VERY AMPLIFIED 12 UTC GFS CONSIDERING LATEST RUN TO RUN VARIANCE
ISSUES FOR THE ALREADY ISSUED FINAL HPC PROGS...BUT I JUST HAD A
QUICK PEEK AT THE NEW 12 UTC ECMWF THROUGH DAY 6 AND ITS MUCH MORE
AMPLIFIED THAN THE 00 UTC ECMWF SO THAT UPS THE ANTE.
OTHERWISE...WE DID STILL PLACE ADDITIONAL MANUALLY ADJUSTED MASS
FIELD AND QPF EMPHASIS ON THE MOIST REMAINS OF DOLLY THAT WE
ANTICIPATE MAY HANG TOGETHER AND BEST FOCUS CONVECTION FROM THE
MID-MS TO THE TN/OH VALLEYS WED/THU.
WHY DID WE CHOOSE THIS PARTICULAR DETERMINNISTIC BLEND OF GUIDANCE
IN THIS PATTERN? IN ADDITION TO THE NEW 12 UTC GFS CHANGES AS
DISCUSSED ABOVE...THE 00/06 UTC GFS RUNS ALSO OFFERED CONTINUITY
ISSUES AS THEY SHOWED LOWER SFC PRESSURES THAN OTHER MODELS FROM
THE GREAT LAKES TO THE NORTHEAST DURING THE WEEK SEEMINGLY AS A
RESULT OF FEEDBACK PROBLEMS. IN CONTRAST...THE 00 UTC ECMWF
SEEMED MORE REASONABLE AND CONSISTENT DURING THE WEEK...BUT THE
NEWER 12 UTC ECMWF CERTAINLY OFFERS A NEW PROBLEM. THE 00 UTC
ECMWF ALSO DID BECOMES A STRONG OUTLIER BY NEXT WEEKEND WITH
DEVELOPMENT OF AN ORGANIZED LOW ACROSS E-CENTRAL CANADA. THIS
ALONG WITH THE EVER PRESENT INCREASED UNCERTAINTY OVER TIME OPENS
THE DOOR FOR INCREASED INCORPORATION OF THE 12 UTC GFS ENSEMBLE
MEAN THAT APPEARS TO HAVE REASONABLE CONTINUITY AND MINIMIZES ANY
OVERLY DEVELOPED FEATURES NOTED ON SOME OF THE OPERATIONAL
GUIDANCE. THIS GFS MEAN ALSO SEEMS TO OFFER SLIGHTLY MORE HINT OF
DOLLY REMAINS THAN THE MORE SHEARED 00 UTC ECMWF ENSEMBLE MEAN.
EFFECTS FROM THE REMNANTS OF DOLLY AND CONTINUED CONVECTION
FARTHER DOWN OVER THE SERN US/FL ALONG WITH RENEWED UPSTREAM
MONSOONAL MOISTURE BACK INTO THE SW/SRN ROCKIES BY LATE WEEK/NEXT
WEEKEND SHOULD PROVE ACTIVE. ANOTHER DEFINED THREAT IN THIS
PATTERN SHOULD RESULT WITH APPROACH AND PASSAGE OF A SERIES OF
HARD TO TIME IMPULSES RIDING THE SRN EDGE OF THE WESTERLIES FROM
THE NRN TIER OF THE NW US/SW CANADA OVERTOP THE RIDGE. THESE
ENERGIES WILL SUBSEQUENTLY STRONGLY TEND TO DIG SEWD ACROSS THE
N-CENTRAL US INTO THE ERN US TROUGH POSITION AND ACT TO ENHANCE
THETA-E CONVERGENCE AND AFTERNOON THUNDERSTORMS AND OVERNIGHT MCS
ACTIVITY LOCALLY AS WAVES DEVELOP/TRACK ALONG A PERSISTENT AND
SLOW MOVING FRONTAL ZONE.
HEDGE/SCHICHTEL
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