757
FXUS02 KWBC 171809
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
208 PM EDT TUE JUN 17 2008
VALID 12Z FRI JUN 20 2008 - 12Z TUE JUN 24 2008
LATEST MODEL AND ENSEMBLE GUIDANCE CONTINUES TO DISPLAY ABOVE
AVERAGE AGREEMENT WITH LARGE SCALE FLOW DURING THE MEDIUM RANGE
PERIOD... FEATURING FAIRLY AMPLIFIED TROFS OVER THE ERN CONUS AS
WELL AS OVER THE ERN PACIFIC NEAR THE WEST COAST. BETWEEN THESE
TROFS THERE WILL BE A MODERATELY STRONG RIDGE WHOSE AXIS WILL
EXTEND NWD FROM NRN MEXICO/SW CONUS/SRN ROCKIES LATE THIS WEEK AND
MAY DRIFT EWD INTO THE PLAINS BY SUN-MON. MULTI-DAY MEANS
INDICATE A GRADUAL RELAXATION OF THE NEGATIVE HGT ANOMALIES ASSOC
WITH THE TWO TROFS. THERE IS SOME HINT OF RETROGRESSION OF THE
ERN PAC MEAN TROF TOWARD THE END OF THE PERIOD.
WITHIN THE AGREEABLE MEAN FLOW THERE ARE TYPICAL SHRTWV DETAIL
DIFFS THAT HAVE LOW PREDICTABILITY AND ARE SUFFICIENTLY CONTAINED
WITHIN THE OVERALL SPREAD OF ENSEMBLES FOR IT TO BE DIFFICULT TO
COMPLETELY DISCOUNT ANY PARTICULAR SOLN. HOWEVER TRENDS OVER THE
PAST WEEK APPEAR TO FAVOR THE LESS EXTREME SOLNS WITH RESPECT TO
HOW MUCH SHRTWV ENERGY REACHES THE PAC NW. THEREFORE PREFER TO
LEAN AWAY FROM THE 00Z/06Z GFS RUNS WHICH ARE ON THE MORE
AGGRESSIVE SIDE OF GUIDANCE WITH SHRTWV ENERGY CROSSING THE NW
DURING THE WEEKEND... AND THE UKMET WHICH BRINGS THE NERN PAC
CLOSED LOW FARTHER SEWD THAN OTHER GUIDANCE. BY DAYS 6-7 MON-TUE
THE GEFS/ECMWF ENSEMBLE MEANS HINT AT EJECTION OF A WEAK SHRTWV
INTO THE PAC NW/WRN CANADA BUT WITH WEST COAST HGTS RISING A
LITTLE LESS THAN THE 00Z ECMWF BY EARLY TUE.
MEANWHILE GUIDANCE IS FAIRLY AGREEABLE WITH THE ERN CONUS MEAN
TROF THRU DAY 5 SUN. THEN RECENT GFS/ECMWF RUNS AND ENSEMBLE
GUIDANCE SHOW A MODERATE DEGREE OF UNCERTAINTY OVER THE PRECISE
EVOLUTION OF UPSTREAM ENERGY THAT MAY TRY TO CONSOLIDATE OVER THE
GRTLKS/NORTHEAST BY THE LATTER HALF OF THE PERIOD. APPARENT
WEAKENING TREND IN LARGE SCALE FLOW BY LATER IN THE PERIOD
RECOMMENDS A SKEPTICAL VIEW OF THE 06Z GFS THAT SHOWS MORE WWD
AMPLIFICATION BY DAY 7 THAN FCST BY MODEL/ENSEMBLE CONSENSUS.
BASED ON PREFERENCE OVER THE ERN PAC/NWRN CONUS AND REASONABLE
CONSENSUS ELSEWHERE EARLY IN THE PERIOD... THE PRELIM FCST USES
THE 00Z ECMWF FOR DAYS 3-5 FRI-SUN. INCORPORATING A 50 PERCENT
BLEND OF THE 00Z GEFS MEAN WITH THE ECMWF... WITH MINOR
ADJUSTMENTS... FOR DAYS 6-7 MON-TUE ACCOUNTS WELL FOR INCREASINGLY
UNCERTAIN DETAIL DIFFS AT THAT TIME AND MAINTAINS A GOOD
INTERMEDIATE SOLN IN LIGHT OF STILL DECENT AGREEMENT FOR LARGE
SCALE FLOW.
12Z UPDATE... THE NEW GFS RUN HAS GENERALLY TRENDED AT LEAST
SOMEWHAT TOWARD PREFERRED CONSENSUS FROM THE ERN PAC THRU THE ERN
CONUS. THE 12Z GEFS MEAN IS FAIRLY CLOSE TO OTHER RECENT GEFS
MEANS. THE UKMET HAS ADJUSTED NWWD WITH THE NERN PAC CLOSED LOW
BY EARLY NEXT WEEK BUT IS STILL ON THE ERN SIDE OF OPERATIONAL
SOLNS... WHILE THE CANADIAN IS WELL WITHIN THE ENVELOPE OF
POTENTIAL SOLNS THRU DAY 6 MON. OVERALL ARRAY OF 12Z GUIDANCE
FAVORS STAYING CLOSE TO HPC CONTINUITY. THE FINAL FCST MAKES
MINOR TWEAKS TOWARD A TYPICALLY BETTER VERIFYING BLEND OF THE 00Z
ECMWF AND NEW 12Z MODELS DAYS 3-4 BUT OTHERWISE THERE ARE NO
CHANGES FROM THE PRELIM FCST.
REGIONAL HIGHLIGHTS...
..WEST/CENTRAL
PIECES OF ENERGY EJECTING FROM THE MEAN TROF ALOFT OVER THE ERN
PACIFIC MAY SUPPORT A FEW PERIODS OF CLOUDS AND PERHAPS SCT RNFL
ACROSS EXTREME NWRN AREAS. THE REMAINDER OF THE WEST SHOULD SEE
FAIR WEATHER WITH ABOVE NORMAL HIGH TEMPS THRU THE PERIOD DUE TO A
MODERATELY STRONG RIDGE ALOFT. SLIGHT EWD DRIFT OF THIS UPPER
RIDGE BY THE WEEKEND OR EARLY NEXT WEEK SHOULD SUPPORT A MODERATE
WARMING TREND OVER PORTIONS OF THE PLAINS AS WELL. IMPULSES
FLOWING AROUND THE PERIPHERY OF THE RIDGE MAY GENERATE AREAS OF
SHWRS/TSTMS TO THE E OF THE ROCKIES. AS INDICATED IN THE SPC DAYS
4-8 OUTLOOK ISSUED TUE... PREDICTABILITY IS TOO LOW AT THIS TIME
TO PINPOINT ANY PARTICULAR AREAS FAVORED FOR SEVERE ACTIVITY.
..EAST
PERSISTENT MEAN TROF ALOFT OVER THE EAST SHOULD SUPPORT UNSETTLED
WEATHER AND BELOW NORMAL TEMPS ON AVERAGE. SOME GRADUAL WEAKENING
OF THE UPPER TROF MAY LEAD TO SOME MODERATION IN TEMPS TOWARD MORE
NORMAL LEVELS EARLY NEXT WEEK. BEST ORGANIZATION AND INTENSITY OF
RNFL SHOULD BE ALONG A MEAN FRONTAL ZONE INITIALLY OVER THE SOUTH
AND GRADUALLY EXTENDING FARTHER NEWD ALONG THE EAST COAST... AND
OVER THE GRTLKS/NORTHEAST WHERE THE COMBINATION OF DAYTIME HEATING
AND COLD AIR ALOFT SHOULD ENHANCE ACTIVITY DURING THE
AFTERNOON/EVENING.
RAUSCH/ROTH
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