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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0639Z Aug 25, 2014)
 
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EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
239 AM EDT MON AUG 25 2014

VALID 12Z THU AUG 28 2014 - 12Z MON SEP 01 2014

...OVERVIEW AND PREFERENCES...

EXPECT MODERATELY PROGRESSIVE FLOW TO CARRY A SERIES OF TROUGHS
ACROSS SRN CANADA AND THE NRN CONUS WHILE A SLOWER MOVING DIFFUSE
SHRTWV SHOULD DRIFT EWD FROM THE CNTRL-SRN ROCKIES.  CURRENTLY THE
GREATEST UNCERTAINTY IN THE FCST INVOLVES AMPLITUDE OF THE SHRTWV
FCST TO REACH ERN CANADA AND NERN QUADRANT OF THE CONUS LATE IN
THE PERIOD... WITH ASSOC SPREAD FOR THE ASSOC SFC FRONT.  SOLNS
ARE COMING INTO BETTER AGREEMENT WITH THE TIMING OF THE DIFFUSE
CNTRL CONUS SHRTWV WHILE GIVEN THE TIME FRAME THERE IS DECENT
AGREEMENT FOR THE FAIRLY VIGOROUS TROUGH CROSSING THE NRN HALF OF
THE WEST SAT-MON AND LEADING SFC SYSTEM.

A CONSENSUS FCST CAPTURES THE MOST LIKELY ELEMENTS OF GUIDANCE
DAYS 3-4 THU-FRI.  AFTER THAT TIME THERE IS ENOUGH UNCERTAINTY IN
MODEL DETAILS... AND SPREAD OVER THE NORTHEAST AND VICINITY
LATE... TO FAVOR A BLEND CONSISTING OF A 60/20/20 WEIGHTING OF THE
12Z ECMWF MEAN/12Z NAEFS MEAN/18Z GEFS MEAN.


...GUIDANCE EVALUATION...

WITH THE MODERATELY AMPLIFIED TROUGH MOVING INTO THE WEST BY THE
WEEKEND... THE 12Z ECMWF RUN IS THE NOTABLE EXTREME WITH ITS
SWD/SEWD AMPLITUDE AFTER THU.  IT IS NOT AN OUTLIER RELATIVE TO
THE FULL ENSEMBLE SPREAD BUT IN THE MINORITY.  SO FAR THERE ARE NO
PRONOUNCED TRENDS WITH THIS TROUGH OR THE SFC SYSTEM EMERGING OVER
THE NRN PLAINS/SRN CANADA SUN-MON.  EMPHASIS ON THE ENSEMBLE MEANS
DOWNPLAYS UNCERTAIN DETAILS FROM INDIVIDUAL OPERATIONAL MODEL
RUNS.  A PERIPHERAL ITEM OF INTEREST WILL BE THE RELATIONSHIP
BETWEEN TIMING/AMPLITUDE OF THIS TROUGH AND EVOLUTION/TRACK OF ERN
PAC TROPICAL SYSTEMS KARINA AND MARIE... AFFECTING THE AMOUNT OF
UPR LVL MSTR/ENERGY THAT MAY ULTIMATELY BE PULLED INTO THE WEST. 
RECENT GFS RUNS AND THE 12Z CMC ARE THE MOST ENTHUSIASTIC IN THIS
REGARD.

GUIDANCE IS SHOWING BETTER CLUSTERING ALOFT FOR THE SHRTWV
DRIFTING EWD FROM THE CNTRL-SRN ROCKIES BUT MEANINGFUL DIFFS EXIST
AT THE SFC.  IN PARTICULAR GFS RUNS THROUGH THE 00Z CYCLE AS WELL
AS THE 00Z GEFS MEAN BRING A SFC WAVE ACROSS THE UPR MS VLY ON A
TRACK THAT IS WELL N/NE OF MOST OTHER SOLNS... PERHAPS UNDER THE
INFLUENCE OF NEARBY CONCENTRATED QPF.

MODELS AND ENSEMBLES DEVELOP CONSIDERABLE SPREAD WITH THE
TIMING/AMPLITUDE OF SHRTWV ENERGY FCST TO CROSS ERN CANADA AND THE
NERN CONUS LATE IN THE PERIOD.  BY DAY 7 MON RECENT ECMWF MEAN
RUNS HAVE BEEN CONSISTENT IN SHOWING AT LEAST MODERATE TROUGH
AMPLITUDE/FARTHER SWD SFC FRONT POSN AS OPPOSED TO SOLNS LIKE THE
12Z GEFS MEAN/CMC THAT SHOW A RIDGE ALOFT/MORE NWD SFC FRONT POSN.
 THE 00Z GEFS MEAN APPEARS TO SHOW SOME TRENDING IN THE ECMWF MEAN
DIRECTION.  OPERATIONAL GFS/ECMWF RUNS DO NOT PROVIDE ANY TREND
SIGNALS YET DUE TO ONLY RECENTLY STABILIZING WITH THE FCST FOR THE
CNTRL-SRN PLAINS SHRTWV.  THE PREFERRED 60/20/20 WEIGHTING OF THE
12Z ECMWF MEAN/12Z NAEFS MEAN/18Z GEFS MEAN BEST ACCOUNTS FOR
ECMWF MEAN CONTINUITY VERSUS OVERALL UNCERTAINTY IN FLOW
AMPLITUDE/SFC FRONT POSN.


...SENSIBLE WEATHER HIGHLIGHTS...

THE SHRTWV EMERGING FROM THE ROCKIES MAY GENERATE SOME AREAS OF
LOCALLY HVY RNFL FROM THE PLAINS EWD DUE IN PART TO THE RELATIVELY
SLOW PROGRESSION OF THE FEATURE.  CURRENTLY LOCATIONS FROM THE
CNTRL/N-CNTRL PLAINS INTO PORTIONS OF THE GRTLKS APPEAR TO HAVE
BEST POTENTIAL TO SEE THE HEAVIEST RNFL WITH THIS SYSTEM.  BY THE
LATTER HALF OF THE PERIOD SOME OF THIS MSTR MAY INTERACT WITH A
FRONT CROSSING THE NRN TIER.  MEANWHILE THE TROUGH CROSSING THE
WEST SAT-MON WILL BRING SOME PCPN TO NRN PARTS OF THE WEST WHILE
DISRUPTING MONSOONAL ACTIVITY TO THE S.  THE WEST WILL ALSO SEE A
COOLING TREND WITH SOME LOCATIONS IN AND NEAR THE NRN ROCKIES
PSBLY SEEING HIGH TEMPS AT LEAST 10F BELOW NORMAL BY NEXT MON. 
ALSO EXPECT A COOL DAY ON THU UNDER THE INITIAL CNTRL-SRN ROCKIES
TROUGH ALOFT.  TEMPS OVER THE SOUTH SHOULD BE NEAR TO ABOVE NORMAL
WITH OTHER LOCATIONS OVER THE CNTRL/EAST VARIABLE DEPENDING ON
FRONTAL PROGRESSION.

RAUSCH