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