EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 244 AM EDT SAT SEP 27 2014 VALID 12Z TUE SEP 30 2014 - 12Z SAT OCT 04 2014 ...OVERVIEW... AN AMPLIFIED MEAN TROUGH WILL GRADUALLY PROGRESS FROM THE W-CNTRL STATES TO NEAR THE MS RIVER VLY BY LATE IN THE FCST WHILE UPSTREAM RIDGING BUILDS OVER THE WEST COAST. COMPARED TO YDAY GUIDANCE APPEARS TO BE CONVERGING FOR SOME OF THE DETAILS WITHIN THE MEAN TROUGH. THE UPSTREAM RIDGE HAS TRENDED STRONGER IN MOST GUIDANCE. MEANWHILE WITH SPREAD ALREADY SIGNIFICANT IN THE SHORT RANGE... CONFIDENCE REMAINS BELOW AVG WITH THE EXACT EVOLUTION OF A DIFFUSE TROUGH NEAR THE EAST COAST AND WITH THE ASSOC WRN ATLC SFC EVOLUTION. ...GUIDANCE EVALUATION/PREFERENCES... MODEL/ENSEMBLE SOLNS DISPLAY IMPROVED CLUSTERING FOR THE MOST IMPORTANT PIECES OF ENERGY COMPRISING THE DOMINANT MEAN TROUGH OF INTEREST. THESE INCLUDE A LEADING UPR LOW OR TROUGH EJECTING THROUGH THE NRN PLAINS TUE-TUE NIGHT... UPSTREAM ENERGY OVER THE NWRN CONUS/WRN CANADA AS OF EARLY TUE... AND SEWD FLOWING ENERGY ORIGINATING FROM NEAR THE SERN COAST OF ALASKA. THE MIDDLE FEATURE HAD BEEN THE MOST CONTENTIOUS PART OF THE FCST. TRENDS OVER THE PAST DAY HAVE YIELDED A TUE-THU COMPROMISE BETWEEN PREVIOUSLY MORE AMPLIFIED ECMWF/EC ENSEMBLE FCSTS AND RELATIVELY FLAT GFS/GEFS SOLNS. LATEST INDICATIONS FROM OPERATIONAL RUNS ARE THAT THE ENERGY NEAR THE SERN COAST OF ALASKA MAY ENCOURAGE SOME SEPARATION OF FLOW ONCE IT REACHES THE CORE OF THE MEAN TROUGH BY THE LATTER HALF OF THE PERIOD. THE MULTIPLE PIECES OF ENERGY CONTAINED WITHIN THE MEAN TROUGH WILL PROMOTE DEVELOPMENT OF AT LEAST A COUPLE WELL DEFINED WAVES ALONG THE LEADING SFC FRONT. DUE TO BEING A LITTLE WEAKER WITH THE UPSTREAM RIDGE ALOFT... THE 12Z GFS/CMC WERE ON THE FAST SIDE OF THE SPREAD WITH LEADING HGT FALLS/SFC FRONT ACROSS THE ERN HALF OF THE CONUS. THE 18Z/00Z GFS RUNS MAY STILL BE A LITTLE FAST BUT COMPARE BETTER TO THE ENSEMBLE MEANS THAN THE 12Z VERSION. CONFIDENCE ALONG THE EAST COAST REMAINS MUCH LOWER THAN DESIRED AS GUIDANCE CONTINUES TO HAVE DIFFICULTY IN RESOLVING IMPORTANT SMALLER SCALE DETAILS... WITH SIGNIFICANT SPREAD ALREADY APPEARING IN THE SHORT RANGE TIME FRAME. ALSO 12Z/18Z/00Z GFS RUNS ARE QUITE VARIED BETWEEN OPEN/PROGRESSIVE AND CLOSED UPR SOLNS WITH CORRESPONDING DIFFS AT THE SFC OVER THE WRN ATLC. AS A WHOLE 12Z/18Z GUIDANCE REPRESENTS A SLIGHTLY MORE SUPPRESSED SFC EVOLUTION FROM THE OLD 00Z CLUSTER... SO ADJUSTED A LITTLE IN THAT DIRECTION. OTHERWISE THE PREFERENCE TO BE CONSERVATIVE WITH ADJUSTMENTS UNTIL GREATER CONFIDENCE CAN BE GAINED LEADS TO EMPHASIZING AN 18Z GFS/12Z ECMWF COMPROMISE. 00Z GUIDANCE CLUSTERING REMAINS POOR. THE OVERALL FCST INCLUDES THE 18Z GFS/00Z ECMWF AND 18Z GEFS/12Z ECMWF MEANS... WEIGHTED MOSTLY TOWARD THE OPERATIONAL RUNS EARLY AND MORE TO THE ENSEMBLE MEANS LATE. THIS SOLN PROVIDES AS MUCH CONTINUITY AS COULD BE HOPED FOR NEAR THE EAST COAST WHILE SOME TIMING/DETAIL DIFFS IN THE OPERATIONAL RUNS UPSTREAM MID-LATE PERIOD AVERAGE OUT FAIRLY WELL WHEN INCLUDING SOME WEIGHTING OF THE MEANS. ...SENSIBLE WEATHER HIGHLIGHTS... WAVY FRONT PROGRESSING FROM THE HIGH PLAINS INTO E-CNTRL STATES WILL SUPPORT AREAS OF ENHANCED RNFL DURING THE PERIOD. NEAR THE START OF THE FCST AN AGREED UPON STRONG SFC WAVE AND UPR DYNAMICS WILL FOCUS HVY RNFL OVER PARTS OF THE NRN PLAINS. BY MID-LATE WEEK EXPECT THE FRONT TO INTERACT WITH GULF INFLOW... LIKELY INCREASING COVERAGE OF MDT-HVY RNFL OVER THE ERN PLAINS/MS VLY AND PSBLY INTO THE WRN GRTLKS. POTENTIAL EXISTS FOR SOMEWHAT HEAVIER ACTIVITY ALONG THE SRN HALF OF THE FRONT THAN FCST BY SOME GUIDANCE. ANOTHER RELATIVE MAXIMUM MAY OCCUR NEAR THE GULF COAST. BEHIND THE FRONT AND UNDER THE CORE OF THE UPR TROUGH DAYTIME TEMPS SHOULD BE WELL BELOW NORMAL WITH SOME MINUS 10-15F ANOMALIES FROM PARTS OF THE INTERIOR WEST EXTENDING NEWD INTO AND BEYOND THE NRN PLAINS. BY LATE IN THE WEEK/EARLY NEXT WEEKEND THE WEST WILL SEE A WARMER/DRIER TREND AS RIDGING ALOFT BUILDS OVER THE WEST COAST. BTWN THE WAVY FRONT AND THE APLCHNS EXPECT ABOVE NORMAL TEMPS WITH GENERALLY GREATER ANOMALIES FOR MINS. ALONG THE EAST COAST GUIDANCE DIFFS SFC/ALOFT CONTINUE TO YIELD LOW CONFIDENCE IN THE COVERAGE/INTENSITY OF RNFL DURING THE PERIOD. RAUSCH