EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 141 AM EST TUE DEC 09 2014 VALID 12Z FRI DEC 12 2014 - 12Z TUE DEC 16 2014 ...OVERVIEW... IN SPITE OF FAIRLY HIGH AMPLITUDE... THERE SHOULD BE SOME DEGREE OF PROGRESSION WITH A WEST COAST TROUGH CONTINUING INTO/ACROSS THE CONUS AND A TRAILING TROUGH LIKELY TO REACH THE WEST COAST LATE IN THE PERIOD. THIS PROGRESSION SHOULD EVENTUALLY HELP TO PUSH AWAY THE DEEP UPR TROUGH/EMBEDDED LOW NEAR THE EAST COAST AS OF FRI. THE TWO WRN TROUGHS EXHIBIT SOME TIMING/DETAIL SPREAD WHILE ONGOING DIFFS WITH THE INITIAL NEW ENGLAND UPR LOW WILL HAVE A MORE LOCALIZED IMPACT ON THE FCST. ...GUIDANCE EVALUATION/PREFERENCES... THROUGH ABOUT DAY 4 SAT THE PAST 24 HRS OF GUIDANCE SHOW BETTER CLUSTERING THAN IN PREVIOUS DAYS FOR THE TROUGH TRACKING EWD FROM THE WEST COAST. FROM SUN ONWARD IS WHEN A COMBINATION OF DIFFS REGARDING TIMING AND THE DISTRIBUTION/EVOLUTION OF ENERGY WITHIN THE OVERALL TROUGH EMERGE. THROUGH THE 12Z-18Z CYCLES THE COMPARISON OF GUIDANCE TO EACH OTHER AND EARLIER RUNS SUGGESTED A SLIGHTLY WEAKER TENDENCY WITH PSBL CLOSED LOW FORMATION OVER THE SRN PLAINS AROUND SUN-MON WITH MOST MODEL/ENSEMBLE MEAN RUNS WITHIN TYPICAL ERROR RANGES FOR TIMING. EVEN WITH THIS TREND GEFS MEAN RUNS WERE GIVEN LESS CONSIDERATION AS THEIR UPR TROUGHS WERE SOMEWHAT LESS SEPARATED THAN MOST OTHER SOLNS. THROUGH DAY 6 MON A BLEND OF THE 12Z ECMWF MEAN AND 12Z GFS/ECMWF WERE PREFERRED. SOME OF THE OPERATIONAL MODEL DETAILS OVER THE SRN AND/OR NRN PLAINS WERE QUESTIONABLE BUT ON AVG ENHANCED THE ECMWF MEAN SOLN. AMONG NEW 00Z GUIDANCE THE GFS/GEFS MEAN STRAY TO THE FAST/NEWD SIDE OF THE SPREAD WITH THE CORE OF THE UPR TROUGH AFTER LATE SAT. BY LATE SUN THE 00Z UKMET AND PARALLEL GFS /GFSP/ SHOW AN UPR LOW NEAR THE 12Z ECMWF MEAN POSN AND THE 00Z CMC IS SLIGHTLY SLOWER. THE GFSP DOES STRAY TOWARD THE OPERATIONAL RUN AFTER LATE SUN THOUGH. THE BIGGEST QUESTION MARK MID-LATE PERIOD WILL BE THE DEGREE OF STREAM INTERACTION WHICH MAY TAKE A WHILE TO RESOLVE. AMPLITUDE/SHARPNESS OF UPSTREAM FLOW OFFERS POTENTIAL FOR SOME INTERACTION WHICH MAY EXPLAIN THE SLIGHTLY FASTER TRENDS VS YDAY BY LATE IN THE PERIOD. BY DAY 7 TUE LEANING AT LEAST 2/3 TOWARD THE SLOWER 12Z ECMWF MEAN RELATIVE TO THE 18Z GEFS MEAN APPEARED REASONABLE. LIKEWISE TO VARYING DEGREES THE GFS/GEFS SOLNS TEND TO RUN A LITTLE FASTER WITH THE TROUGH REACHING THE WEST COAST EARLY NEXT WEEK. SHRTWV DETAILS WITHIN THE OVERALL TROUGH BECOME AMBIGUOUS... FAVORING A BLENDED SOLN... AND TIMING AGAIN MORE BUT NOT COMPLETELY TO THE SLOWER 12Z ECMWF MEAN VS RECENT GEFS MEANS. ECMWF MEAN RUNS HAVE BEEN NUDGING THE TROUGH A LITTLE FASTER OVER THE LAST 1-2 DAYS WHICH COULD BE FEASIBLE IN LIGHT OF FAST NATURE OF UPSTREAM FLOW. THE FAVORED 12Z GFS/ECMWF/EC MEAN BLEND THROUGH DAY 6 MON ALSO PROVIDES A GOOD INTERMEDIATE SOLN FOR THE UPR LOW GRADUALLY DEPARTING FROM THE NORTHEAST... WITH RECENT ECMWF RUNS TENDING TO BE A LITTLE ON THE SWRN SIDE OF THE SPREAD AND GFS RUNS ON THE ERN SIDE. THE 00Z GEFS MEAN/GFSP ARE REASONABLY CLOSE TO THE ECMWF MEAN THROUGH SUN WHILE THE 00Z GFS/CMC ARE FARTHER N/E. THE BLOCKY NATURE OF THE ATLC PATTERN SUPPORTS LEANING AWAY FROM THE FASTER SIDE OF THE SPREAD. ...SENSIBLE WEATHER HIGHLIGHTS... ENHANCED PCPN OVER CALIFORNIA LATE THIS WEEK WILL RAPIDLY TAPER OFF AS THE SUPPORTING UPR TROUGH MOVES INLAND. THIS TROUGH WILL SPREAD AREAS OF LESS INTENSE PCPN THROUGH THE WEST INTO THE WEEKEND... AND THEN INCREASE RNFL OVER THE SRN PLAINS BY SUN-MON WHILE LOW LVL FLOW FROM THE WRN GULF OF MEXICO INTERACTS WITH A SFC FRONT. POTENTIAL FOR AN UPR LOW TO CLOSE OFF WITHIN THE TROUGH MAY FURTHER ENHANCE THE ACTIVITY FROM THE PLAINS INTO MS VLY. UPSTREAM ERN PAC TROUGH WILL BRING ANOTHER AREA OF MSTR TO THE WEST COAST BY EARLY NEXT WEEK. RNFL/HIGH ELEV SNOW SHOULD BE LIGHTER THAN WITH THE PRECEDING TROUGH ALOFT BUT SOME LOCALIZED ENHANCEMENT IS PSBL DEPENDING ON PRECISE EVOLUTION OF ENERGY WITHIN THE TROUGH. MEANWHILE PCPN OVER THE NORTHEAST SHOULD BE FAIRLY LGT BY THE MEDIUM RANGE TIME FRAME THOUGH IT MAY PERSIST AT SOME LOCATIONS FOR A WHILE UNTIL THE SUPPORTING UPR LOW FINALLY TRACKS INTO THE ATLC. CONTINUE TO EXPECT THE MOST EXTREME TEMPS RELATIVE TO NORMAL FROM THE ROCKIES INTO THE PLAINS AND UPR MIDWEST FRI-SUN... WITH SOME DAILY RECORDS PSBL IN PARTS OF THE PLAINS AS ANOMALIES REACH PLUS 20-30F AT SOME LOCATIONS. RAUSCH