EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 145 AM EST SUN DEC 04 2016 VALID 12Z WED DEC 07 2016 - 12Z SUN DEC 11 2016 ...OVERVIEW... AN AMPLIFIED/COLD UPR TROUGH WILL PROGRESS FROM THE W-CNTRL U.S. THROUGH THE EAST MID-LATE WEEK. UPSTREAM FLOW WILL BE FLATTER IN THE MEAN WITH SOME DEGREE OF CYCLONIC SHAPE PERSISTING OVER THE LOWER 48 AS SUGGESTED BY TELECONNECTIONS RELATIVE TO FAIRLY STRONG RIDGING FCST TO BE JUST NW OF ALASKA. MODELS/ENSEMBLES ARE STILL IN THE PROCESS OF RESOLVING IMPORTANT DETAILS OF THE CNTRL-ERN UPR TROUGH WHILE THE SMALLER SCALE OF UPSTREAM FEATURES FURTHER LOWERS FCST CONFIDENCE LATER IN THE PERIOD. ...MODEL EVALUATION/PREFERENCES... FROM DAY 3 WED INTO EARLY DAY 4 THU THE FCST STARTS WITH A BLEND OF HALF 12Z ECMWF AND THE REMAINDER 12Z-18Z GFS. THERE IS CONSIDERABLE MODEL/ENSEMBLE SPREAD WITH WEAK LOW PRES FCST OFF THE EAST COAST EARLY IN THE PERIOD. A STARTING POINT BTWN THE SLOWER/SWWD ECMWF AND SOMEWHAT FARTHER NEWD GFS PROVIDES THE BEST ACCOUNT FOR THE DISPARITY IN THE GUIDANCE. SOME INDIVIDUAL RUNS SUCH AS THE 12Z UKMET AND NEW 00Z CMC ARE QUITE PROGRESSIVE AND THE NEW 00Z GFS/GEFS MEAN ARE FASTER THAN THE 12Z/00Z RUNS SO FASTER TIMING MAY BE THE MORE PROBABLE ADJUSTMENT. THE INITIAL BLEND AVERAGES OUT TIMING DIFFS FOR THE DEEP CNTRL CONUS TROUGH... WITH RECENT FASTER TRENDS IN THE ECMWF BRINGING ITS SOLN CLOSER TO THE MIDDLE OF THE SPREAD. LEADING FRONTAL EVOLUTION OVER THE EAST IS SOMEWHAT COMPLEX WITH AN INITIAL BNDRY LIKELY DISSIPATING AFTER EARLY WED FOLLOWED BY A REFORMING FRONT REACHING A SIMILAR LOCATION EARLY THU. MEANWHILE RECENT GFS RUNS ARE ON THE ERN SIDE OF THE SPREAD WITH STRONG LOW PRES OFF THE PAC NW COAST AT LEAST INTO THU. CONSENSUS LIES MORE IN LINE WITH THE 12Z ECMWF WED-THU. THE REMAINDER OF THE FCST FAVORS A 75/25 WEIGHTING OF THE 18Z GEFS MEAN VS 12Z ECMWF MEAN. OVER THE EAST THERE ARE STILL SIGNIFICANT DIFFS IN HOW ENERGY EVOLVES ON THE ERN SIDE OF THE UPR TROUGH. GFS RUNS ARE AMONG THE DEEPEST WITH MID LVL EVOLUTION/SFC DEVELOPMENT FROM NEW ENGLAND INTO THE CANADIAN MARITIMES. THE NEXT CLOSEST SOLN IS THE 00Z UKMET THAT IS ESSENTIALLY A WEAKER VERSION OF THE GFS. THE 12Z ECMWF SHOWS STRONGEST DEVELOPMENT FARTHER NEWD. THE ECMWF MEAN IS ON THE SRN/SWRN SIDE OF THE CURRENT SPREAD FOR NEW ENGLAND LOW PRES AROUND FRI AND MULTI-DAY TRENDS HAVE BEEN TOWARD FARTHER EWD SFC DEVELOPMENT. THUS PREFER LEANING MORE TO THE 18Z GEFS MEAN AS THE BEST ALTERNATIVE TO LOWER CONFIDENCE OPERATIONAL MODEL RUNS. DIFFS THAT ARISE WITH ENERGY TRACKING S OF THE ALEUTIANS AROUND DAY 4 THU ARE THE STARTING POINT FOR INCREASING SPREAD THAT IS OBSERVED FROM THE PAC INTO THE LOWER 48 AFTER DAY 5 FRI. THE FACT THAT SHRTWV ENERGY COMING INTO THE NORTHWEST ON FRI IS HEADING TOWARD THE MEAN TROUGH POSN DURING THE WEEKEND WOULD SEEM TO FAVOR MAINTAINING A STRONGER REFLECTION THAN SEEN IN THE 12Z ECMWF/CMC MEANS. EVOLUTION OF PLAINS TO E-CNTRL U.S. SFC LOW PRES SAT-SUN WILL BE SENSITIVE TO EXACTLY HOW MID LVL ENERGY IS DISTRIBUTED... A LOW PREDICTABILITY ENDEAVOR TO RESOLVE AT THAT DISTANT OF A TIME FRAME. ENSEMBLE GUIDANCE THROUGH THE 12Z/18Z CYCLE SHOWS BEST POTENTIAL FOR A DEFINED SFC LOW TO TRACK INTO THE GRTLKS RATHER THAN INTO THE SOUTHEAST AS IN THE 12Z ECMWF. BEHIND THIS SYSTEM GUIDANCE IS SIGNALING THE ARRIVAL OF ANOTHER FEATURE TO THE WEST COAST BY SUN. AGAIN CONFIDENCE IS LOW IN THE SPECIFICS. ...WEATHER HIGHLIGHTS/THREATS... VERY COLD TEMPS OVER THE WRN-CNTRL STATES AS OF WED WILL PUSH EWD WITH TIME. DURING WED-THU SOME LOCATIONS FROM THE ERN SLOPES OF THE ROCKIES INTO THE NRN-CNTRL PLAINS SHOULD SEE HIGHS 20-30F BELOW NORMAL. COLD AIR REACHING THE EAST LATE IN THE WEEK SHOULD BRING A COUPLE DAYS OF MINUS 10-20F ANOMALIES. A WARMING TREND WILL COMMENCE BEHIND THE CORE OF COLDEST AIR BUT CNTRL-ERN U.S. TEMPS SHOULD TEND TO REMAIN SOMEWHAT BELOW NORMAL THROUGH NEXT WEEKEND WITH THE NRN PLAINS LIKELY TO SEE THE MOST PERSISTENT COLD ANOMALIES. THERE MAY BE A BAND OF PCPN ACCOMPANYING THE LEADING COLD FRONT... FOLLOWED BY MEANINGFUL LAKE EFFECT SNOW IN STRONG CYCLONIC FLOW DURING THE LATTER HALF OF THE WEEK. POTENTIAL SYSTEM NEXT WEEKEND MAY SPREAD A SHIELD OF PCPN FROM THE PLAINS INTO THE EAST WITH SNOW N/RAIN S. AFTER WED EXPECT A MULTI-DAY PERIOD OF PCPN FOCUSED OVER WINDWARD TERRAIN FROM THE NRN-CNTRL WEST COAST INTO THE ROCKIES. HIGHEST TOTALS WILL LIKELY BE FROM THE PAC NW COAST AND CASCADES INTO THE NRN SIERRA NEVADA RANGE. TEMPS OVER THE WEST WILL MODERATE FROM A CHILLY START TO THE PERIOD WED BUT SHOULD REMAIN COOL ENOUGH TO KEEP SNOW LEVELS FAIRLY LOW. DEPENDING ON OFFSHORE SFC LOW TRACK/STRENGTH THE PAC NW MAY SEE A PERIOD OF ENHANCED WINDS MID-LATE WEEK. RAUSCH