EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1057 AM EST SUN NOV 23 2014 VALID 12Z WED NOV 26 2014 - 12Z SUN NOV 30 2014 ...OVERVIEW... THE PASSAGE OF TIME IS DOING REMARKABLY LITTLE TO RESOLVE THE AREA OF SIGNIFICANT SPREAD THAT GUIDANCE HAS PERSISTENTLY BEEN SHOWING OVER THE NWRN CONUS/SWRN CANADA AS OF EARLY DAY 3 WED WITH DOWNSTREAM IMPACTS ON THE MID LVL ENERGY/SFC SYSTEM TRACKING INTO THE GRTLKS AND SYSTEM THAT MAY TRACK NEAR THE EAST COAST EARLY IN THE PERIOD. SOME ASPECTS OF UPSTREAM ERN PAC FLOW ALSO CHANGE FROM DAY TO DAY WITH CONSIDERABLE MODEL/ENSEMBLE SPREAD AS WELL. A GENERAL EVOLUTION TOWARD LOWER AMPLITUDE MEAN FLOW ACROSS THE LOWER 48 BY NEXT WEEKEND MAY FURTHER REDUCE PREDICTABILITY... SO NEEDLESS TO SAY THE CONFIDENCE IN A SPECIFIC SOLN IS BELOW AVG. ...GUIDANCE EVALUATION/PREFERENCES... EARLY IN THE PERIOD RECENT MODEL/ENSEMBLE RUNS APPEAR TO BE CONVERGING TOWARD THE IDEA OF A REX BLOCK CONFIGURATION OVER THE ERN/NERN PAC BUT WITH INDIVIDUAL MODEL/ENSEMBLE SOLNS STILL UNDECIDED OVER THE RELATIVE STRENGTH OF STREAMS DOWNSTREAM FROM THE NRN RIDGE AND SRN TROUGH/UPR LOW. THUS OVER THE PAST DAY THERE HAS BEEN SURPRISINGLY LITTLE IMPROVEMENT IN MODEL/ENSEMBLE SPREAD FROM SWRN CANADA/NWRN CONUS INTO THE PLAINS WED-FRI. THE FULL SOLN ENVELOPE SPANS THE FULL RANGE OF RIDGE TO TROUGH... WITH 00Z GEFS MEMBERS TRENDING MORE TOWARD THE RIDGE SIDE OF THE SPREAD THAN 24 HRS AGO AND A DECENT NUMBER OF ECMWF MEMBERS REVERSING COURSE FROM THE MIDDLE TO RIDGE SIDE OF THE SPREAD MORE TOWARD A MODERATE RIDGE OR TROUGH. RESULTING SFC DIFFS ARE DRAMATIC OVER SOME LOCATIONS... FOR EXAMPLE 35-40MB VALID 12Z FRI IN THE 06Z GFS VS 00Z ECMWF. FCST SENSITIVITY IS FURTHER DISPLAYED IN GFS/ECMWF RUNS THAT HAVE SWITCHED PLACES WITH SOME DETAILS OVER THE PAST DAY. GIVEN THIS UNCERTAINTY IT IS DIFFICULT TO HAVE MUCH CONVICTION IN A SPECIFIC SOLN FOR THE MIDWEST-GRTLKS SYSTEM AND LEADING EAST COAST WAVE DURING WED-THU. AT THE VERY LEAST THE 00Z ECMWF/ECMWF MEAN AND A SLIGHTLY SLOWER VERSION OF THE 00Z/06Z PARALLEL GFS PROVIDE THE BEST INTERMEDIATE SOLN RELATIVE TO THE FULL ENSEMBLE SPREAD WITH THE MIDWEST-GRTLKS SYSTEM... WHILE THE 06Z GFS/00Z ECMWF ENSEMBLE MEAN OFFER THE MOST REASONABLE TRACK FOR THE EAST COAST SYSTEM. REGARDING THE LATTER... WIDE SPREAD PERSISTS WITH ECMWF RUNS ON THE WEST EDGE OF THE FULL GUIDANCE SPREAD AND CURRENTLY JOINED BY THE 00Z UKMET. GFS/PARALLEL GFS/CMC RUNS HAVE TENDED TO WAFFLE IN THE MIDDLE TO ERN HALF OF THE SPREAD. THE ONE IMPROVEMENT TODAY IS MUCH BETTER TIMING AGREEMENT. THE PREFERRED 06Z GFS/00Z ECMWF MEAN TRACK MAINTAINS DECENT CONTINUITY FROM PRIOR PREFS AND THE TRACK PARTIALLY INTO THE WRN HALF OF THE ENVELOPE SEEMS REASONABLE GIVEN MULTI-DAY TRENDS SUGGESTING AT LEAST MODERATE DEVELOPMENT OF THIS SYSTEM. RECENT TRENDS TOWARD A BETTER DEFINED REX BLOCK PATTERN IN THE ERN PAC DURING THE FIRST HALF OF THE PERIOD LEAD TO A SLOWER TREND WITH THE EVENTUAL PROGRESSION OF ERN PAC TROUGH ENERGY TOWARD THE WEST COAST BY NEXT WEEKEND IN RESPONSE TO UPSTREAM PAC FLOW. 06Z GEFS/00Z ECMWF MEANS ARE FAIRLY SIMILAR ALOFT OVER THE ERN PAC/WRN NOAM AS WELL AS WITH RESPECT TO FLOW DOWNSTREAM. EXACT EVOLUTION OF THE UPSTREAM FLOW IS STILL FAIRLY UNCERTAIN SO SOLNS MAY WELL CHANGE FURTHER FOR THE TROUGH NEARING THE WEST COAST. DAYS 3-4 WED-THU START PLACE PRIMARY EMPHASIS ON THE 00Z ECMWF MEAN/06Z GFS TO REFLECT PREFS FOR THE EAST COAST SYSTEM AND YIELD AN INTERMEDIATE SOLN ELSEWHERE... WITH MINOR ADJUSTMENTS TOWARD CONSENSUS FOR THE SYSTEM TRACKING INTO THE GRTLKS. DAYS 5-7 START WITH AN EVEN BLEND OF THE 06Z GEFS/00Z ECMWF MEANS TO STAY NEAR THE MIDDLE OF THE GUIDANCE SPREAD WHILE AWAITING FOR BETTER CLUSTERING/CONTINUITY. ...SENSIBLE WEATHER HIGHLIGHTS... FCST REMAINS QUITE UNCERTAIN OVER THE MID ATLC/NORTHEAST WED INTO THU BUT THE MOST LIKELY SCENARIO AT THIS TIME IS TOWARD SOME DEGREE OF MEANINGFUL PCPN. DEPENDING ON EXACT TRACK THERE SHOULD BE A BAND OF RAIN AND/OR SNOW WITH ABOVE FREEZING SFC TEMPS CLOSER TO THE COAST AND SNOW/BELOW FREEZING SFC TEMPS FARTHER INLAND. EXPECT PERIODS OF SNOW OVER THE GRTLKS REGION IN ASSOC WITH THE SYSTEM TRACKING INTO THE REGION EARLY IN THE PERIOD AND LAKE EFFECT ACTIVITY DURING PERIODS OF COLD CYCLONIC FLOW. AS WITH SOME ASPECTS OF THE FCST OVER THE EAST... CONFIDENCE IS NOT ESPECIALLY HIGH WITH WRN DETAILS EITHER. HOWEVER FOR THE 5-DAY PERIOD AS A WHOLE THERE ARE DECENT SIGNALS TOWARD ENHANCED PCPN POTENTIAL OVER FAVORED TERRAIN FROM THE CNTRL-NRN WEST COAST INTO THE NRN ROCKIES. FOR TEMPS... CNTRL-SRN PARTS OF THE WEST MAY SEE SOME PLUS 10-20F ANOMALIES FOR HIGHS WED-FRI BEFORE HGT FALLS ALOFT BRING A MODERATING TREND. FARTHER EWD THE NRN TIER SHOULD BE MOST CONSISTENTLY BELOW NORMAL WITH MULTIPLE DAYS OF SIMILARLY NEGATIVE ANOMALIES FROM THE NRN TIER INTO UPR GRTLKS. THE ERN STATES SHOULD ALSO BE BELOW NORMAL WITH A LITTLE MORE VARIABILITY IN ANOMALIES. RAUSCH