EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1147 AM EDT SUN SEP 18 2016 VALID 12Z WED SEP 21 2016 - 12Z SUN SEP 25 2016 ...OVERVIEW AND GUIDANCE EVALUATION... A RIDGE IN THE PROCESS OF BUILDING INTO WRN CANADA AND UPPER LOW DROPPING INTO THE NWRN U.S. WILL YIELD A REX BLOCK CONFIGURATION. THIS PATTERN SHOULD MAKE GRADUAL PROGRESS TO THE CNTRL PART OF THE CONTINENT... PUSHED ALONG BY FLOW ENTERING WRN CANADA AND A RIDGE BUILDING TOWARD THE WEST COAST. CYCLONIC FLOW DOWNSTREAM FROM THE WRN-CNTRL CANADIAN RIDGE SHOULD EXTEND INTO THE NERN STATES TO SOME DEGREE. GUIDANCE AGREES IN PRINCIPLE ON THE LARGE SCALE EVOLUTION. HOWEVER MODELS/ENSEMBLES HAVE BEEN DISPLAYING MEANINGFUL SPREAD AND RUN TO RUN VARIABILITY FOR TIMING OF THE OVERALL WRN TROUGH/TRACK OF THE EMBEDDED UPR LOW... PLUS THE AMPLITUDE OF NERN U.S. TROUGHING. BASED ON THE CURRENT RANGE OF DATA PREFS WENT TO A SOLN CLOSEST TO A COMPROMISE AMONG THE 00Z-06Z GFS/00Z ECMWF. THIS SCENARIO ACCOUNTS FOR OFFSETTING TIMING CONSIDERATIONS... WITH THE AMPLIFIED/BLOCKY NATURE OF THE PATTERN FAVORING SLOW PROGRESSION BUT TELECONNECTIONS RELATIVE TO STRONG POSITIVE HGT ANOMALIES OVER NRN CANADA SUPPORTING FASTER MEAN FLOW OVER THE LOWER 48. MINORITY WEIGHTING OF THE 00Z ECMWF MEAN AND WPC CONTINUITY WERE ADDED TO THE BLEND DAYS 6-7 SAT-SUN AS CONFIDENCE IN A SPECIFIC SOLN DECREASED. TRENDS OVER THE PAST COUPLE DAYS IN THE ENSEMBLE MEANS HAVE BEEN TOWARD GREATER DEPTH AND DEFINITION OF THE WRN TROUGH/UPR LOW... WHICH ASIDE FROM TIMING/TRACK ISSUES HAVE BEEN DEPICTED WITH BETTER LEAD TIME IN THE OPERATIONAL RUNS. THIS SUPPORTS AN OPERATIONAL COMPROMISE IN MOST RESPECTS. COMPARING RECENT 12-HRLY GFS/ECMWF RUNS... THE GFS HAS BEEN MUCH MORE ERRATIC WITH THE TIMING AND TRACK OF THE UPR LOW AND THUS ALSO MORE VARIABLE WITH THE DEGREE OF DOWNSTREAM UPR TROUGHING THAT EXTENDS INTO THE NERN STATES. CURRENT CONSENSUS SEEMS TO LIE BTWN THE 00Z GFS AND FASTER 06Z RUN. FASTER ADJUSTMENT OF THE 06Z GEFS MEAN TOWARD THE CONSENSUS ADDS SUPPORT FOR HEDGING FASTER THAN THE SLOW END OF THE SPREAD... BUT AGAIN THE BLOCKY NATURE OF THE PATTERN DOES NOT SUPPORT GOING TOO FAR INTO THE FASTER HALF OF THE ENVELOPE EITHER. INTERESTS ALONG THE SOUTHEAST/CAROLINA COAST AND SWRN U.S. SHOULD MONITOR NATIONAL HURRICANE CENTER FORECASTS FOR JULIA AND PAINE RESPECTIVELY. 1500 UTC ADVISORIES HAVE BOTH SYSTEMS WEAKENING TO POST-TROPICAL/REMNANT LOW STATUS BY EARLY WED. ...WEATHER HIGHLIGHTS/THREATS... THE WRN U.S. TROUGH/UPR LOW HEADING TOWARD THE PLAINS WILL PROVIDE A LARGE SCALE FOCUS FOR ACTIVE WEATHER DURING THE PERIOD. GIVEN THE PROJECTED TRACK OF THE UPR LOW AND A PERIOD OF LOW LVL UPSLOPE FLOW OVER THE NRN HIGH PLAINS... THERE IS DECENT POTENTIAL FOR HVY PCPN FROM THE NRN ROCKIES THROUGH THE NRN HIGH PLAINS WITH HIGHEST 5-DAY TOTALS CENTERED OVER THE ERN HALF OF MONTANA. SOME SNOW MAY BE PSBL OVER HIGH ELEVS OF THE NRN ROCKIES. BELOW NORMAL TEMPS WILL SPREAD ACROSS THE WEST AND THEN INTO THE PLAINS. THE CORE OF COLDEST AIR SHOULD BE OVER THE NRN ROCKIES/HIGH PLAINS WHERE HIGHS MAY BE 15-20F BELOW NORMAL ON ONE OR MORE DAYS. BY NEXT WEEKEND EXPECT CONVECTION... SOME POTENTIALLY HVY AND/OR STRONG... ALONG THE LEADING COLD FRONT AS IT REACHES THE PLAINS AND MS VLY/MIDWEST. TEMPS ALONG THE WEST COAST SHOULD WARM UP LATE IN THE PERIOD AS RIDGING ALOFT BUILDS IN AND SFC HIGH PRES OVER THE INTERIOR WEST SUPPORTS STRENGTHENING OFFSHORE FLOW. WELL AHEAD OF WRN SYSTEM EARLY IN THE PERIOD... ENERGY/MSTR STREAMING NEWD FROM ERN PAC TROPICAL CYCLONE PAINE AS WELL AS A NEARBY COMPACT UPR LOW OFF NRN BAJA CALIF IN THE SHORT RANGE MAY CONTRIBUTE TO A PERIOD OF HVY RNFL OVER PARTS OF THE UPR MS VLY/UPR GRTLKS AROUND WED-THU. LOCATIONS FROM THE SRN MID ATLC TO FLORIDA MAY SEE PERIODS OF DIURNALLY FAVORED CONVECTION. THE EXTENT TO WHICH MSTR FROM JULIA MAY CONTRIBUTE TO RNFL WILL NEED TO BE MONITORED. MUCH OF THE EAST WILL LIKELY SEE ABOVE NORMAL TEMPS WITH PLUS 10-15F ANOMALIES LIKELY TO BE COMMON FROM THE MS VLY INTO THE APLCHNS. EARLY IN THE PERIOD THIS WARMTH WILL EXTEND ACROSS THE PLAINS AS WELL WITH SOME RECORD WARM MIN TEMPS PSBL. BY THE LATTER HALF OF THE PERIOD THE GRTLKS AND NORTHEAST MAY COOL DOWN TO AROUND NORMAL. SWD EXTENT OF THE COOLER AIR BECOMES INCREASINGLY UNCERTAIN BY NEXT SAT-SUN GIVEN SPREAD IN DETAILS OF UPSTREAM FLOW ALOFT. RAUSCH