EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 243 AM EDT SUN APR 30 2017 VALID 12Z WED MAY 03 2017 - 12Z SUN MAY 07 2017 ...PATTERN OVERVIEW AND GUIDANCE EVALUATION... LATEST GUIDANCE CONTINUES TO SHOW AN EVOLUTION TOWARD AN AMPLIFIED BLOCKY PATTERN BY THE WEEKEND, CONSISTING OF EAST AND WEST COAST TROUGHS WITH A STRONG RIDGE OVER THE CENTRAL U.S.. OPERATIONAL MODEL RUNS FOR DETAILS WITHIN THE ERN MEAN TROUGH HAVE BEEN REMARKABLY ERRATIC AND DIVERGENT. INDIVIDUAL ENSEMBLE MEMBERS HAVE BEEN SIMILARLY DIVERSE BUT THE RESULTING MEANS CONTINUE TO PROVIDE A MORE CONSISTENT FCST. MEANWHILE EARLY-PERIOD GFS TRENDS OVER THE NRN PAC IN THE PAST DAY HAVE YIELDED BETTER CLUSTERING FOR THE WEST COAST TROUGH ALBEIT WITH TYPICAL EMBEDDED DETAIL UNCERTAINTIES REMAINING. THE FCST FOR AMPLIFYING CNTRL-ERN U.S. TROUGH ENERGY REMAINS VERY SENSITIVE IN TERMS OF THE DEGREE OF FLOW SEPARATION THAT OCCURS BY LATE IN THE WEEK AND THEN TO WHAT EXTENT FLOW AROUND THE CANADIAN EXTENSION OF THE CNTRL U.S. RIDGE ULTIMATELY INTERACTS WITH THE SYSTEM ASSOC WITH THE INITIAL BUNDLE OF ENERGY. 12Z GFS/ECMWF RUNS THAT SEPARATED INITIAL ENERGY ENOUGH TO LEAVE BEHIND A SLOW MOVING SRN TIER SYSTEM WERE IN THE SLOWEST 10-15 PCT OF ENSEMBLE MEMBERS. THE 12Z UKMET WAS SIMILAR IN PRINCIPLE. THE 18Z GFS REVERTED BACK TO A MORE PHASED/CONSOLIDATED SYSTEM, CLOSER TO WHAT THE 00Z/29 ECMWF RUN SHOWED AND WHAT MOST ENSEMBLE MEAN RUNS HAVE BEEN ADVERTISING IN RECENT DAYS. MODEL/ENSEMBLE SPREAD AND POOR CONTINUITY IN OPERATIONAL RUNS KEEP CONFIDENCE LOWER THAN DESIRED, AND MAINTAIN PREFS FOR AN ENSEMBLE MEAN APPROACH THAT MAINTAINS BETTER CONTINUITY THAN OTHER SOLNS. THE ONE TREND OF NOTE IN THE LATEST MEANS AS OF THE 12Z/18Z CYCLE WAS TOWARD SLIGHTLY SLOWER PROGRESSION, WHICH MAY BE A REASONABLE ADJUSTMENT GIVEN THE PATTERN AMPLITUDE. NOT SURPRISINGLY THE NEW 00Z MODEL RUNS THUS FAR OFFER SOME FURTHER OPTIONS. THE 00Z GFS HAS A SRN CLOSED LOW BUT ONE THAT IS FASTER THAN THE 12Z RUN AND EVENTUALLY PHASES WITH NRN STREAM FLOW OVER THE NORTHEAST. THE UKMET HAS JUMPED TO THE CONSOLIDATED CLUSTER WHILE THE CMC ALSO HAS THE CORE OF ITS SYSTEM ALOFT N OF THE GFS BY MID PERIOD. MEANWHILE SLOWER TRENDS IN THE GFS FOR SHORTER TERM NRN PAC FLOW OVER THE LAST 24 HRS HAVE BROUGHT ITS RUNS INTO BETTER ALIGNMENT WITH CONSENSUS FOR THE GENERAL AXIS AND AMPLITUDE OF THE UPR TROUGH AMPLIFYING NEAR THE WEST COAST. FOR AT LEAST THE PAST DAY ECMWF/ECMWF MEAN/CMC RUNS HAVE BEEN ADVERTISING POTENTIAL FOR AN EMBEDDED LOW TO CLOSE OFF OVER OR NEAR CA BY DAY 7 SUN AND 12Z/18Z GFS RUNS CLOSED OFF A LOW AS WELL, WHILE CORRESPONDING GEFS MEANS WERE A LITTLE LATER TO CLOSE OFF A WEAKER LOW. OVERALL A MODEL/ENSEMBLE MEAN BLEND PROVIDED A FAIR REPRESENTATION OF BEST CLUSTERING/TRENDS THROUGH THE 12Z/18Z CYCLES. INCOMING 00Z RUNS CAST A LITTLE MORE DOUBT ON EXACTLY WHEN/WHERE AN UPR LOW MAY CLOSE OFF. WITH THE LEADING RIDGE STILL TRENDING STRONGER IN THE MEANS AS WAS THE CASE YDAY, LEADING HGT FALLS ALOFT/SFC FRONT PUSHING INTO THE WEST HAVE SLOWED DOWN SOMEWHAT. BASED ON THE ABOVE CONSIDERATIONS, THE UPDATED FCST STARTED WITH A BLEND OF 12Z NAEFS/ECMWF MEANS AND TO A LESSER EXTENT 18Z GFS/GEFS MEAN THROUGH THE PERIOD WITH MINOR EDITING TO ENHANCE DETAIL. ...WEATHER HIGHLIGHTS... WITH SOME IMPORTANT DETAILS STILL IN DOUBT, GENERALLY EXPECT THE SYSTEM ASSOC WITH THE AMPLIFYING CNTRL-ERN U.S. UPR TROUGH TO GENERATE SOME LOCALLY MDT-HVY RNFL OVER PARTS OF THE CNTRL-SRN PLAINS/W-CNTRL GULF COAST ON WED WITH ACTIVITY THEN SPREADING EWD-NEWD TO THE EAST COAST BY LATE IN THE WEEK. AMPLIFIED/BLOCKY NATURE OF THE LARGE SCALE PATTERN COULD PROMOTE FAIRLY SLOW SYSTEM MOTION AND LINGERING PRECIP OVER PARTS OF THE EAST INTO THE WEEKEND. COMBINING THE VARIOUS POSSIBILITIES FOR SYSTEM EVOLUTION THE LWR MS VLY/CNTRL GULF COAST REGION APPEARS TO HAVE THE BEST POTENTIAL FOR SEEING HIGHEST RNFL TOTALS WITH THIS SYSTEM. SOME LOCATIONS EXTENDING INTO THE MID ATLC/NORTHEAST, AND PERHAPS WITHIN ANOTHER BAND FARTHER NW OF THIS AXIS, MAY SEE SIGNIFICANT AMOUNTS AS WELL. WARM SECTOR CONVECTION MAY BE STRONG TO SEVERE WITH GUIDANCE DETAILS STILL TOO UNCERTAIN TO SPECIFY MOST LIKELY AREAS TO BE AFFECTED. THE COLD SECTOR OF THIS SYSTEM WILL BRING BELOW NORMAL TEMPS FROM THE HIGH PLAINS INTO THE EAST WITH SOME AREAS SEEING ONE OR MORE DAYS OF AT LEAST MINUS 10F ANOMALIES FOR HIGHS. AHEAD OF THE UPR TROUGH AMPLIFYING TOWARD THE WEST COAST, ABOVE TO MUCH ABOVE NORMAL TEMPS WITH MANY ANOMALIES OF PLUS 10F OR GREATER WILL PROGRESS FROM THE WEST COAST STATES THROUGH THE INTERIOR WEST AND INTO THE ROCKIES/HIGH PLAINS. BASED ON LATEST FCST FOR TEMPS THERE STILL APPEARS TO BE A BETTER CHANCE FOR SCATTERED DAILY RECORD WARM LOWS THAN RECORD HIGHS. ARRIVAL OF THE UPR TROUGH WILL BRING NEAR TO BELOW NORMAL READINGS TO THE WEST COAST FOR NEXT WEEKEND. MOST PRECIP SHOULD BE IN THE LGT-MDT RANGE OF INTENSITY AND BE CONFINED MOSTLY TO THE NRN-CNTRL WEST COAST INTO THE NRN ROCKIES, THOUGH SOME LGT/SCT ACTIVITY OVER THE S LATE IN THE PERIOD CANNOT BE RULED OUT. RAUSCH