EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 300 AM EDT THU SEP 15 2016 VALID 12Z SUN SEP 18 2016 - 12Z THU SEP 22 2016 ...GUIDANCE AND UNCERTAINTY ASSESSMENT... TONIGHTS WPC MEDIUM RANGE PRODUCT SUITE IS DERIVED FROM A COMPOSITE BLEND OF REASONABLY WELL CLUSTERED GUIDANCE FROM THE 18 UTC GFS/GEFS AND 12 UTC ECMWF/ECMWF ENSEMBLES SUNDAY/MONDAY IN A PERIOD WITH RELATIVELY LOW FORECAST SPREAD. INCREASING FORECAST SPREAD THEN PROMPTED A QUICK TRANSITION BY TUESDAY-THURSDAY INTO A 18 UTC GEFS AND RELATIVELY COMPATABLE 12 UTC ECMWF ENSEMBLE MEAN FORECAST BLEND SCENARIO BY DAYS 6/7. THIS MAINTAINS MAX WPC CONTINUITY FOR THIS PERIOD. THERE IS A GUIDANCE TREND FOR MORE AMPLIFIED FLOW UPSTREAM FROM THE NERN PACIFIC INTO THE NWRN US OVER THE PAST FEW RUNS. THIS MAY PROVE REASONABLE CONSIDERING THE CHOATIC FLOW ACROSS THE NRN PACIFIC FROM RUN TO RUN...INCLUDING POTENITAL DOWNSTREAM EFFECTS FROM AN UNCERTAIN TRACK OF WRN PACIFIC TYPHOON MALAKAS INTO THE EXTRATROPICAL WESTERLIES NEXT WEEK. ...WEATHER PATTERN HIGHLIGHTS/THREATS... LEAD TROUGH ENERGY ALOFT AND A SURFACE LOW EJECT NEWD OVER ERN CANADA AND THE NERN US SUNDAY INTO MONDAY...LEAVING A TRAILING FRONT SOUTHWESTWARD ALL THE WAY BACK TO THE SRN PLAINS/SW US. THE NRN PORTION OF THE FRONT WITH A PROGRESSIVE SWATH OF CONVECTION REACHES THE WRN ATLANTIC MONDAY...BUT THE WAVY/STALLED TRAILING FRONT WILL DRAPE/LINGER ACROSS THE US SRN TIER TO FOCUS SOME HEAVY CONVECTION INTO MIDWEEK. WEEKEND TO EARLY NEXT WEEK ACTIVITY BACK OVER THE SW US TO SRN ROCKIES/HIGH PLAINS MAY BE ENHANCED IN PROXIMITY OF A MID-UPPER LEVEL TROUGH DUG BACK TO BAJA. THIS TROUGH MAY TAP SOME MID-LEVEL MOISTURE NEXT WEEK FROM AN ERN PACIFIC TROPICAL DISTURBANCE OFF MEXICO THAT IS BEING MONITORED FOR DEVELOPMENT BY NHC/WPC. FORECAST SPREAD STILL MEANWHILE SUGGESTS LESS CERTAIN SUBSEQUENT TIMING OF A MID-UPPER LEVEL TROUGHS AND SURFACE FRONTAL SYSTEM AND PCPN PASSAGES ACROSS THE NWRN US SUNDAY...THEN NRN PLAINS/MIDWEST/GREAT LAKES AND SRN CANADA MONDAY-WEDNESDAY...POSSIBLY SPREADING MORE IN ERNEST OVER THE WRN US DAYS 6/7 WITH FLOW AMPLIFCATION. SCHICHTEL