EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1145 AM EDT FRI SEP 25 2015 VALID 12Z MON SEP 28 2015 - 12Z FRI OCT 02 2015 ...OVERVIEW... HIGH-LATITUDE FLOW ALOFT WILL PREVAIL FROM THE BERING SEA TO HUDSON BAY...WITH A CLEAR 'DETACHMENT' FROM THE FLOW PATTERN ACROSS A LARGE PORTION OF THE LOWER 48. A DISTINCT SUBTROPICAL RIDGE AND WEAKNESS IN THIS RIDGE (OVER THE GULF OF MEXICO) WILL PREVAIL OVER THE SOUTHERN PORTION OF THE CONTINENT. GIVEN THIS SCENARIO...A MORE 'TROPICAL' OR SUMMER-LIKE FEEL TO THE SENSIBLE WEATHER PATTERN VERSUS---WHAT SHOULD BE A MORE 'AUTUMN-LIKE' SCENARIO HEADING INTO EARLY OCTOBER. ...MODEL PREFERENCES... THROUGH MID-POINT DAY 4 (29/12Z)...THE 25/00Z DETERMINISTIC GFS/ECMWF SEEMED TO HAVE THE BETTER CONTINUITY WITH RESPECT TO (1) CURRENT CONDITIONS...(2) FROM THE INDIVIDUAL RUN-TO-RUN PERSPECTIVE CONCERNING A DISTINCT NORTHERN STREAM SEPARATION ACROSS EASTERN NORTH AMERICA... AND (3) TROUGH CONFIGURATION---THEN MIGRATION---ALONG THE WEST COAST TO...AND ACROSS THE NORTHERN ROCKIES. WILL UTILIZE A BLEND OF THE 25/00Z GEFS/ECENS MEANS (50/50 RATIO) BY 30/12Z WHICH MAINTAINS THE DISTINCT 'DETACHMENTS' BETWEEN AIR STREAMS ALOFT OVER CANADA AND ANY 'PHASED' SHORTWAVE ENERGY EMBEDDED WITHIN A WEAKNESS IN THE SUBTROPICAL RIDGE INVOF 90W AND 130W. ...UNCERTAINTY ASSESSMENT... DEVIATIONS IN THE DETERMINISTIC GUIDANCE INCREASES CONSIDERABLY BY 30/00Z---AS A DEEP HIGH-LATITUDE TROUGH AND A SERIES OF MEANDERING MID-LEVEL VORTICES OVER THE SOUTHEAST PORTION OF THE CONTINENT INTERACT OVER THE OHIO VALLEY. FOR THE GULF COAST...THE SYNOPTIC-SCALE SCENARIO UNFOLDING HAS NOT CHANGED---WITH ANOTHER MID-LEVEL CIRCULATION ATTEMPTING TO 'BRIEFLY' ANCHOR ALONG THE TEXAS GULF COAST AND EJECT SHORTWAVE ENERGY (TIED TO LOWER SURFACE PRESSURE) NORTHWARD INTO THE FLORIDA PANHANDLE. THE DETERMINISTIC GUIDANCE AGREES IN PRINCIPLE WITH THIS SCENARIO---THAT THE SUBSEQUENT SURFACE WAVE FORMATION IS TIED TO MID-LATITUDE SHORTWAVE INTERACTION. THE CHALLENGE IS...TO USE THE GFS/ECMWF SOLUTION OR TO BELIEVE THE UKMET/CANADIAN SOLUTION AS A 'TREND' BEYOND DAY 4 WITH ONE OR MORE OF THE 'VORTICES' AND CLUSTERS OF CONVECTION. ALONG THE WEST COAST... TO ME---THE FORECAST CHALLENGE IS---WHEN TO 'LET GO' OF A DETERMINISTIC APPROACH AND ACCEPT THE 'DEMISE' OF THE CLOSED 500MB CIRCULATION SOLUTION OVER NORTHERN CALIFORNIA. HERE...CONCEDE THAT THE ISSUE CONCERNS THE STRENGTH OF THE DOWNSTREAM UPPER-LEVEL RIDGE AND ITS 'PROJECTED' TRAJECTORY---IS DICTATING DETERMINISTIC DEVELOPMENTS/OUTCOMES ALONG THE PACIFIC NORTHWEST COAST. THE CANADIAN/UKMET SOLUTIONS VS THE 25/00Z ECMWF AND 24/18Z GFS SOLUTION ILLUSTRATE THE 'TUSSLE' BETWEEN UPPER RIDGE AND UPPER TROUGH (CLOSED LOW) DOMINANCE. NOT SURE THE ANSWERS ARE THERE EVEN NOW (FOR THE DAY 4-5 PERIOD)...GIVEN THE ECMWF/GFS SOLUTIONS DIFFER CONSIDERABLY (WITH MID-LEVEL SHORTWAVE ENERGY) AND DOWNSTREAM MIGRATION OVER THE NORTHERN GREAT BASIN AND NORTHERN ROCKIES. ...SENSIBLE WEATHER HIGHLIGHTS... UNSETTLED WEATHER FOR THE EAST...ESPECIALLY THE GULF COAST WHERE CONVECTION HAS HEAVY...RAINFALL POTENTIAL THROUGH DAY 5. AFTER DAY 5...LOOKS LIKE ANOTHER 'FRESH' CANADIAN AIRMASS WILL SPREAD ACROSS THE GREAT LAKES AND NORTHEAST QUARTER OF THE NATION. A GRADUAL DEEPENING OF THE ONSHORE FLOW WILL SEEP SOUTHWARD FROM THE ALASKA PANHANDLE TO THE CASCADES OF OREGON...WITH THE MARITIME AIRMASS ANTICIPATED TO REACH A MAXIMUM DEPTH AND SOUTHWARD PUSH AROUND THE DAY 5 TIME FRAME. VOJTESAK