EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1212 PM EDT FRI SEP 05 2014 VALID 12Z MON SEP 08 2014 - 12Z FRI SEP 12 2014 ...'NORBERT' MOISTURE TO EMCOMPASS MORE TERRITORY DOWNSTREAM THROUGH TIME... ...COLD CANADIAN AIR/UPSLOPE CONDITIONS RETURN TO THE ROCKIES/HIGH PLAINS... ...OVERVIEW... PERSISTENT UPPER-LEVEL RIDGING ALONG THE WEST COAST OF CANADA AND EASTERN GULF OF ALASKA PRODUCES AN AMPLIFIED PATTERN DOWNSTREAM IN THE CENTRAL PORTION OF THE NORTH AMERICAN CONTINENT. MID-PERIOD---REMNANT 'NORBERT' MID-LEVEL MOISTURE STREAKS INTO THE BASE OF THE UPPER-LEVEL TROUGH AND PROCEEDS DOWNSTREAM ACROSS THE CENTRAL PLAINS...MID-MISSISSIPPI VALLEY AND OHIO VALLEY. MID-LEVEL MOISTURE ASSOCIATED WITH 'NORBERT' WILL COME IN THE FORM OF A SERIES OF PREDECESSOR RAINFALL EVENTS (OR PRE's) THAT FANS OUT ACROSS THE WESTERN MOUNTAINS OF MEXICO AND MIGRATE DOWNWIND INTO THE FOUR CORNERS. BY 9/12Z...THE MIGRATION OF AN UPPER-LEVEL TROUGH ALONG 125W---CURRENTLY INVOF NORTHERN CALIFORNIA---WILL IMPINGE A DRIER AIRMASS UPON THE NORTHERN PERIPHERY OF NORBERT CLOUD SHIELD AND PROVIDE THE UPPER-LEVEL STEERING MECHANISM FOR DOWNSTREAM SUBTROPICAL MOISTURE TRANSPORT. BEHIND THE EXITING TROUGH ALONG THE WEST COAST...A SECONDARY SHORTWAVE WILL DESCEND THE CANADIAN ROCKIES AND FORM A DEEPER...COLDER AND MORE UNSTABLE UPPER-LEVEL TROUGH OVER THE UPPER MIDWEST AND SOUTH CENTRAL CANADA AT PERIOD'S END. ...MODEL PREFERENCES/UNCERTAINTY ASSESSMENT... MUCH IF NOT ALL THE RUN-TO-RUN VARIABILITY OVER THE PAST 4 DAYS---REMAINED TIED TO THE MOISTURE AND EMBEDDED VORTICITY EJECTING OUT OF NORBERT'S NORTHEAST QUADRANT. THE DETERMINISTIC 5/00Z & 5/06Z GFS HAVE A 'TRACK-ABLE' SHORTWAVE STREAKING INTO EASTERN COLORADO @ 9/12Z---WITH THE 5/00Z ECMWF CARRYING THE ENERGY INVOF THE WESTERN OKLAHOMA/NORTHERN TEXAS PANHANDLES. AT LEAST THE ECMWF IS ACKNOWLEDGING THE FACT THAT A TREMENDOUS AMOUNT OF WESTERLY SHEAR OVER CENTRAL CALIFORNIA AND NEVADA WILL BE 'SHAVING OFF' THE NORTHERN PERIPHERY OF NORBERT'S MID-LEVEL MOISTURE SHIELD. BY 10/12Z...THIS WEST COAST UPPER-LEVEL/NORTHERN STREAM TROUGH AXIS WILL BE MIGRATING THROUGH SOUTHERN UTAH AND THE FOUR CORNERS...ENDING THE EFFECTS OF WESTERLY SHEAR UPON 'NORBERT'. FROM FOUR DAYS AGO...THE MODEL CONTENTION SURROUNDED HOW THE NORTHERN STREAM (VALID FOR THE 8/00Z TO 9/00Z) TROUGH WOULD BE CHARACTERIZED INVOF NORTHERN CALIFORNIA AND THE PACIFIC NORTHWEST. TO ME ANYWAY...THE 5/00Z RUNS SEEM TO HAVE ESSENTIALLY CAPTURED THE FLOW CORRECTLY FROM A MASS FIELD PERSPECTIVE OVER THE NORTHWEST QUARTER OF THE NATION. OVER THE HIGHER LATITUDES...THE MODEL CONSENSUS WAS SUFFICIENT IN CAPTURING THE DETAILS OF THE ALBERTA/EASTERN BC SHORTWAVE CARVING INTO THE BASE OF THE UPPER-LEVEL TROUGH BEYOND DAY 5...WITH A CLOSED 500MB FEATURE EMERGING OVER SOUTHERN MANITOBA AROUND DAY 6. THE 5/00Z CANADIAN IS ALMOST A DAY (24 HOURS) QUICKER WITH ITS FORMATION AND THAT SEEMED OUT OF PLACE GIVEN THE ANTICIPATED SUBTROPICAL PLUME STREAKING ALONG THE BASE OF THE TROUGH. DOWNSTREAM ACROSS EASTERN CANADA TO MID-ATLANTIC CONUS...THE 5/00Z GFS/ECMWF SEEMED TO BE THE MORE REASONABLE SOLUTIONS WITH THE DEMISE OF THE SURFACE RIDGE HUGGING THE NORTH AMERICAN COASTLINE AND ANY SURFACE WAVE DEVELOPMENT OVER THE GULF STREAM. WITH THESE MODEL HIGHLIGHTS AND KEY FEATURES DISCUSSED...THE 5/00Z GEFS/ECENS--BLENDED AT A 50/50 RATIO---BEST FIT THE KEY FLOW PATTERN FEATURES AND PREVIOUS WPC CHOICES FOR DAYS 3 THROUGH 7. IF I HAD A NOTION TO USE ANY ONE PARTICULAR DETERMINISTIC SOLUTION VERBATIM---AND MAYBE TIL DAY 4--IT'D BE THE 5/00Z GFS. IN GENERAL...THE MEANS HANDLE THE 1024MB + SURFACE HIGH BACK FILLING THE UPPER-LEVEL TROUGH ON DAY 5-6...AND THE SOUTHEASTWARD SPREAD OF THE MUCH COOLER CANADIAN AIRMASS OVER THE CENTRAL PORTION OF THE LOWER 48. ...SENSIBLE WEATHER HIGHLIGHTS... CONSIDERABLY COLDER CANADIAN HIGH PRESSURE MIGRATING SOUTHEASTWARD AND SPREADING OUT ACROSS MUCH OF THE LOWER 48 NEXT WEEK. AN UNSTABLE AND TURBULENT UPSLOPE FLOW COMPONENT APPEARS TO RETURN IN EARNEST ALONG THE DIVIDE AND EASTERN SLOPES FROM CUTBANK TO DENVER. RESIDUAL 'NORBERT' MID-LEVEL MOISTURE WILL BE INFLUENTIAL IN GENERATING A VARIED OVER-RUNNING ENVIRONMENT ALONG/AHEAD OF THE MAIN FRONT. DOWNSTREAM...WPC PROGS HOLD ONTO A ERN US SURFACE WEDGE INTO MIDWEEK AND ALLOW A MOIST WAVE TO CARRY DOWNWIND INTO THE WRN ATLANTIC. THIS ALLOWS COOLER AND WETTER WEATHER TO LINGER MAINLY FROM SERN US AND MID-ATLANTIC COAST LINE INTO NEW ENGLAND PRIOR TO LATER NEXT WEEK UPSTREAM FLOW AMPLIFICATION AND FRONTAL APPROACH. VOJTESAK