EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 200 AM EDT FRI SEP 04 2015 VALID 12Z MON SEP 07 2015 - 12Z FRI SEP 11 2015 ...OVERVIEW... AN ACTIVE HIGH-LATITUDE JET STREAM WILL PREVAIL AND DOMINATE THE FLOW PATTERN FOR A LARGE PORTION OF THE WESTERN HEMISPHERE. THIS PROMINENT FLOW WILL GRADUALLY 'SUPPLANT' THE PREVAILING UPPER-LEVEL RIDGE OVER THE SOUTHEAST PORTION OF THE CONTINENT. VERY UNUSUAL FLOW PATTERN IN THE SUBTROPICAL PACIFIC---WITH 4 TROPICAL SYSTEMS MEANDERING---IN ONE WAY...SHAPE OR FORM--- BETWEEN 20N AND 40N LATITUDE THIS MEDIUM RANGE PERIOD. SEVERAL OF THESE SYSTEMS WILL ENTRAIN VARYING PORTIONS OF THEIR MOISTURE PLUMES INTO THE MID-LATITUDE FLOW AND PROVIDE SOME SENSIBLE WEATHER CHANGES ACROSS NORTH AMERICA. OVERALL...A 3/12Z ECENS/GEFS AND DETERMINISTIC BLEND OF THE GFS/ECMWF WORKS THROUGH THE MID-POINT (9/12Z) OF THE 'NEW DAY 5'---THEN A 70/30 BLEND OF THE TWO MEANS THEREAFTER WILL KEEP GOOD CONTINUITY AND NOT PRODUCE MUCH 'CHANGE' FOR THE MEDIUM RANGE FORECAST. ....UNCERTAINTY ASSESSMENT... ASIDE FROM THE 3/12Z CANADIAN ENSEMBLE MEAN...THE MODEL ENSEMBLE CONSENSUS IS DEPICTING A BROAD HEMISPHERIC TROUGH AND COOLER UPPER-LEVEL FLOW PATTERN AT/ABOVE 55N LATITUDE FROM THE BERING SEA TO HUDSON BAY AND ATLANTIC CANADA. THIS PATTERN WILL KEEP 'SEPARATION' FROM THE SUBTROPICAL RIDGE PATTERN PREVAILING ACROSS THE SOUTHERN HALF OF THE LOWER 48---WITH THE EXCEPTION OF A SERIES OF MID-LEVEL LONG WAVES PROJECTED TO TRAVEL ALONG THE NORTHERN STREAM AND MIGRATE EASTWARD---STARTING IN THE SHORT RANGE AND CONTINUING THROUGHOUT THIS MEDIUM RANGE PERIOD. THE UNCERTAINTY LIES IN THE ABILITY OF THESE SHORTWAVES TO ENTRAIN SUBTROPICAL MOISTURE AND GRADUALLY WEAKEN THE NORTHERN PERIPHERY OF THE UPPER-LEVEL RIDGE. AND PRIMARILY ACROSS EAST AND SOUTHEAST NORTH AMERICA...THE CHALLENGE WILL BE TO TIME THE EVACUATION OF THE WARM/HUMID AND DRY WEATHER NOW IN PLACE---AND THE LATITUDE WHERE A COOLER...WETTER WEATHER PATTERN CAN 'DISPLACE' THE STAGNANT TROUGH INVOF THE GULF OF MEXICO AND 'MOVE IT' DURING THE MEDIUM RANGE PERIOD. ...MODEL PREFERENCES... ACROSS THE WEST COAST OF NORTH AMERICA...THE NORTHERN STREAM---WITH ORIGINS IN NORTHEAST SIBERIA WILL HAVE ENTRAINED SOME OF 'IGNACIO' (BETWEEN 5/12-7/12Z) PER CPHC/OPC COORDINATION---WITH THE MOISTURE STREAM FOCUSING BETWEEN 55N-60N LATITUDE THROUGH THE END OF THE MEDIUM RANGE PERIOD. THE 3/12Z GEFS/ECENS AND THEIR DETERMINISTIC RUNS REASONABLY HANDLE THE SYSTEM MIGRATION AND SENSIBLE WEATHER---WITH A BROAD WARM ADVECTION SCENARIO FOR THE AK PANHANDLE/BRITISH COLUMBIA AND BRUSHING ACROSS PUGET SOUND (DAY3-4). THIS WARM FRONT AND MODIFIED NORTHEAST PACIFIC MOISTURE PLUME APPEARS TO HAVE SOME 'ROOM' TO EXPAND DOWNSTREAM ALONG AND EAST OF THE DIVIDE TO THE GREAT LAKES...BUT THE DETERMINISTIC RUNS (ALL 4 OF THEM...UK/EC/GFS/CMC) ALL HAVE DIFFERENT 'LOOKS' WITH THE FORWARD SPEED AND AREAL EXTENT OF THE WARM ADVECTION ALONG THE CANADIAN BORDER ASSOCIATED WITH THE MOISTURE PLUME. ACROSS THE DESERT SOUTHWEST...THE TROPICAL MOISTURE PLUME ASSOCIATED WITH THE SYSTEM CURRENTLY KNOWN AS 'KEVIN' WILL BE MIGRATING NORTHWARD---AND PER THE NHC 4/03Z FORECAST ADVISORY...THE SYSTEM IS NOT EXPECTED TO LOOK LIKE THE 3/12Z CANADIAN AND UKMET SOLUTIONS. THAT SAID...A PORTION OF ITS MOISTURE PLUME SHOULD BE LIFTING NORTHEASTWARD AHEAD OF THE DAY 2-3 TROUGH...AND THE PREVIOUS WPC FORECAST PHILOSOPHY AND MODEL CHOICES WERE CARRIED FORWARD BY THE 3/12Z ECENS/GEFS MEANS. NOT SURE THE DETAILS OF THE 3/12Z GFS/ECMWF DETERMINISTIC RUNS (AS PREFECT PROGS) CAPTURE THE INTERACTION BETWEEN THE MID-LATITUDE AND SUBTROPICAL MOISTURE/ENERGY EXCHANGE BETWEEN 30N-40N ALONG 125W---AT/BEYOND 9/12Z. CAUGHT IN THE 'CROSSFIRE' WITH THE DEVELOPMENTS IN THE PACIFIC...IS THE QUASI-STATIONARY UPPER-LEVEL TROUGH LODGED IN THE SOUTHWESTERN GULF OF MEXICO AND THE DECAYING STATIONARY FRONTAL DRAPE ALONG 30N FROM THE MS DELTA EASTWARD TO THE FIRST COAST OF FLORIDA. THE PREVAILING UPPER-LEVEL RIDGE TO ITS NORTH--- RESPONSIBLE FOR THE WARM/HUMID EARLY-SEPTEMBER WEATHER IN THE EAST---WILL BE SLOW TO BREAK DOWN---WITH A SLOWER-MOVING FRONTAL PROGRESSION. THE 3/12Z ECENS/GEFS MEANS GAVE THE IMPRESSION THAT THE MEAN UPPER-LEVEL TROUGH RETROGRADES VERY SLOWLY WESTWARD THROUGH TIME. BUT THE PATTERN APPEARS TO BE MORE OF A PERSISTENT-TYPE FORECAST BENEATH THE RIDGE (IN THE LOWER LAYERS) ACROSS THE SOUTHEAST. AND ALOFT IN THE WESTERN GULF---LOOKS FAR MORE DEPENDENT UPON WHAT IMPACT THE SUBTROPICAL JET-LET OVER MEXICO AND MOISTURE PLUME FROM 'KEVIN' HAVE ACROSS SOUTH TEXAS AND THE MOISTURE TRANSLATION DOWNSTREAM OVER THE LOWER MISSOURI VALLEY AHEAD OF THE FRONT. OVERALL...THE 3/12Z DETERMINISTIC ECMWF WOULD BE THE FASTER/STRONGER SOLUTION ACROSS THE SOUTHERN PLAINS...LOWER MS VALLEY AND THROUGH THE NORTHEAST WITH THE PROGRESSION OF THE LOW-LEVEL FRONT VERSUS THE 3/12Z GFS. AT THIS POINT...IT SEEMS TO BE A TOSS UP...IE WHICH SOLUTION WILL PREVAIL AND WHETHER THE FRONT CAN ACTUALLY MAKE A STRONG PUSH INTO THE GULF OF MEXICO IN THE DAY6-7 PERIOD. ...SENSIBLE WEATHER HIGHLIGHTS... THE DAY 3-6 FRONTAL PROGRESSION AND SURFACE CYCLONE TRACK APPEAR TO BE THE PRIMARY REGIONS FOR CONVECTIVE PRECIPITATION ACROSS THE PLAINS AND MIDWEST. RESIDUAL MOISTURE FROM 'KEVIN' WILL HAVE THE GREATEST IMPACTS ON THE UPPER RIO GRANDE VALLEY AND FOUR CORNERS/SOUTHERN ROCKIES---PRIOR TO DAY 5---BEFORE THE MOISTURE TRANSLATES DOWNSTREAM INTO THE OZARKS AND MID-MS VALLEY. VOJTESAK