EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 131 AM EDT MON SEP 07 2015 VALID 12Z THU SEP 10 2015 - 12Z MON SEP 14 2015 ...OVERVIEW... PREVAILING UPPER-LEVEL RIDGE PATTERN IN THE EAST---BEGINS TO BREAK DOWN DURING THE FIRST HALF OF THE FORECAST PERIOD AS HIGH-LATITUDE NORTHERN STREAM TROUGH FROM ALASKA/WESTERN CANADA (WITH REMNANTS OF IGNACIO) RACES QUICKLY ACROSS CANADA. THE SOUTHERN END OF THIS TROUGH---SWEEPS A 'FRESH' CANADIAN AIRMASS ACROSS THE EASTERN THIRD OF THE LOWER 48---AND LIFTS A WEAK MID-LEVEL DISTURBANCE CURRENTLY IN THE SOUTHEAST---NORTHEASTWARD ALONG THE EASTERN SEABOARD. IN THE WEST...A 'POST-IGNACIO' RESPONSE TO THE POST-FRONTAL TROUGH MIGRATION ALONG/NORTH OF 50N LATITUDE WILL BE TO BUILD A DEEP-LAYERED EAST PACIFIC RIDGE INTO THE PACIFIC NORTHWEST---WITH PORTIONS OF THE WASHINGTON...OREGON AND CALIFORNIA COAST AND COASTAL RANGES BECOMING ENTRENCHED IN A VERY WARM...AND LOCALIZED---OFFSHORE FLOW PATTERN. FROM THE CONTINENTAL DIVIDE TO THE MISSISSIPPI RIVER---GENERALLY A DRY...SEASONAL SEPTEMBER PATTERN WITH PACIFIC HIGH PRESSURE MIGRATING ACROSS THE ROCKIES---AND A PREVAILING WIND THAT HAS A DRY DOWNSLOPE WEST TO NORTHWEST FLOW COMPONENT. THE ONLY THREAT OF RAIN TIED TO A WEAK MID-LEVEL DISTURBANCE AND LEE TROUGH ORIGINATING IN SOUTHEAST ALBERTA AND MIGRATING DOWNSTREAM IN THIS WEST-TO-NORTHWEST FLOW. EXPECT THE LEE TROUGH TO 'FORM' AND DROP SOME OF THE LOWS IN THE 'BELOW NORMAL' CATEGORY. ...MODEL PREFERENCES/UNCERTAINTY ASSESSMENT... THROUGH DAY 4 (12/00Z)...THE 6/12Z DETERMINISTIC GFS/ECMWF AND THEIR ENSEMBLE MEANS LOOKED LIKE SOLID PIECES OF GUIDANCE WITH THE PRIMARY SYNOPTIC-SCALE FEATURES AND MASS FIELD FORECASTS ALONG/NORTH OF 50N LATITUDE. EVEN TO 12/12Z---BOTH DETERMINISTIC RUNS SEEM TO CAPTURE THE NATURE OF THE DEEP CYCLONIC FLOW PATTERN EMERGING OVER NORTH CENTRAL CANADA ALONG 100W INVOF 56N. THEN... A 60/40 RATIO OF THE ECENS/GEFS MEANS FOR THE LATTER HALF OF THE FORECAST PERIOD TO ACCOUNT FOR THE FACT THAT THE NORTHERN JET STREAM AXIS OVER SOUTHERN ALASKA HAS AN 'AUTUMN' PROGRESSIVENESS LOOK TO IT---WITH EMBEDDED SHORTWAVE ENERGY THAT ATTEMPTS TO FILL IN BEHIND THE DEEPENING CYCLONE. FOR THE LOWER 48--- ALONG AND SOUTH OF 40N LATITUDE---THE ECENS/GEFS MEAN CONTINUE TO 'SUPPLANT' THE UPPER-LEVEL RIDGE IN THE EAST BUT THEIR DETERMINISTIC RUNS CONTINUE TO VARY---WITH HOW TO HANDLE THE WEAK MID-LEVEL CIRCULATION MEANDERING ACROSS THE SOUTHEAST (AND CURRENTLY TRAPPED TO THE SOUTH OF THE UPPER-LEVEL RIDGE). NOW THAT THE TWO MODELS HAVE COME INTO BETTER AGREEMENT WITH THE LOW-LEVEL FRONTAL PROGRESSION AND DEPTH OF THE CANADIAN AIRMASS INTRUSION INTO THE GREAT LAKES AND CENTRAL MIDWEST---THE QUESTION IS HOW THE ATMOSPHERE WILL 'EVACUATE' THE 'WARM' RIDGE AND WHAT THE WEAK CIRCULATION'S 'ROLE' WILL BE IN SLOWING DOWN THE SOUTHEASTWARD MIGRATION OF THE COLD FRONT. THE GFS CONTINUES TO BE A 'SLOWER' SOLUTION---AND THE ECMWF (THOUGH FAR LESS AGGRESSIVE VERSUS 24 HOURS)--- STILL WANTS TO FASHION SOME SORT OF LOW-LEVEL CYCLOGENESIS ALONG THE ONTARIO/QUEBEC BORDER ON DAY 4 AND TRACK IT ACROSS NEW ENGLAND ON DAY 5. IN BOTH SCENARIOS---THE WEAK CIRCULATION OVER THE SOUTHEAST WILL ATTEMPT TO DRIFT NORTHEASTWARD AHEAD OF THE FRONT---WITH ENHANCED RAINFALL POTENTIAL FROM THE SOUTHERN APPALACHIANS/PIEDMONT TO THE DELMARVA AND SOUTHERN NEW ENGLAND. PREFER TO USE THE TWO MEANS AT A 30/30 RATIO AND THE TWO DETERMINISTIC RUNS AT 20/20...FOR NOW---TO FORMULATE THE QPF FROM 10/12Z TO 12/12Z---AND TRY TO FOCUS THE HIGHER QPFS ALONG THE COLD FRONT. ALONG THE WASHINGTON/BRITISH COLUMBIA BORDER--- THE 6/12Z DETERMINISTIC ECMWF/GFS MAY NOT BE MUCH HELP ON/AFTER 12/00Z WITH THE AMPLITUDE OF A NW-TO-SE MOVING SHORTWAVE AND ITS INCORPORATION IN THE FLOW ALONG THE SOUTHWEST QUADRANT OF THE UPPER-LEVEL CIRCULATION FORMING OVER NORTH CENTRAL CANADA. NOTING THAT THE AMPLITUDE OF THE 250MB FLOW ALONG 130W AND NORTH OF 50N LATITUDE---THE 6/12Z ECMWF AND GFS BOTH QUICKLY BECOME OUT-OF-PHASE IN THE GULF OF ALASKA---THOUGH BOTH HAVE A CLOSED 250MB LOW OVER NORTHERN MANITOBA BY 13/12Z (MID-POINT DAY 6). IMPLICATIONS FOR THE NORTHERN ROCKIES?...EITHER ANOTHER NORTHWEST FLOW EVENT EAST OF THE DIVIDE NEXT WEEKEND (SUNDAY) OR NOT. HERE...THE 6/12Z ECENS/GEFS MEANS SHOULD BE A DECENT 'PLACEHOLDER' FOR THE TEMPS/POPS...UNTIL THE WAVE CAN BE RESOLVED. ...SENSIBLE WEATHER HIGHLIGHTS... EXPECT THE GREATER CONCENTRATIONS OF RAINFALL TO DEVELOP ALONG THE SLOW-MOVING FRONT ACROSS THE SOUTHEAST/MID ATLANTIC AND NORTHEAST---WITH BELOW NORMAL TEMPERATURES ANTICIPATED BY NEXT WEEKEND FOR THE GREAT LAKES...OHIO VALLEY AND THE LOWER MS VALLEY. GIVEN THE CURRENT HEAT AND HUMIDITY---THE LOWER TEMPERATURES MAY ACTUALLY BE A WELCOMED SIGHT---NOT NECESSARILY A 'BAD THING'. IN THE WEST...A PACIFIC HIGH (NOT UNUSUALLY STRONG FOR SEPTEMBER STANDARDS)---DOES BUILD EAST ACROSS THE PACIFIC NORTHWEST AND GREAT BASIN---WITH AN OFFSHORE FLOW AND ABOVE NORMAL HIGH TEMPERATURES ALONG THE I-5 CORRIDOR AND COAST. MOST OF THE PERIOD...TEMPERATURES WILL REMAIN ABOVE NORMAL---UPWARDS OF 10-15F ABOVE SEPTEMBER NORMALS IN SOME LOCALES. VOJTESAK