EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1139 AM EDT THU SEP 08 2016 VALID 12Z SUN SEP 11 2016 - 12Z THU SEP 15 2016 ...OVERVIEW AND GUIDANCE ASSESSMENT/PREFERENCES... ONE UPPER TROUGH RETREATS NORTHEASTWARD ACROSS SOUTHEASTERN CANADA ON SUNDAY/DAY 3...DRAGGING A COLD FRONT THAT ENDS THE RECENT PERIOD OF HEAT IN THE EAST WITH A BUILDING HIGH PRESSURE SYSTEM THAT BRINGS A MODEST COOLDOWN. THE SURFACE HIGH THEN MOVES EASTWARD FROM THE MIDWEST ON DAY 3 TO THE NORTHEAST ON DAY 4/MONDAY AND THEN OFF THE SOUTHERN NEW ENGLAND COAST BY DAY 5/TUESDAY. THE NEXT TROUGH MOVES SOUTHEAST FROM SOUTHWESTERN CANADA AND SPLITS INTO TWO DISTINCT SHORT WAVES...ONE WHICH DIGS SOUTHWARD ACROSS THE WESTERN UNITED STATES AND APPEARS TO HAVE GOOD OPERATIONAL AND ENSEMBLE AGREEMENT THROUGH THE END OF THE FORECAST PERIOD THE NEXT FRONTAL SYSTEM CROSSES THE COUNTRY FROM DAYS 3/SUNDAY TO DAY 7/THURSDAY...CROSSING THE NORTHWEST ON SUNDAY/DAY 3 WITH MUCH COOLER AIR DRIVING SOUTHWARD ACROSS THE NORTHERN ROCKIES INTO THE NORTHERN PLAINS ON MONDAY/DAY 4. HIGH PRESSURE BUILDS EASTWARD ACROSS THE PLAINS AND MIDDLE TO UPPER MISSISSIPPI VALLEY ON TUESDAY/DAY 5 WHILE THE COLD FRONT CONTINUES EASTWARD ACROSS THE MIDWEST AND OHIO VALLEY AND SOUTHWARD ACROSS THE SOUTHERN PLAINS ON WEDNESDAY/DAY 6. THE COLD FRONT THEN CROSSES THE EAST COAST BY DAY 7/THURSDAY WITH A LARGE COOL HIGH PRESSURE SYSTEM BUILDING ACROSS THE MIDWEST. WHILE THERE IS SIGNIFICANT MODEL AGREEMENT THROUGH THE FIRST 3 DAYS OF THE FORECAST...SOME SIGNIFICANT DIFFERENCES BEGIN TO APPEAR ON DAY 5/TUESDAY WITH THE SPEED OF A SHORTWAVE TROUGH MOVING EASTWARD ACROSS SOUTHERN CANADA...WITH THE LATEST OPERATIONAL ECMWF THE SLOWEST AND THE GFS THE FASTEST WITH THE CANADIAN/UKMET IN BETWEEN. THE ISSUE WITH THESE DIFFERENCES WILL ULTIMATELY BE ON HOW FAST THE COLD FRONT ASSOCIATED WITH THIS SYSTEM PROGRESSES EASTWARD ACROSS THE NORTH CENTRAL TO NORTHEAST TIER OF STATES IN THE LAST HALF OF THE FORECAST. GIVEN THE KNOWN FAST BIAS OF THE GFS...HAVE RELIED MORE HEAVILY ON THE ECMWF/ECMWF MEANS IN THE DURING THIS PERIOD WITH THE FRONT MOVEMENT FAIRLY CONSISTENT WITH THE OVERNIGHT FORECAST. ...WEATHER HIGHLIGHTS/THREATS... COOLER WEATHER ACROSS THE MIDWEST AND NORTHEAST WILL START THE PERIOD ALTHOUGH TEMPERATURE ANOMALIES ARE SMALL...BUT TEMPERATURES AND HUMIDITY WILL BE LOWER THAN AT PRESENT. WHILE A WARMUP WILL INVARIABLY FOLLOW AS THE HIGH DRIFTS OFFSHORE ESPECIALLY BY MIDWEEK...THE NEXT FRONTAL SYSTEM APPEARS TO BE A QUITE SIGNIFICANT EARLY AUTUMNAL PUSH OF COOL AIR WITH MAXIMUM TEMPERATURE DEPARTURES OF 10 TO 20 DEGREES BELOW AVERAGE ACROSS THE NORTHERN ROCKIES AND PLAINS ON MONDAY INTO TUESDAY. UPSLOPE RAINFALL AND HIGH ELEVATION SNOWFALL ACROSS THE NORTHERN ROCKIES WILL GREET THIS COOL AIR SURGE. AS THE FRONT MAKES ITS WAY EASTWARD...SOME OF THE HEAVIEST RAINFALL IS EXPECTED FROM THE SOUTHERN TO CENTRAL PLAINS/MID MISSISSIPPI VALLEY WITH HEAVIER SHOWERS COMMON ALONG THE WESTERN GULF COAST AND OVER CENTRAL AND SOUTHERN FLORIDA. SHOWERS AND THUNDERSHOWERS WILL ACCOMPANY THE PASSAGE OF THE COLD FRONT AS IT MOVES FROM THE CENTER OF THE NATION TO THE EAST COAST. KOCIN