EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1113 AM EDT TUE SEP 06 2016 VALID 12Z FRI SEP 09 2016 - 12Z TUE SEP 13 2016 ...OVERVIEW AND GUIDANCE ASSESSMENT/PREFERENCES... MODEL ENSEMBLES ALL SHOW A PROGRESSIVE SHORTWAVE SYSTEM MOVING ACROSS THE NORTHERN TIER OF THE US ON FRIDAY THROUGH SUNDAY/DAYS 3 TO 5). TODAYS FORECASTS APPEAR SLIGHTLY SLOWER THAN LAST NIGHT BUT ARE STILL CONSISTENT WITH A SIGNIFICANT SHORTWAVE/COLD FRONT PASSAGE FROM THE WEST CENTRAL TO EASTERN UNITED STATES VERY MUCH IN KEEPING WITH THE APPROACH OF AUTUMN. THIS SYSTEM LIFTS EAST NORTHEASTWARD ACROSS SOUTHEASTERN CANADA BY BY DAY 6/MONDAY WHILE ANOTHER SIGNIFICANT SHORTWAVE TROUGH MOVES ACROSS SOUTHWESTERN CANADA AND THE NORTHWEST US ON SUNDAY/DAY 5. THIS NEXT SYSTEM APPEARS TO SPLIT INTO TWO DISTINCT SHORTWAVES...ONE MORE PROGRESSIVE THAT MOVES NORTHEASTWARD ACROSS THE US/CANADA BORDER DRAGGING ANOTHER SIGNIFICANT COLD FRONT ACROSS THE ROCKIES/NORTHERN PLAINS AND UPPER MISSISSIPPI VALLEY BY EARLY TUESDAY/DAY 7. ANOTHER SHORTWAVE TROUGH SPLITS MOVING MORE SOUTHWARD ACROSS THE WESTERN UNITED STATES...CARVING A TROUGH OVER THE WESTERN US WHILE HEIGHTS REMAIN RELATIVELY HIGH ACROSS THE EASTERN US. EARLY IN THE FORECAST...ALL THE OVERNIGHT OPERATIONAL MODELS ARE QUITE SIMILAR TO EACH OTHER WITH NO OBVIOUS OUTLIER ON DAYS 3 AND 4...WITH THE ONLY SIGNIFICANT DIFFERENCES BEING THE EXACT LOCATION OF A SURFACE LOW OVER THE MIDWEST. THOSE DIFFERENCES INCREASE ON DAY 5/SUNDAY WITH THE LOCATION/STRENGTH OF THE SURFACE LOW THAT HAS NOW MOVED INTO QUEBEC. HOWEVER...DESPITE THE DIFFERENCES...THE RETREATING COLD FRONT AND SURFACE HIGH BEHIND THE SYSTEM MATCHING VERY WELL WITH THE ENSEMBLE MEANS. WITH REGARD TO THE NEXT SYSTEM/SYSTEMS MOVING ACROSS THE NORTHWEST/DOWN THE WEST COAST...THE OPERATIONAL RUNS MATCH WELL WITH THE MEANS FROM THE NORTHWEST ACROSS THE PLAINS ON DAY 5/SUNDAY THROUGH DAY 6/MONDAY WITH THE ONLY SOMEWHAT SIGNIFICANT DIFFERENCES OCCURRING NEAR THE MAIN SURFACE LOW PRESSURE AREA THAT IS LOCATION NEAR THE BORDER OF MANITOBA AND ONTARIO. OTHERWISE...THE PLUNGE OF COOLER AIR ACROSS THE NORTHWEST INTO THE NORTHERN ROCKIES IS HANDLED QUITE SIMILARLY BY THE 00Z ECMWF AND THE 06Z GFS. BEYOND DAY 6...DIFFERENCES ACROSS CANADA WITH THE SPEED OF THE SHORTWAVE SYSTEM MOVING EASTWARD ACROSS QUEBEC START BECOMING SIGNIFICANT BUT THE RESULTANT FORECAST OF THE COLD FRONT MOVING EASTWARD DOWN THE EASTERN SLOPES OF THE ROCKIES AND ACROSS THE PLAINS STATES TOWARD THE MIDWEST ARE STILL QUITE REASONABLE IN THE GFS/ECMWF COMPARED TO THE MEANS. AS A RESULT...A PRETTY STANDARD BLENDING OF FORECAST MODELS INCLUDING THE 00Z ECMWF/06Z GFS AND THE CORRESPONDING ENSEMBLE MEANS ARE USED THROUGHOUT THE FORECAST...WITH THE USUAL PREFERENCES GIVEN TO THE OPERATIONAL RUNS EARLIER IN THE FORECAST AND TO THE MEANS IN THE LATTER FORECASTS...WITH THE OPERATIONAL FORECASTS PROVIDING INSIGHT INTO SEA LEVEL PRESSURE MINIMA/MAXIMA POSSIBLE SMOOTHED IN THE BLENDING PROCESS. ...WEATHER HIGHLIGHTS/THREATS... VERY WARM/HOT TEMPERATURES WILL START THE FORECAST IN THE EAST BEFORE THE COLD FRONT PROGRESSES EASTWARD FROM THE MISSISSIPPI VALLEY ON DAY 3/FRIDAY ACROSS THE OHIO VALLEY TO SOUTHERN PLAINS ON DAY 4/SATURDAY TIL IT REACHES THE EAST COAST ON SUNDAY/DAY 5. RESIDUAL MOISTURE FROM FORMER TROPICAL CYCLONE NEWTON WILL BE DRAWN INTO THE FRONTAL SYSTEM CROSSING THE MISSISSIPPI VALLEY AND MIDWEST BY FRIDAY. SHOWERS AND THUNDERSTORMS WILL ACCOMPANY THE FRONT ACROSS THE MIDWEST TO THE SOUTHERN PLAINS ON SATURDAY/DAY 5 WHILE THE FRONT PRESENTLY APPEARS TO CROSS THE EAST COAST ON SUNDAY MORNING/DAY 6...A DIURNAL MINIMUM FOR CONVECTIVE PRECIPITATION ALTHOUGH CONVECTION WILL FLARE ALONG THE FRONT ALONG THE GULF COAST LATER ON SUNDAY AFTERNOON TO EVENING. COOLER AND FAIR WEATHER WILL CROSS THE MIDWEST AND NORTHEAST LATER SUNDAY INTO MONDAY AND TUESDAY/DAYS 5 THROUGH 7 AND THE SURFACE HIGH MOVES EASTWARD AND THEN RETREATS EASTWARD OVER THE WESTERN ATLANTIC. THE NEXT SERIES OF TROUGHS/FRONTS CROSSES THE WESTERN HALF OF THE US STARTING ON SUNDAY AND CONTINUES SOUTH AND EASTWARD ACROSS THE WEST/THE ROCKIES AND THE PLAINS/UPPER MISSISSIPPI VALLEY BY TUESDAY MORNING/DAY 7. COLD RAIN WILL DEVELOP ACROSS THE NORTHERN ROCKIES WITH EVEN SOME HIGH ELEVATION SNOW AND THE TWO TROUGHS SPLIT ACROSS THE WEST AND AS COOLER AIR MOVES SOUTH AND EASTWARD FROM SOUTHWESTERN CANADA ACROSS THE NORTHWEST AND ROCKIES. SOME UPSLOPE RAINFALL FROM MONTANA SOUTHWARD ACROSS WYOMING AND EVEN INTO COLORADO WILL OCCUR...ANOTHER SIGN THAT AUTUMN IN COMING. KOCIN