EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1154 AM EDT THU SEP 01 2016 VALID 12Z SUN SEP 04 2016 - 12Z THU SEP 08 2016 ...OVERVIEW AND GUIDANCE/UNCERTAINTY ASSESSMENT... TWO AREAS OF PRIMARY FOCUS EXIST FOR THE MEDIUM RANGE, HERMINE WHICH IS FORECAST BY NHC TO BE EXTRATROPICAL BY DAY 3, AND A POTENTIAL MULTI-DAY CONVECTION/HEAVY RAIN EVENT OVER PORTIONS OF THE NORTHERN PLAINS AND MIDWEST. MODEL SOLUTIONS HAVE SHOWN A FAIR AMOUNT OF RUN-TO-RUN CONSISTENCY WITH THE PATTERN ACROSS THE NORTHERN-TIER, WITH TRENDS BEING GRADUAL AND RELATIVELY SMALL. ON THE OTHER HAND, SOLUTIONS HAVE VARIED SIGNIFICANTLY FROM RUN-TO-RUN WITH RESPECT TO HERMINE DUE TO THE COMPLEX EXTRATROPICAL TRANSITION AND SUBSEQUENT POSITION BETWEEN TWO RIDGES. WITH RESPECT TO EXTRATROPICAL HERMINE, A BLEND OF THE 00Z ECMWF/ECMWF ENSEMBLE MEAN AND THE 06Z GEFS MEAN RESULTED IN A REASONABLE FORECAST STARTING POINT, WITH MANUAL ADJUSTMENTS TO MATCH THE OFFICIAL NATIONAL HURRICANE CENTER FORECAST. THIS FORECAST PLACES EXTRATROPICAL HERMINE OFF THE MID-ATLANTIC COASTLINE ON DAY 3, AND KEEPS THE SYSTEM SLOWLY MOVING/MEANDERING OFFSHORE THROUGH DAY 7. ELSEWHERE, MODEL/ENSEMBLE SOLUTIONS HAVE TRENDED A BIT SLOWER AND MORE AMPLIFIED WITH THE LARGE-SCALE TROUGH ACROSS THE WESTERN U.S. ON DAYS 3-6. BY DAY 7 SOME SPREAD EMERGES WITH THE 06Z GFS AND GEFS MEMBERS KEEPING THE TROUGH SLOWER AND MORE AMPLIFIED RELATIVE TO THE 00Z ECMWF/ECMWF ENSEMBLE MEMBERS. ADDITIONAL SHORTWAVE ENERGY IS FORECAST BY THE MODEL CONSENSUS TO ENTER THE NORTHWESTERN U.S. ON DAY 7, WITH RELATIVELY SMALL DIFFERENCES IN THE TIMING AND AMPLITUDE OF THE FEATURE. A BLEND OF THE 00Z ECMWF/ECMWF ENSEMBLE MEAN ALONG WITH THE 06Z GEFS WAS USED AS A STARTING POINT FOR THE WPC FORECAST ACROSS THESE AREAS. THE FORECAST TRENDED MORE TOWARD THE ENSEMBLE MEANS BY DAYS 6 AND 7 AS SPREAD INCREASES WITH THE WESTERN U.S. TROUGH. ...WEATHER HIGHLIGHTS/THREATS... SOME AREAS ALONG THE CENTRAL/NORTHERN EAST COAST COULD SEE A PERIOD OF STRONG WINDS AND RAINFALL ASSOCIATED WITH EXTRATROPICAL HERMINE. FARTHER WEST, MULTIPLE EPISODES OF HEAVY RAINFALL AND POTENTIALLY STRONG CONVECTION ARE POSSIBLE FROM THE NORTHERN PLAINS INTO THE UPPER MIDWEST. CONVECTIVE ACTIVITY OVER THESE REGIONS WILL LIKELY PERSIST THROUGH MULTIPLE DAYS DUE TO THE SLOW (AND TRENDING SLOWER) MOVEMENT OF THE LARGE-SCALE UPPER TROUGH ACROSS THE WEST, AND RELATIVELY LITTLE MOVEMENT FROM DAY TO DAY OF THE SURFACE FRONTAL BOUNDARY ACROSS THE PLAINS AND MIDWEST. THE EXACT TIMING AND PLACEMENT OF CONVECTION WILL BE GOVERNED LARGELY BY RELATIVELY SMALL-SCALE MID/UPPER-LEVEL SHORTWAVES WITH LOW PREDICTABILITY IN THE MEDIUM RANGE, AS WELL AS MESOSCALE CONVECTIVE/THERMODYNAMIC PROCESSES. ADDITIONALLY, A PERIOD OF UPSLOPE FLOW MAY PROMOTE A PERIOD OF ENHANCED RAINFALL POTENTIAL ACROSS MONTANA. ANY POSSIBLE EFFECTS FROM HERMINE ON THE EAST COAST COULD RESULT IN A PERIOD OF BELOW AVERAGE TEMPERATURES EARLY IN THE MEDIUM RANGE, BUT TEMPERATURES WILL WARM ACROSS THE ENTIRE EASTERN HALF OF THE NATION THROUGH DAY 4-7 PERIOD AS MID/UPPER-LEVEL RIDGING BUILDS. BY MID-WEEK, HIGH TEMPERATURES ARE FORECAST TO BE 10 TO 15 DEGREES ABOVE AVERAGE ACROSS MUCH OF THE EASTERN U.S. CONVERSELY, ACROSS THE WEST, TROUGHING WILL RESULT IN BELOW AVERAGE TEMPERATURES THROUGH THE MEDIUM RANGE. THE CORE OF THE NEGATIVE TEMPERATURE ANOMALIES IS FORECAST TO BE FROM THE NORTHERN ROCKIES INTO THE NORTHERN HIGH PLAINS OF MONTANA WHERE HIGH TEMPERATURES EARLY NEXT WEEK MAY BE 10 TO 20 DEGREES BELOW AVERAGE. RYAN