EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1153 AM EDT THU SEP 28 2017 VALID 12Z SUN OCT 01 2017 - 12Z THU OCT 05 2017 ...OVERVIEW... GUIDANCE HAS BEEN CONSISTENT IN DEPICTING A STRONG RIDGE BUILDING OVER THE SOUTHEASTERN HALF OF THE LOWER 48 BUT SENSITIVITY IN DETAILS OF NORTHERN PACIFIC EVOLUTION IN THE LATE WEEK/WEEKEND TIME FRAME HAD BEEN CAUSING MUCH DIFFICULTY IN RESOLVING SPECIFICS WITHIN A WESTERN U.S. MEAN TROUGH. FOR THE MOST PART THESE NORTHERN PACIFIC ISSUES APPEAR TO BE RESOLVED, CONSIDERABLY INCREASING FORECAST CONFIDENCE THROUGH AT LEAST MID-PERIOD. THIS PATTERN WILL FAVOR ACTIVE WEATHER FROM PARTS OF THE ROCKIES INTO THE UPPER GREAT LAKES WITH MUCH BELOW NORMAL TEMPS FROM THE INTERIOR WEST THROUGH THE NORTHERN PLAINS, IN CONTRAST TO DRY AND WARM CONDITIONS OVER MUCH OF THE EAST. THE ONE EXCEPTION WILL BE WITH RAINFALL ALONG THE GULF/SOUTHEAST COASTS. ...GUIDANCE EVALUATION/PREFERENCES... AFTER MULTIPLE DAYS OF MODEL/ENSEMBLE SYSTEMS DIFFERING WIDELY AND FLIPPING BACK AND FORTH BETWEEN SOLUTION CLUSTERS OVER THE NORTHERN/EASTERN PACIFIC AND THEN DOWNSTREAM INTO THE LOWER 48, LATEST GFS/GEFS ADJUSTMENTS HAVE LED TO IMPROVED AGREEMENT IN THE LATEST CYCLE OF GUIDANCE. LATEST CONSENSUS WOULD HAVE NORTHERN TIER ENERGY EJECTING EARLY IN THE PERIOD HELPING TO DEEPEN LOW PRESSURE JUST NORTH OF THE CANADIAN BORDER BY MON WITH SUBSEQUENT NORTHEAST PROGRESSION OF THIS LOW. THEN TRAILING FLOW DIGGING INTO THE WEST WILL LIKELY CLOSE OFF AN UPPER LOW BY DAY 5 TUE. SPREAD INCREASES BY DAYS 6-7 WED-THU AS THE 00Z/06Z GFS RUNS BEGIN TO EJECT THE UPPER LOW FASTER THAN RECENT ECMWF RUNS AND 00Z CMC. GEFS/ECMWF/CMC MEANS MAY BE SOMEWHAT WEAK IN SHOWING A MERE OPEN TROUGH AT THIS TIME FRAME BUT AGREE ON A MORE PERSISTENT FEATURE THAN THE OPERATIONAL GFS RUNS. AMONG GEFS RUNS THE SLOWER 00Z VERSION COMPARED BETTER TO THE ECMWF/CMC MEANS. THE OTHER ISSUE THAT ARISES INVOLVES PROGRESSION OF HEIGHT FALLS INTO/ACROSS EASTERN CANADA AND NORTHEAST U.S. LATE IN THE PERIOD. RECENT RUNS FROM OPERATIONAL MODELS INCLUDING THE GFS/ECMWF HAVE BEEN INCONSISTENT. THE 06Z GFS TRENDED NOTICEABLY SLOWER THAN THE 00Z RUN BUT STILL STRAYS FASTER THAN THE ENSEMBLE MEANS. ECMWF RUNS HAVE ALTERNATED BETWEEN FAST/SLOW TIMING. THE ECMWF MEAN HAS BEEN THE MOST STABLE PIECE OF GUIDANCE OVER THE PAST DAY. WITH THE 06Z GFS COMPARING BETTER TO CONSENSUS FOR SOME DETAILS VERSUS THE 00Z RUN, DAYS 3-5 SUN-TUE STARTED WITH A BLEND BASED MOSTLY ON THE 06Z GFS/00Z ECMWF ALONG WITH MINORITY INPUT FROM THE 00Z UKMET. THEN THE FORECAST ADJUSTED TO THE PAST TWO ECMWF RUNS ALONG WITH COMPONENTS OF CONTINUITY AND 00Z GEFS/ECMWF MEANS. THIS YIELDED MAINTENANCE OF THE WESTERN U.S. CLOSED LOW INTO DAY 7 THU WHILE TEMPERING CONTINUITY CHANGES FOR THE SOUTHEAST CANADA/GREAT LAKES FRONT AT THAT TIME. ...WEATHER HIGHLIGHTS... THE MEAN PATTERN WILL SUPPORT ONE AREA OF ENHANCED PRECIP POTENTIAL FROM THE ROCKIES THROUGH THE NORTHERN-CENTRAL PLAINS INTO UPPER GREAT LAKES. HIGHER ELEVATIONS OF THE NORTHERN ROCKIES SHOULD SEE PERIODS OF SNOW. CURRENTLY THE BEST SIGNAL FOR HIGHEST RAINFALL TOTALS DURING THE FORECAST PERIOD EXTENDS FROM THE CENTRAL PLAINS INTO THE UPPER GREAT LAKES, WITH A NEARLY STATIONARY FRONT EXTENDING NORTHEAST FROM THE CENTRAL HIGH PLAINS PROVIDING A MULTI-DAY LOW LEVEL FOCUS FOR REPEATING/TRAINING ACTIVITY. THE SOUTHEAST COAST/FLORIDA WILL BE ANOTHER REGION OF HEAVY RAINFALL POTENTIAL IN EASTERLY LOW LEVEL FLOW TO THE SOUTH OF STRONG HIGH PRESSURE. EARLY IN THE PERIOD THERE MAY ALSO BE A SURFACE LOW ALONG/NEAR A WEAKENING FRONT. MUCH OF THE GULF COAST MAY SEE PERIODS OF RAIN WITH VARYING INTENSITY. THE REMAINDER OF THE EAST ALONG WITH THE SOUTHWEST SHOULD SEE DRY WEATHER DURING THE PERIOD. IN THE VICINITY OF THE WESTERN MEAN TROUGH ALOFT, COOLEST TEMPS OF AT LEAST 10-20F BELOW NORMAL SHOULD EXTEND FROM THE INTERIOR WEST INTO THE NORTHERN PLAINS WITH SOME HIGHS MORE THAN 20F BELOW NORMAL POSSIBLE OVER MT/WY EARLY NEXT WEEK. MEANWHILE IN THE WARM SECTOR EXPECT THE HIGHEST ANOMALIES OF PLUS 10-20F TO STRETCH FROM APPROXIMATELY THE MID MS VALLEY THROUGH THE GREAT LAKES/OH VALLEY INTO THE NORTHEAST. RAUSCH