EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1159 AM EDT MON SEP 25 2017 VALID 12Z THU SEP 28 2017 - 12Z MON OCT 02 2017 ...OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES... THE EARLY/MIDDLE PART OF THE FORECAST CURRENTLY SHOWS DECENT CLUSTERING OF SIGNIFICANT FEATURES BUT WITH TRENDS IN SOME GUIDANCE OVER THE PAST 24-36 HOURS. UNCERTAINTIES REGARDING PACIFIC TROUGH EVOLUTION TOWARD 160-170W LONGITUDE AROUND DAYS 4-5 FRI-SAT LEAD TO A RAPID INCREASE OF MODEL/ENSEMBLE SPREAD AND RUN TO RUN VARIABILITY FROM THE EASTERN PACIFIC ACROSS THE LOWER 48 BY DAYS 6-7 SUN-MON. FROM THU INTO THE WEEKEND EXPECT SOUTHERN CANADA ENERGY TO AMPLIFY A TROUGH THAT NEARS THE EAST COAST WHILE AN EASTERN PACIFIC TROUGH WILL APPROACH/REACH THE WEST COAST. THE EASTERN TROUGH HAS EXHIBITED A FASTER TREND IN SOME GUIDANCE, INCLUDING THE 06Z GFS RELATIVE TO ITS 00Z RUN. ECMWF RUNS HAVE BEEN GRADUALLY TRENDING AWAY FROM EARLIER RUNS THAT HAD BEEN MUCH SLOWER AND MORE CLOSED. BY LATE FRI-SAT THE 00Z GFS HOLDS ONTO A SLOWER/STRONGER SURFACE LOW OVER THE NORTHEAST COMPARED TO MOST OTHER SOLUTIONS. MEANWHILE THERE HAS BEEN A STRONGER TREND WITH THE UPPER TROUGH REACHING THE WEST ALONG WITH SOME MODERATE DIFFERENCES IN AMPLITUDE BY SAT. THIS TROUGH WILL HELP TO EJECT UPPER LOW ENERGY EXPECTED TO CLOSE OFF OVER THE SOUTHWEST DURING THE SHORT RANGE. OVERALL PREFER AN INTERMEDIATE SOLUTION INCORPORATING THE MOST AGREEABLE ASPECTS OF THE 00Z-06Z GFS/00Z ECMWF AND TO A MINOR EXTENT 06Z GEFS/00Z ECMWF MEANS, AS THE BEST WAY TO NUDGE CONTINUITY TOWARD LATEST GUIDANCE IDEAS. AROUND FRI-SAT ENERGY DROPPING SOUTH FROM THE BERING SEA MAY INTERACT WITH A MID-LATITUDE PACIFIC TROUGH. BEYOND THE QUESTION OF HOW MUCH INTERACTION OCCURS, THERE IS ALSO RAPIDLY INCREASING MODEL/ENSEMBLE SPREAD WITH RESPECT TO HOW MUCH ENERGY MAY GET PULLED OFF THE SOUTHERN PART OF THE OVERALL TROUGH. THESE ISSUES ULTIMATELY LEAD TO MAJOR DIFFERENCES/RUN TO RUN CHANGES FROM THE EASTERN PACIFIC INTO THE LOWER 48. FOR EXAMPLE BY DAY 7 MON 00Z GFS/ECMWF RUNS FROM 24 HOURS AGO SUGGESTED DIFFERING DEGREES OF EASTERN PACIFIC TROUGH/WESTERN U.S. RIDGE WHILE 12Z/24 RUNS FLIPPED TO AN EASTERN PACIFIC RIDGE, AND ARE NOW BACK TO A STRONG EASTERN PACIFIC TROUGH. THE PREVAILING ENSEMBLE MEAN SCENARIO LEANS TOWARD AN EASTERN PACIFIC TROUGH THAT SUPPORTS A LEADING NORTHEAST PACIFIC SURFACE SYSTEM BUT ONE THAT IS SOMEWHAT NORTH OF THE 06Z GFS. THESE DIFFERENCES TRANSLATE DOWNSTREAM INTO A WIDENING GUIDANCE SPREAD FOR WHAT HAPPENS WITH UPPER TROUGH ENERGY CROSSING THE WEST DURING THE WEEKEND. SELECTED OPERATIONAL MODEL SOLUTIONS BY LATE DAY 7 MON RANGE FROM THE SOUTHERN HIGH PLAINS (00Z GFS/CMC) TO THE MID MS VALLEY (00Z ECMWF) OR NORTHWESTERN GREAT LAKES (06Z GFS). D+8 MULTI-DAY MEANS AGREE FAIRLY WELL WITH A CORE OF POSITIVE HEIGHT ANOMALIES CENTERED NEAR JAMES BAY, WITH TELECONNECTIONS SEEMING TO RECOMMEND LEANING TOWARD SOLUTIONS THAT HAVE HIGHER HEIGHTS OVER THE EAST. THE 06Z GFS COULD BE PROGRESSIVE ENOUGH WITH ITS LATE PERIOD CENTRAL U.S. ENERGY TO FIT INTO THAT THEME FOR A MULTI-DAY AVERAGE AND THE SLOWER 00Z ECMWF SEEMS MORE QUESTIONABLE, BUT EITHER DIFFERS TOO MUCH FROM THE ENSEMBLE MEANS TO HAVE ANY CONFIDENCE LATE IN THE FORECAST. GIVEN THE INCREASING FORECAST UNCERTAINTY AND ERRATIC NATURE OF RECENT OPERATIONAL RUNS, FORECAST PREFERENCE GOES FROM AN EVEN MODEL/MEAN BLEND DAY 6 SUN TO THE ENSEMBLE MEANS EXCLUSIVELY BY DAY 7 MON. ...WEATHER HIGHLIGHTS... SOME ENHANCED PRECIP MAY PERSIST INTO LATE WEEK OVER THE SOUTHERN HALF OF THE ROCKIES/SOUTHERN PLAINS AS SOUTHWEST/GREAT BASIN ENERGY ALOFT EJECTS NORTHEASTWARD AND HIGH PRESSURE OVER THE CENTRAL HIGH PLAINS PROMOTES LOW LEVEL UPSLOPE FLOW AT SOME LOCATIONS. MOIST FLOW NEAR AHEAD OF A SLOW MOVING FRONT MAY BRING PERIODS OF MODERATE-HEAVY RAINFALL TO THE FLORIDA PENINSULA. MEANWHILE A WEAKENING WAVE WILL LIKELY BRING A PERIOD OF LIGHT-MODERATE PRECIP TO THE GREAT LAKES/NORTHEAST AND POSSIBLY NORTHERN MID ATLANTIC THU-SAT. THE NORTHWEST WILL SEE SOME MODEST ACTIVITY WITH THE UPPER TROUGH REACHING THE WESTERN STATES BY THE WEEKEND. THIS TROUGH AND EJECTING SOUTHWEST ENERGY AHEAD OF IT SHOULD BRING SOME RAINFALL INTO THE PLAINS BY THE WEEKEND BUT DECREASING PREDICTABILITY IN SPECIFICS OF FLOW ALOFT REDUCES CONFIDENCE IN COVERAGE/INTENSITY/TIMING. LIKEWISE CONFIDENCE IS CURRENTLY BELOW AVERAGE REGARDING HOW MUCH MOISTURE REACHES THE NORTHWEST SUN-MON. THE MOST EXTREME TEMPS SHOULD BE FOR BELOW NORMAL HIGHS WITH CLOUDS/RAIN OVER THE SOUTHERN HALF OF THE HIGH PLAINS DURING THE FIRST HALF OF THE PERIOD. THE NORTHWEST WILL BE QUITE WARM ON THU BEFORE MODERATING TO NEAR NORMAL LEVELS THEREAFTER. RAUSCH