EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1213 PM EDT TUE SEP 19 2017 VALID 12Z FRI SEP 22 2017 - 12Z TUE SEP 26 2017 ...OVERVIEW/GUIDANCE EVALUATION/PREFERENCES... THE SYNOPTIC-SCALE PATTERN IS NOT EXPECTED TO CHANGE APPRECIABLY DURING THE PERIOD AS AN ANOMALOUS UPPER TROUGH AFFECTS THE WESTERN HALF OF THE COUNTRY WHILE POSITIVE HEIGHT ANOMALIES CENTER OVER THE EASTERN U.S. THE 00Z GFS 500-MB HEIGHT FORECAST INDICATES DEPARTURES FROM NORMAL ON THE ORDER OF 2.5 TO 3 STANDARD DEVIATIONS BELOW CLIMATOLOGY ACROSS THE SOUTHWESTERN U.S. THIS WEEKEND. EQUALLY IMPRESSIVE...A POWERFUL 588-DM RIDGE IS EXPECTED TO SITUATE OVER THE GREAT LAKES/OH VALLEY AND INTO THE MID-ATLANTIC WHICH WILL ULTIMATELY AFFECT THE PATH OF WHAT IS CURRENTLY HURRICANE JOSE. THE EASTWARD EXTENSION OF THIS RIDGE AXIS WILL THWART AN ESCAPE OUT INTO THE OPEN ATLANTIC. BASED ON THE LATEST NHC FORECAST...THE CIRCULATION SHOULD BECOME POST-TROPICAL WHICH IS EXPECTED TO LINGER IN THE OFFSHORE WATERS OF THE MID-ATLANTIC POSSIBLY INTO EARLY NEXT WEEK. OF COURSE THIS WILL ALSO HAVE RAMIFICATIONS ON THE TRACK OF HURRICANE MARIA GIVEN JOSE PROVIDES SUFFICIENT WEAKNESS IN THE MID-LEVEL RIDGE TO SUPPORT A POSSIBLE NORTHEASTWARD TURN AWAY FROM THE EASTERN SEABOARD. FOR THE LATEST INFORMATION ON THESE TROPICAL CYCLONES...PLEASE CHECK OUT THE NHC WEBSITE AT WWW.NHC.NOAA.GOV. THROUGHOUT THE PERIOD...THE LONGWAVE TROUGH ACROSS THE WESTERN U.S. WILL SLOWLY MOVE EASTWARD WITH A GENERAL WESTWARD TREND NOTED IN THE PAST SEVERAL ENSEMBLE SPAGHETTI PLOTS. THIS IS PARTICULARLY THE CASE WITH THE 00Z GEFS MEMBERS WHICH EVENTUALLY FAVORS PINCHING OFF A CLOSED LOW SOMEWHERE IN THE VICINITY OF NORTHERN BAJA CA BY EARLY NEXT WEEK. THIS IS ALSO SUPPORTED BY OTHER MODEL SOLUTIONS AS A BROAD UPPER RIDGE BEGINS TO EXPAND OVER THE FAR EASTERN PACIFIC. MEANWHILE...THE PRIMARY BAND OF HEIGHT FALLS FINALLY EJECT EASTWARD TOWARD THE GREAT PLAINS WITH A MULTITUDE OF SURFACE WAVES TRAVERSING NORTH-NORTHEASTWARD ALONG THE SLOW-MOVING FRONTAL ZONE. TIMING DIFFERENCES ARE OF COURSE PREVALENT DEEPER INTO THE FORECAST BUT SOLUTIONS ARE GENERALLY ALIGNED REASONABLY WELL WITH ONE ANOTHER. DURING THE FORECAST...TOOK AN APPROACH OF SPLITTING THE DIFFERENCE BETWEEN THE 06Z/00Z GFS AND 00Z ECMWF SOLUTIONS WITH MORE LIMITED USE OF THE 06Z GEFS MEAN AND 00Z ECMWF/NAEFS ENSEMBLE MEANS THROUGH DAY 4/SATURDAY. BEYOND THIS PERIOD...STARTED TO DECREASE OPERATIONAL INFLUENCES GIVEN GROWING UNCERTAINTY IN THE STATE OF JOSE AS WELL DETAILS OF THE EVOLVING WESTERN U.S. TROUGH. BY DAY 7/SEPTEMBER 26...FAVORED ALMOST PURE USE OF THE MENTIONED ENSEMBLE MEANS WITH 20 PERCENT OPERATIONAL GUIDANCE...06Z/00Z GFS AND 00Z ECMWF TO GAIN SOME MODEST DETAILS. ...SENSIBLE WEATHER/HIGHLIGHTS... ACROSS THE WESTERN U.S...WELL BELOW NORMAL TEMPERATURES WILL PREVAIL GIVEN THE PERSISTENCE OF UPPER TROUGHING OVER THE REGION. THROUGH THIS WEEKEND...CURRENT FORECAST DEPARTURES SHOULD BE ON THE ORDER OF 20 TO 25 DEGREES BELOW CLIMATOLOGY ACROSS SECTIONS OF THE UPPER INTERMOUNTAIN WEST/NORTHERN ROCKIES. THIS WILL SUPPORT WIDESPREAD HIGH TEMPERATURES IN THE UPPER 40S TO LOWER 50S WITH EVEN A POCKET OF MID 30S LIKELY ACROSS WESTERN WY. IN ADDITION...THERE WILL BE A THREAT FOR SOME HIGHER ELEVATIONS SNOWS ACROSS THE SECTIONS OF EASTERN ID/SOUTHWESTERN MT/WESTERN WY GIVEN ACTIVE PRECIPITATION OVER THE REGION. FARTHER EAST...CONTINUED RIDGING CENTERED OVER THE GREAT LAKES WILL KEEP CONDITIONS QUITE MILD AS HIGHS SIT IN THE MID/UPPER 80S AS FAR NORTH AS WI/MI THIS WEEKEND. SUCH FORECAST HIGH TEMPERATURES RATE ABOUT 15 TO 20 DEGREES ABOVE AVERAGE. SOME RECORD WARM MINIMA ARE POSSIBLE ON SATURDAY MORNING WITH LOW 70S BEING FORECAST ACROSS THE UPPER MIDWEST. HEAVY RAINFALL IS LIKELY FROM THE CENTRAL PLAINS INTO THE UPPER MIDWEST GIVEN FAVORABLE SOUTH-SOUTHWESTERLY FLOW ALOFT PERSISTING THROUGH MONDAY. THERE MAY BE SOME HYDROLOGIC ISSUES WITH THIS SET UP AS MULTIPLE DAYS OF PRECIPITATION ARE LIKELY TO AFFECT THIS REGION. BY EARLY NEXT WEEK...THE UPPER TROUGH PROGRESSING EASTWARD SHOULD HELP ADVANCE THE BAND OF RAINFALL TOWARD THE GREAT LAKES. HOWEVER...WITH AN UPPER LOW SPINNING IN THE VICINITY OF BAJA CA...SUFFICIENT UPPER DIFFLUENCE TOWARD THE SOUTHERN PLAINS MAY SUPPORT ANOTHER REGION OF HEAVY RAINFALL ON MONDAY/TUESDAY. DETAILS ARE A BIT MORE UNCERTAIN GIVEN IT IS A DAY 6/7 FORECAST. RUBIN-OSTER