EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1135 AM EDT MON SEP 11 2017 VALID 12Z THU SEP 14 2017 - 12Z MON SEP 18 2017 ...15Z THOUGHTS... THE MEAN UPPER-LEVEL PATTER STARTS OFF WITH A LARGE POSITIVE ANOMALY STRETCHING FROM THE ROCKIES TO THE NORTHERN TIER OF THE COUNTRY TO EASTERN CANADA/NEW ENGLAND AND A NEG ANOMALY MOSTLY TRAPPED OVER THE OH/TN VALLEYS TO THE GULF COAST STATES, WHAT IS LEFT OF IRMA. BY D+8, NEG HEIGHTS WILL STRETCH FROM THE BERING SEA/AK TO THE PAC NW/NORTHERN ROCKIES, PERHAPS ENOUGH POSITIVE HEIGHTS FROM NORTH OF HUDSON BAY TO FILTER SOUTH INTO THE GREAT LAKES AND NEW ENGLAND/CANADIAN MARITIMES AND A SMALL POCKET OF LOWER HEIGHTS OFF THE MID-ATL/SOUTHEAST COAST, WHICH COULD CONTAIN JOSE. SO THE PATTERN ACTUALLY BECOMES SOMEWHAT PROGRESSIVE WITH SHORT WAVE UPPER DYNAMICS FROM THE PAC TEAMING UP WITH A STRONG SYSTEM FROM NEAR AK TO THE NW QUAD OF THE COUNTRY, THE STURDY BUT SMALL UPPER RIDGE OVER THE SOUTHERN ROCKIES SHIFTS INTO TX AND SOUTH AND PAC NW TROUGH WITH CA SHORT WAVE DYNAMICS TEAM FOR WEAK UPPER TROUGH OVER GREAT LAKES EXTENDING INTO MID-ATL WITH JOSE HOPEFULLY WELL OFFSHORE. SPEAKING OF THE TROPICAL ENTITY, THE 06Z GFS TRENDED LEFT AT THE END OF THE FORECAST PERIOD FROM ITS 00Z RUN, WHICH SHIFTS IT CLOSER TO THE COAST NEXT WEEKEND INTO EARLY NEXT WEEK BUT THIS IS AN OUTLYING SOLUTION. IN FACT, THE 06Z GEFS MEAN IS ACTUALLY RIGHT OF BOTH OPERATIONAL GFS SOLUTIONS AND MIRROR MORE THE OPERATIONAL ECMWF AND ITS MEAN OF BEING WELL OFFSHORE AND NO THREAT FOR THE MID-ATL/NORTHEAST SEABOARD. WPC FOLLOWED A 60 PERCENT OPERATIONAL GUIDANCE VERSUS 40 PERCENT MEANS THROUGH D5/SAT AND A FLIPPED 60/40 BLEND FAVORING THE MEANS NEXT SUN AND MON. THE HEAVIEST QPF DURING THE FIVE DAY PERIOD WILL BE ACROSS FROM THE NRN INTER-MOUNTAIN WEST/NORTHERN ROCKIES INTO THE NORTHERN PLAINS AND UPPER MIDWEST/UPPER MS VALLEY WITH WIDESPREAD AREAL AVG 2 TO 4 INCH AMOUNTS WITH LOCALLY HIGHER VALUES WILL BE POSSIBLE WITH EVEN A SLIGHT PROBABILITY OF HIGHER ELEVATION SNOW AT THE HIGHEST PEAKS. THE WPC MEAN MAX TEMP ANOMALY FOR THE FIVE DAYS FEATURES WELL BELOW AVG HIGHS ACROSS MUCH OF THE WEST WITH THE COLDEST HIGHS EXTENDING FROM THE INTERIOR PAC NW THROUGH THE NORTHERN ROCKIES INTO THE NORTHERN PLAINS, AOA 10-15 DEG BELOW AVG. WHILE WELL ABOVE AVG HIGHS WILL BE POSSIBLE FROM TX/LOWER MS VALLEY THROUGH THE CENTRAL PLAINS/MID MS VALLEY INTO THE GREAT LAKES AND NEW ENGLAND, AOA 10 DEG ABOVE AVG FROM WEST TX THROUGH CENTRAL OK INTO EASTERN KS. IN FACT, A FEW RECORD HIGHS COULD BE POSSIBLE ACROSS WEST TX NEAR DRT/MAF. MUSHER EARLIER DISCUSSION... OVERVIEW ~~~~~~~~ THE TRANSITION OF THE UPPER PATTERN WILL INTRODUCE A DEEP TROUGH ACROSS THE NORTHWESTERN QUADRANT OF THE NATION LATER THIS WEEK. AT THE SURFACE, A STRONG SURFACE COLD FRONT WILL DROP TOWARDS THE SOUTHEAST THROUGH THE PERIOD BUT REMAIN QUASI-STATIONARY OVER THE GREAT LAKES REGION AND THE NORTHEAST. THE REMNANT CIRCULATION OF IRMA WILL DISSIPATE BY FRIDAY BUT HURRICANE JOSE IS FORECAST TO MEANDER IN THE ATLANTIC TO THE NORTHEAST OF THE BAHAMAS. ANOTHER LARGE TROUGH IS LIKELY TO APPROACH THE PACIFIC NORTHWEST AROUND THE END OF THE FORECAST PERIOD NEXT MONDAY. GUIDANCE EVALUATIONS/PREFERENCES ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ THE DETERMINISTIC MODELS AND ENSEMBLES ARE GENERALLY INDICATING GOOD CLUSTERING AT THE BEGINNING OF THE FORECAST PERIOD ON THURSDAY, SO A MULTI-MODEL BLEND INCLUDING THE GFS/ECMWF/UKMET WAS USED AS A STARTING POINT. THE 12Z ECMWF CONTINUED TO APPEAR MORE AMPLIFIED WITH THE TROUGH OVER THE GREAT BASIN BY FRIDAY, WHICH IS SIMILAR TO EARLIER RUNS. SHORT WAVELENGTH SPACING FROM THE PACIFIC ACROSS THE NORTHERN TIER STATES CAN OFTEN PRECLUDE SUCH AMPLIFICATION SO THEREFORE IT WAS PRUDENT TO SPLIT THE DIFFERENCE BETWEEN IT AND THE LESS AMPLIFIED GFS SOLUTION FOR NOW. DIFFERENCES IN THE FLOW ACROSS CENTRAL CANADA ALSO EMERGE BY THIS UPCOMING WEEKEND WHICH LEAD TO DECREASED CONFIDENCE IN THE DETERMINISTIC SOLUTIONS FOR THE SECOND HALF OF THE FORECAST PERIOD FOR THE WESTERN AND CENTRAL STATES. THE ENSEMBLE CONSENSUS STILL SUGGESTS THE RESULTING SURFACE LOW TO LIFT ACROSS THE NORTHERN PLAINS TOWARD LAKE SUPERIOR, BUT THERE REMAINS SIGNIFICANT SPREAD IN THE GUIDANCE AND LIMITS THE AMOUNT OF CONFIDENCE IN THE FORECAST. A SECOND SURFACE LOW IS FORECAST TO DEVELOP ALONG THE FRONT BY FRIDAY AND SLOWLY MAKE ITS WAY TOWARDS THE MIDWEST, WITH GROWING DIFFERENCES BY THE WEEKEND. HURRICANE JOSE WILL NEED TO BE CLOSELY MONITORED NEXT WEEKEND AND BEYOND. MODEL DIFFERENCES WITH THAT STORM ARE QUITE LARGE, WITH THE UKMET SHOWING IT OVER THE BAHAMAS BY THE END OF ITS FORECAST, AND THE CMC MUCH FASTER TOWARDS THE NORTHEAST. THE ECMWF HAS TRENDED FARTHER WEST OVER ITS LAST FEW RUNS, AND THE GFS HAS TRENDED TO A LESSER DEGREE TO THE EAST. MULTIPLE CHANGES TO THE EXTENDED FORECAST WITH HURRICANE JOSE ARE EXPECTED IN THE COMING DAYS AS THE MODELS EVENTUALLY COME INTO BETTER AGREEMENT. WEATHER HIGHLIGHTS ~~~~~~~~~~~~~~~~~~ AFTER RECENT WARM WEATHER ACROSS MUCH OF THE NORTHWESTERN U.S., A BIG PATTERN CHANGE IS COMING COURTESY OF THE LARGE UPPER LEVEL TROUGH BUILDING OVER THE WESTERN U.S. AND A STRONG COLD FRONT SETTLING SOUTHWARD ACROSS THE ROCKIES AND TOWARDS THE CENTRAL AND NORTHERN PLAINS. IT WILL FEEL MORE LIKE LATE OCTOBER ACROSS MUCH OF MONTANA, WYOMING, AND IDAHO BY THE END OF THE WEEK WITH HIGH TEMPERATURES ONLY IN THE 40S FOR MANY AREAS, WHICH IS ON THE ORDER OF 20 DEGREES BELOW NORMAL. WIDESPREAD LIGHT TO MODERATE RAIN IS LIKELY NEAR AND BEHIND THE FRONT AS THE UPPER TROUGH SETTLES IN WITH LOCALLY HEAVIER AMOUNTS IN WESTERN MONTANA WITH UPSLOPE FLOW FROM THE EAST. THIS TYPE OF ATMOSPHERIC SET-UP IS CONDUCIVE FOR ACCUMULATING SNOW FOR THE HIGHEST ELEVATIONS OF WESTERN MONTANA, WESTERN WYOMING, AND NORTHERN IDAHO, ESPECIALLY ABOVE 7500 FEET ELEVATION. IT COULD BE THE FIRST NOTEWORTHY WINTER WEATHER EVENT OF THE SEASON FOR THE NORTHERN ROCKIES. ELSEWHERE ACROSS THE CONTINENTAL U.S., ABOVE NORMAL TEMPERATURES ARE EXPECTED FROM TEXAS TO THE OHIO VALLEY. A LATE SEASON HEAT WAVE IS LIKELY OVER CENTRAL AND WESTERN TEXAS LATE IN THE WEEK, WITH SOME 100+ DEGREE TEMPERATURES FROM THE RIO GRANDE TO THE OKLAHOMA BORDER. SCATTERED SHOWERS AND A FEW THUNDERSTORMS WILL BE POSSIBLE FOR THE EAST COAST STATES FROM THE REMNANTS OF IRMA AND A FRONTAL BOUNDARY OVER THE NORTHEAST U.S. SHOWERS SHOULD ALSO RETURN TO WESTERN WASHINGTON AND OREGON BY NEXT MONDAY AS THE NEXT TROUGH APPROACHES THE REGION. HAMRICK