EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1038 AM EST MON DEC 11 2017 VALID 12Z THU DEC 14 2017 - 12Z MON DEC 18 2017 ...OVERVIEW... UPPER RIDGING IN THE WEST WILL FINALLY BE KNOCKED DOWN LATE THIS WEEK BUT BROAD CYCLONIC FLOW WILL STILL DOMINATE THE EAST. THIS WILL GUIDE A SERIES OF MAINLY PACIFIC SYSTEMS ACROSS THE LOWER 48 AS THE ARCTIC BOUNDARY WOBBLES NEAR THE US/CANADIAN BORDER. A SLOW MODERATING TREND WILL UNFOLD ACROSS THE EAST THROUGH THE WEEKEND WHILE THE WEST WILL SEE NEAR TO ABOVE AVERAGE TEMPERATURES. PRECIPITATION WILL FINALLY COME BACK TO THE PACIFIC NORTHWEST BY FRIDAY BUT LIMITED TO WA/OR. ...GUIDANCE/PREDICTABILITY ASSESSMENT... LATEST 00Z GUIDANCE SUITE OFFERED A REASONABLE STARTING POINT TO THE FORECAST FOR THU-FRI (BLEND OF THE GFS/ECMWF/UKMET/CANADIAN). 00Z ECMWF/CANADIAN WERE IN THE MIDDLE OF THE SPREAD ACROSS THE LOWER MS VALLEY AS A POSITIVELY TILTED TROUGH SWINGS THROUGH THE REGION (GFS QUICKER, UKMET SLOWER) WHICH FIT BEST WITH THE ENSEMBLE MEANS. TO THE NORTHWEST THERE WAS MUCH MORE DISAGREEMENT ON TIMING OF THE SHORTWAVE INTO WESTERN WASHINGTON FRIDAY THOUGH THE ECMWF AND ITS ENSEMBLES HAVE BEEN MORE CONSISTENT THAN THE GFS/GEFS. FOR NEXT SUN/MON, UPPER TROUGH WILL PROGRESS EASTWARD OUT OF THE CENTRAL PLAINS AND CARRY THAT FRONT TO THE EAST COAST WHILE ANOTHER FLOWS THROUGH THE NORTHWEST INTO THE HIGH PLAINS. AGAIN, TIMING DIFFERENCES EXPAND WITH TIME PER THE DETERMINISTIC MODELS AS THEY SPLIT THE TROUGH AND EITHER FOCUS ON THE NORTHERN OR SOUTHERN PORTION. A CONSENSUS BLEND SERVED TO MINIMIZE CHANGES FROM OVERNIGHT. THIS SUGGESTED LEAVING ANOTHER SOUTHERN POTENTIAL VORTICITY TAIL JUST WEST OF BAJA CALIFORNIA AS HEIGHTS BUILD THROUGH THE WESTERN CARIBBEAN. FRACASSO ...WEATHER HIGHLIGHTS/THREATS... THERE WILL ONLY BE A BRIEF PERIOD OF BELOW AVERAGE TEMPERATURES AND THIS APPEARS TO BE CONFINED TO LOCATIONS EAST OF THE MS RIVER ON THURSDAY/FRIDAY. THE FINAL ARCTIC SURGE SHOULD BRING A REINFORCEMENT OF COLD AIR TO THE REGION WITH THE MOST ANOMALOUS TEMPERATURES BEING OVER NEW ENGLAND WITH ANOMALIES IN THE 10 TO 20 DEGREE BELOW CLIMATOLOGY RANGE. THIS SUGGESTS HIGHS ON THURSDAY SHOULD STRUGGLE TO GET OUT OF THE UPPER SINGLE DIGITS AND TEENS OVER INTERIOR NEW ENGLAND. OTHERWISE...MUCH OF THE COUNTRY SHOULD SEE ABOVE AVERAGE TEMPERATURES AS THE FLOW FLATTENS AND BECOMES MORE PACIFIC DOMINANT. DOWNSLOPING FLOW IN THE LEE OF THE ROCKIES WILL RAISE TEMPERATURES INTO THE 50S AS FAR NORTH AS EASTERN MT ON FRIDAY. THIS IS EASILY 15 TO 20 DEGREES ABOVE AVERAGE. LOOKING AT PRECIPITATION DURING THE PERIOD...SOME LAKE EFFECT ACTIVITY IS POSSIBLE ALTHOUGH IT SHOULD BE MORE TAME GIVEN A CHANGING SYNOPTIC-SCALE PATTERN. WHILE PRECIPITATION CHANCES WILL LOOM THROUGHOUT THE PERIOD...IT SHOULD BE WARM ADVECTION DRIVEN VERSUS MESOSCALE BANDING RELATED. OUT WEST...CONDITIONS WILL BE MUCH MORE ACTIVE THAN THEY HAVE BEEN THIS MONTH GIVEN THE FLATTENING OF THE RIDGE. WIDESPREAD RAIN/SNOW SHOULD BLANKET THE PACIFIC NORTHWEST AND LOCATIONS DOWNSTREAM INTO THE CENTRAL GREAT BASIN/UPPER INTERMOUNTAIN WEST/NORTHERN ROCKIES. GIVEN A LACK OF COLD AIR...SNOWFALL SHOULD BE RELEGATED TO THE HIGHER ELEVATIONS. THIS THREAT COULD CONTINUE THROUGH DAY 7/DECEMBER 18 GIVEN THE SEMI-ZONAL FLOW IN PLACE. ELSEWHERE...WARM ADVECTION SHOWERS ARE POSSIBLE FROM THE WESTERN GULF COAST TOWARD THE TN VALLEY/SOUTHEASTERN U.S. THIS WEEKEND ALTHOUGH MODEL/ENSEMBLE SPREAD OF QPF IS HIGH. RUBIN-OSTER