EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 142 AM EST MON DEC 11 2017 VALID 12Z THU DEC 14 2017 - 12Z MON DEC 18 2017 ...PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT... THERE WILL BE A SIGNIFICANT SHIFT IN THE SYNOPTIC-SCALE PATTERN DURING THE PERIOD WITH A MOVEMENT FROM HIGHLY AMPLIFIED TO MORE PROGRESSIVE WITH LOWER AMPLITUDE TRANSIENT FEATURES. INITIALLY...THERE WILL STILL BE ROBUST 500-MB HEIGHT ANOMALIES...GENERALLY IN THE 2 TO 2.5 STANDARD DEVIATION RANGE. THIS CONSISTS OF A BROAD LONGWAVE TROUGH EXTENDING OVER MUCH OF NORTHEASTERN NORTH AMERICA ALTHOUGH THE LOWEST HEIGHTS WILL BE CENTERED WITHIN A POWERFUL UPPER LOW OVER NORTHERN QUEBEC ON 14/1200Z. ONE PRIMARY SHORTWAVE REMAINS IN THE WAVE TRAIN WITH SOME GAIN OF AMPLITUDE ON APPROACH TOWARD THE EAST COAST. HOWEVER...MANY SOLUTIONS DO NOT SHOW A WHOLE LOT OF SURFACE DEEPENING OFFSHORE AS THE WAVE EXITS THE EASTERN SEABOARD. THERE IS SOME HINT THAT A LOBE OF SHORTWAVE ENERGY WILL SPLIT OFF AND PUSH THROUGH THE SOUTHERN U.S. ON FRIDAY/SATURDAY ALTHOUGH THIS SCENARIO SEEMS TO BE A LOW PROBABILITY OUTLIER. LOOKING BACK TO THE WEST COAST...A STAGNANT UPPER RIDGE IS FORECAST TO FINALLY BUCKLE BY FRIDAY AS UPSTREAM HEIGHT FALLS SWING INTO THE PACIFIC NORTHWEST BY EARLY FRIDAY. THIS SYSTEM IN PARTICULAR APPEARS TO BE THE ONE WHICH FINALLY SHIFTS THE PATTERN TOWARD MORE PROGRESSIVE AND OF SOMEWHAT LOWER AMPLITUDE WAVES. THE SCENARIO OF SOME OF THIS ENERGY CUTTING OFF OVER THE DESERT SOUTHWEST APPEARS TO BE DISAPPEARING RELATIVE TO THE GUIDANCE YESTERDAY. GENERALLY SPEAKING...THIS SYSTEM SHOULD QUICKLY TRANSLATE EASTWARD WITH SOME POTENTIAL SURFACE DEEPENING OVER THE NORTHERN TIER STATES IF ANY NORTHERN INTERACTION TAKES PLACE. WHILE THIS PROGRESSIVE SHORTWAVE MOVES TOWARD THE EASTERN SEABOARD BY DAY 7/DECEMBER 18...ADDITIONAL AMPLIFICATION IS LIKELY OVER THE NORTH-CENTRAL U.S. WHILE QUASI-ZONAL FLOW IS ESTABLISHED BACK TOWARD THE EASTERN PACIFIC. REGARDING THE SHORTWAVE MOVING THROUGH THE OH VALLEY LATE THURSDAY/EARLY FRIDAY...MODELS ARE STILL AT ODDS WITH ONE ANOTHER REGARDING SURFACE EVOLUTION. ENSEMBLE SCATTER LOW PLOTS SHOW A VAST MAJORITY OF THE 12Z ECMWF ENSEMBLES FAVORING A MORE SOUTHERN WAVE ACROSS THE OH VALLEY ON 14/1200Z. MEANWHILE...THE LAST SEVERAL RUNS OF THE GFS ARE MORE CONSISTENT WITH A NORTHERN LOW PUSHING THROUGH THE LOWER GREAT LAKES AT THE SAME TIME. THEN THERE ARE MODELS LIKE THE 00Z CMC WHICH FAVOR THE EXISTENCE OF BOTH OF THESE SOLUTIONS. MEANWHILE...SOME SPREAD EXISTS ON THE SOUTHERN EXTENT OF THE EJECTING LONGWAVE TROUGH AS THE 12Z ECMWF/UKMET ATTEMPT TO SEPARATE SOME OF THIS ENERGY. THIS PARTICULAR SCENARIO WOULD ALLOW FOR A GULF COAST WAVE TO FORM SOUTH OF THE MS/AL COAST ON FRIDAY MORNING ALTHOUGH ITS SOLUTION IS A DISTINCT MINORITY AMONG THE 90 AVAILABLE ENSEMBLE MEMBERS. FURTHER...THE 00Z UKMET BACKED OFF ON THIS AS WELL WHICH WAS EXPECTED GIVEN THE DIVERGENCE FROM THE ENSEMBLE SPAGHETTI CLUSTERING. SHIFTING THE FOCUS WEST...GENERAL AGREEMENT EXISTS IN THE EVENTUAL EROSION OF THE MEAN UPPER RIDGE ACROSS THE WESTERN U.S. BY SOMETIME FRIDAY. MULTI-DAY ENSEMBLE SPAGHETTI PLOT COMPARISONS SHOW A SHIFT AWAY FROM PINCHING OFF A CLOSED LOW OVER THE SOUTHWESTERN STATES BY EARLY SATURDAY. THE 12Z CMC STILL SUGGESTED ITS EXISTENCE BUT HAS SINCE BACKED OFF VIA THE 00Z RUN. THE BIGGEST ISSUE NOW IS TIMING WITH THE 12Z ECMWF AS WELL AS PREVIOUS RUNS BEING ON THE QUICKER SIDE OF THE ENVELOPE. THERE IS THE POTENTIAL FOR A FAIRLY IMPRESSIVE SURFACE CYCLONE ACROSS THE GREAT LAKES BY SUNDAY MORNING BUT MODELS ARE INCONSISTENT WITH INTENSITY AND PLACEMENT. FOR INSTANCE...THE 12Z GFS TOOK THE SYSTEM DOWN TO A 983-MB PRESSURE ON 17/1200Z WHILE THE RUNS AFTERWARD WERE 1003/1002-MB. AS THIS SYSTEM ACCELERATES EASTWARD...ANOTHER DEEP CYCLONE MAY EMERGE OVER THE NORTH-CENTRAL BY DAY 7/DECEMBER 18 BUT DETAILS ARE AGAIN FUZZY AT BEST. INITIALLY WENT WITH AROUND 60 TO 80 PERCENT OF 12Z GFS/ECMWF/CMC FOR DAYS 3/4...THURSDAY/FRIDAY TO ACCOUNT FOR THE SYSTEM MOVING TOWARD THE EAST COAST. GENERALLY STAYED AWAY FROM ENSEMBLE MEANS AS THEY AVERAGED THE TWO LOWS MOVING THROUGH THE OH VALLEY/GREAT LAKES ON THURSDAY. DID QUICKLY REDUCE CONTRIBUTIONS OF THE 12Z ECMWF ON DAYS 4/5...FRIDAY/SATURDAY TO AROUND 10 TO 15 PERCENT GIVEN THE OUTLYING SOLUTION OVER THE SOUTHEASTERN U.S. BEYOND THIS PERIOD...PUSHED TOWARD A FULL ENSEMBLE MEAN APPROACH GIVEN UNCERTAINTIES WITH THE TIMING OF THE PROGRESSIVE TROUGH AS WELL AS THE POTENTIAL FOR DEEP SURFACE LOWS ALONG THE NORTHERN TIER. GAVE A SLIGHT EDGE TO THE 12Z GEFS MEAN RELATIVE TO THE 12Z ECMWF ENSEMBLE MEAN DURING THE END OF THE PERIOD. ...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 SPREAD IS HIGH WITH THE THE 00Z GFS BEING WET WHILE THE 12Z ECMWF IS COMPLETELY DRY. RUBIN-OSTER