EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 153 AM EST WED DEC 13 2017 VALID 12Z SAT DEC 16 2017 - 12Z WED DEC 20 2017 ...OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT... THE SYNOPTIC-SCALE PATTERN WILL FEATURE A SPLIT FLOW REGIME WITH A NUMBER OF MODEL DIFFERENCES WHICH MAKE THIS A CHALLENGING AND LOW CONFIDENCE FORECAST AHEAD. TO START WITH...A PRONOUNCED UPPER TROUGH WILL BE IN THE PROCESS OF EXITING NEW ENGLAND ON 16/1200Z WHILE RENEWED HEIGHT FALLS LURK UPSTREAM IN THE VICINITY OF THE NORTHERN/CENTRAL ROCKIES. DETACHED FROM THE PREVAILING WESTERLIES IS AN ELONGATED UPPER LOW INITIALLY POSITIONED OVER CENTRAL MX ON SATURDAY MORNING. THIS LATTER FEATURE IS EXPECTED TO QUICKLY BECOME ABSORBED BY THE MID-LATITUDE TROUGH PUSHING THROUGH THE CENTRAL U.S. BY EARLY SUNDAY. AS WILL BE MENTIONED IN THE NEXT SECTION...MODEL UNCERTAINTY PLAGUES THE EASTWARD PROGRESSION OF THIS LONGWAVE TROUGH WITH THE POTENTIAL FOR SOME SHORTWAVE ENERGY TO PEEL OFF AND SEPARATE OVER THE FOUR CORNERS REGION. THE SYSTEM SHOULD EVENTUALLY REACH THE EAST COAST BY ANYWHERE BETWEEN DAY 5-7...DECEMBER 18-20...DEPENDING ON THE SELECTED MODEL. A MYRIAD OF SURFACE WAVES ARE NOTED ON OR IN THE VICINITY OF THE PROPAGATING BOUNDARY WITH PERHAPS CYCLONES OVER THE GREAT LAKES/LOWER MS VALLEY/RED RIVER ON SUNDAY MORNING. WEAKENING/SHEARING OF THE MID-LEVEL THROUGH WILL WEAKEN THE SURFACE REFLECTION LATER INTO THE PERIOD. ALL THE WHILE...AMPLIFIED FLOW WITHIN THE NORTHERN STREAM WILL TAKE SHAPE OVER THE NORTHERN PLAINS/UPPER MIDWEST BY MONDAY ACCOMPANIED BY A DEEP SURFACE LOW OVER CENTRAL CANADA. BACK WEST...IN THE WAKE OF THE SYSTEM TRACKING TOWARD THE SOUTHERN HIGH PLAINS...A RETURN OF UPPER RIDGING IS EXPECTED THROUGH DAY 6/TUESDAY ALTHOUGH THERE ARE SOME HINTS THAT A CUT-OFF LOW MAY AFFECT SOUTHERN CA. EVENTUALLY A PATTERN CHANGE WILL BE UNDERWAY AS A POWERFUL POSITIVELY-TILTED TROUGH SWINGS SOUTHWARD FROM BRITISH COLUMBIA TOWARD THE PACIFIC NORTHWEST ON DAY 7/DECEMBER 20. AS DISCUSSED PREVIOUSLY...A LOW CONFIDENCE FORECAST AT BEST WITH ENSEMBLE SPAGHETTI PLOTS FROM DAY 4 LOOKING CLOSER TO WHAT ONE WOULD EXPECT ON DAY 6/7. WHILE THE UPPER TROUGH EXITING NEW ENGLAND APPEARS TO BE WELL AGREED UPON...THE OPPOSITE CAN BE SAID FOR THE EVOLVING SYSTEM PUSHING TOWARD THE HIGH PLAINS ON SATURDAY. INITIALLY...AN UPPER LOW OVER CENTRAL MX IS A NEW FEATURE ON THE MAP AS PREVIOUS DAYS HAD IT WELL WEST BACK TOWARD BAJA CA. WHILE EARLY PLACEMENT IS VARIABLE...ALL OF THE GUIDANCE AGREES ON LIFTING THIS CLOSED LOW NORTH-NORTHEASTWARD WITH GRADUAL ABSORPTION INTO THE APPROACHING LONGWAVE TROUGH. REGARDING THIS LATTER FEATURE...THE MOST STABLE SOLUTION ON A RUN-TO-RUN BASIS HAS BEEN THE GFS. IT HAS STUCK WITH ITS SLOWER FORECAST ALL ALONG AS IT ALLOWS FOR A LOBE OF VORTICITY TO PEEL OFF AND DRIFT EASTWARD FROM THE FOUR CORNERS REGION TO THE OZARKS FROM SUNDAY THROUGH TUESDAY. SUCH A SCENARIO HAS SOME NEIGHBORS...MOST NOTABLY THE 12Z CMC/UKMET. MEANWHILE...THE ECMWF SYSTEM HAS BEEN THE FASTEST GLOBAL MODEL ALL ALONG ALTHOUGH IT HAS MADE A DRAMATIC WESTWARD SHIFT DURING THE 12Z/00Z RUNS. UNFORTUNATELY...IT IS STILL THE QUICKEST PIECE OF GUIDANCE WHILE ALSO BEING THE MOST VIGOROUS IN TERMS OF POSITIVE VORTICITY ADVECTION OVER THE SOUTHEASTERN U.S. SOMEWHAT UNNERVING...THE 00Z CMC/UKMET HAVE TRENDED QUICKER BUT STILL REMAIN WEST OF THE MENTIONED ECMWF SOLUTIONS. FROM WHAT HAS BEEN SEEN OF THE 00Z ECMWF...IT HAS MOVED WESTWARD WHICH IS A GOOD SIGN AS THE EARLIER RUNS WERE FOR THE MOST PART REJECTED AS OUTLIERS. BUT IT STILL IS THE QUICKEST 00Z OPERATIONAL MODEL AT THIS TIME. CONSIDERING THE REMAINDER OF THE MAP...AMPLIFIED FLOW WITHIN THE NORTHERN STREAM WILL BEGIN TO AFFECT THE NORTHERN PLAINS/UPPER MIDWEST BY AS EARLY AS MONDAY. BASED ON THE ENSEMBLE SPAGHETTI PLOTS...TEMPORAL DIFFERENCES ARE ALSO EVIDENT HERE WITH THE 12Z ECMWF ENSEMBLE SYSTEM WELL AHEAD OF THE OTHER GUIDANCE. OVER THE WEST COAST...THE PAST THREE RUNS OF THE ECMWF ARE THE ONLY MODELS TO SHOW A CUT-OFF UPPER LOW MOVING TOWARD SOUTHERN CA ON DAY 6/7...DECEMBER 19/20. BACKTRACKING SUGGESTS THIS SYSTEM ACTUALLY PEELS OFF FROM THE INITIAL HEIGHT FALLS MOVING THROUGH THE WESTERN U.S. ON SATURDAY. GIVEN NO OTHER MODEL SHOWS THIS EVOLUTION...WILL CONTINUE TO DISMISS IT AS AN OUTLIER. LOOKING NORTHWARD...A POSITIVELY-TILTED TROUGH IS FORECAST TO DIP DOWN FROM BRITISH COLUMBIA INTO THE PACIFIC NORTHWEST LATE IN THE PERIOD. RELATIVE TO THE REST OF THE PERIOD...IT CONSISTS OF THE LOWEST MODEL SPREAD DESPITE BEING A DAY 7 FORECAST. RUN-TO-RUN CONTINUITY IS A LITTLE SKETCHY THOUGH AS THE 00Z GFS SLOWED BY LEAST 12 HOURS WHICH WOULD KEEP HEIGHT FALLS OUT OF WESTERN WA ON DECEMBER 20. GIVEN THE ROBUST AMOUNT OF MODEL SPREAD...IT WAS NECESSARY TO LEAN MORE HEAVILY ON THE ENSEMBLE MEANS THAN USUAL. THROUGH DAY 4/SUNDAY...TOOK A MULTI-MODEL BLEND APPROACH WITH THE 12Z GFS/GEFS MEAN/UKMET BEING 80 PERCENT OF THE BLEND WITH ONLY 20 PERCENT OF THE 12Z ECMWF/ECMWF ENSEMBLE MEAN BEING UTILIZED. THEREAFTER...QUICKLY REMOVED THE 12Z ECMWF WHILE LEANING MUCH MORE HEAVILY ON THE 12Z GFS/GEFS MEAN/NAEFS ENSEMBLE MEAN. DID KEEP 20 PERCENT OF THE 12Z ECMWF ENSEMBLE MEAN IN THE MIX DURING THE DAY 5-7...DECEMBER 18-20 PERIOD TO ACCOUNT FOR GENERAL MODEL UNCERTAINTY. ...WEATHER HIGHLIGHTS/THREATS... GENERALLY SPEAKING...TEMPERATURES SHOULD BE AT OR ABOVE CLIMATOLOGY THROUGH THE FORECAST PERIOD. EXCEPTIONS INCLUDE THE EAST COAST ON SATURDAY IN THE WAKE OF A STRONG FRONTAL PASSAGE EXITING INTO THE WESTERN ATLANTIC. THIS EXTENDS DOWN ALONG THE GULF COAST AS THE BOUNDARY SWEEPS SOUTHWARD THROUGH CUBA ON 16/1200Z. DEPARTURES FROM NORMAL WILL LIKELY ONLY BE IN THE 5 TO 10 DEGREE RANGE. BY SUNDAY...COOLER WEATHER IS EXPECTED UNDERNEATH THE EVOLVING UPPER TROUGH ACROSS THE FOUR CORNERS/SOUTHERN HIGH PLAINS. OTHERWISE...THE ENTIRE COUNTRY CAN EXPECT MILD CONDITIONS FOR THE THIRD WEEK OF DECEMBER GIVEN THE DOMINANCE OF PACIFIC AIR MASSES. DOWNSLOPING FLOW IN THE LEE OF THE ROCKIES WILL ALLOW FOR HIGHS TO BE ANYWHERE FROM 10 TO 15 DEGREES ABOVE NORMAL OVER THE CENTRAL U.S. BY THE MIDDLE OF NEXT WEEK...HIGH TEMPERATURES MAY REACH THE MID 50S ALL THE WAY UP TO THE NEB PANHANDLE. WITH A LACK OF ARCTIC AIR TO WORK WITH...ANY WINTRY PRECIPITATION WILL LIKELY BE CONFINED TO HIGHER ELEVATIONS OVER THE PACIFIC NORTHWEST/UPPER INTERMOUNTAIN WEST/ROCKIES AS WELL AS ACROSS THE GREAT LAKES AND INTERIOR NEW ENGLAND. CONSIDERING PRECIPITATION IN GENERAL...A STREAK OF LIGHT/MODERATE RAIN/SNOW IS LIKELY OVER THE CENTRAL/NORTHERN ROCKIES AS THE UPPER TROUGH TRACKS SOUTHEASTWARD ON SATURDAY. INCREASING WARM ADVECTION AHEAD OF THIS SYSTEM WILL LEAD TO ENHANCED HEAVY RAINFALL CHANCES FROM THE ARKLATEX EASTWARD TO THE SOUTHEASTERN U.S. COAST. THERE ARE MANY MODELS SHOWING 2 TO 3 INCH 24-HOUR AMOUNTS DURING PART OF THE PERIOD. TIMING IS OF COURSE AN ISSUE GIVEN THE PREPONDERANCE OF SOLUTIONS IN PLAY. BACK TO THE PACIFIC NORTHWEST...AN ATMOSPHERIC RIVER SIGNATURE SHOWS UP IN THE GUIDANCE BY THE EARLY/MIDDLE PART OF NEXT WEEK. HOWEVER...THERE ARE CHANCES THIS SETS UP NORTH OF THE INTERNATIONAL BORDER WITH CANADA AS SHOWN IN THE 00Z GFS. RUBIN-OSTER