EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1100 AM EST FRI DEC 22 2017 VALID 12Z MON DEC 25 2017 - 12Z FRI DEC 29 2017 ...ARCTIC AIR TO GRIP MOST OF THE LOWER 48 NEXT WEEK ALONG WITH SOME SNOW THREAT... ...PATTERN OVERVIEW... DEEP CYCLONIC FLOW WILL CONTINUE TO DOMINATE THE PATTERN FROM THE INTERMOUNTAIN WEST TO THE EASTERN SEABOARD. LITTLE CHANGE IN SUCH A SYNOPTIC REGIME IS EXPECTED AS ADDITIONAL SHORTWAVES HELP ASSERT REINFORCING SHOTS OF ARCTIC AIR TO THE REGION. WITHIN THIS CONVEYOR BELT OF EMBEDDED SHORTWAVE...THE INITIAL ONE WILL BE RESPONSIBLE FOR A COASTAL STORM IN THE PROCESS OF EXITING NEW ENGLAND CHRISTMAS MORNING. IN ITS WAKE...ARCTIC HIGH PRESSURE SHOULD EXERT ITS DOMINANCE FROM THE ROCKIES EASTWARD AS TEMPERATURES CRASH OVER MUCH OF THE COUNTRY. WITHIN SOME OF THESE SURGES...FORECAST 700-MB TEMPERATURES MAY DROP BELOW -30C ALONG THE NORTHERN TIER OF THE COUNTRY AS ANOMALIES DROP TO AROUND 2 TO 3 STANDARD DEVIATIONS BELOW AVERAGE. OVER THE EASTERN PACIFIC AND INTO THE WEST COAST...AN UPPER RIDGE IS FORECAST TO GENERALLY REMAIN IN PLACE FOR MUCH OF THE PERIOD. THERE IS A BRIEF MOMENT ON TUESDAY WHERE A POTENTIAL SHORTWAVE WILL MOVE INTO THE PACIFIC NORTHWEST BEFORE SHEARING THEREAFTER. WHILE THE UPPER RIDGE RE-ESTABLISHES ITS PRESENCE...A PROGRESSIVE SYSTEM IS EXPECTED TO SLIDE THROUGH WESTERN BRITISH COLUMBIA EARLY WEDNESDAY WHILE ACCELERATING SOUTHEASTWARD ALONG THE EXTERN EXTENT OF THE MEAN RIDGE. IT SHOULD EVENTUALLY ADJOIN THE LONGWAVE TROUGH OVER THE CENTRAL/EASTERN U.S. WHICH WILL PROVIDE A NOTABLE SOUTHERN STREAM FEATURE TO WORK WITH. GIVEN TREMENDOUS COLD AIR IN PLACE...THIS WAVE IS DEFINITELY WORTH MONITORING TOWARD THE DAY 7/DECEMBER 29 AND BEYOND REALM GIVEN POTENTIAL WINTER WEATHER IMPLICATIONS. ...GUIDANCE/PREDICTABILITY ASSESSMENT/PREFERENCES... AT THE START OF THE PERIOD ON CHRISTMAS DAY...AN EXITING COASTAL STORM IS WELL AGREED UPON BUT WITH SOME SPATIAL/INTENSITY DIFFERENCES. MOST NOTABLY...THE 00Z UKMET SITS ON THE WESTERN SIDE OF THE SPREAD WHILE THE 06Z/00Z GFS AND 00Z ECMWF DISAGREE ON INTENSITY. RECENT RUNS OF THE GFS HAVE TRENDED DEEPER WHILE THE ECMWF HAS MOVED IN THE OPPOSITE DIRECTION. FOR INSTANCE...THE 00Z GFS HAS A 993-MB CYCLONE OFF THE COAST OF MAINE WHILE THE 00Z ECMWF IS APPROXIMATELY 10-MB WEAKER AT THE SAME TIME. REGARDING TRAILING FEATURES TRAVERSING THE DEEP CYCLONIC FLOW...THE 00Z UKMET IS AN OUTLIER HERE AS WELL AS IT IS MARKEDLY QUICKER WITH SHORTWAVE ENERGY SURGING TOWARD THE NORTH-CENTRAL U.S. ON TUESDAY. OVERALL...DETAIL DIFFERENCES ARE EVIDENT IN THE MID/UPPER-LEVEL FLOW...BUT THE GENERAL IDEA OF ARCTIC HIGH PRESSURE SWEEPING THROUGH THE CENTRAL U.S. IS AGREED UPON WHILE A BOUNDARY REMAINED LOCKED UP AGAINST THE HIGHER TERRAIN OUT WEST. ONE OF THE BIGGER AND MOST CONTENTIOUS AREAS OF THE FORECAST REMAINS WITH THE EASTERN PACIFIC RIDGE. MULTI-DAY COMPARISONS HAVE SHOWN PERIODS OF TIME WHERE OPERATIONAL MODELS MOVE A MODEST SHORTWAVE THROUGH WHICH ESTABLISHES A BRIEF PATTERN SHIFT. HOWEVER...ENSEMBLE SPAGHETTI PLOTS HAVE STRONGLY PUSHES TOWARD A MORE SEMI-PERMANENT UPPER RIDGE WHICH AGREES WITH THE PATTERN SETTING UP. THE 06Z/00Z GFS ARE STRONGER WITH THE TUESDAY SYSTEM APPROACHING THE PACIFIC NORTHWEST RELATIVE TO THE FLATTER CONSENSUS. EVENTUALLY THE MENTIONED SYSTEM SWINGING SOUTHEASTWARD OUT OF WESTERN BRITISH COLUMBIA ON WEDNESDAY IS HANDLED QUITE DIFFERENTLY. COMPARING THE GFS/ECMWF ON A MULTI-RUN BASIS...THE GFS HAS TENDED TO BE THE SLOWER SOLUTION WHICH FAVORS A SHORTWAVE MOVING THROUGH THE FOUR CORNERS ON THURSDAY BEFORE ACCELERATING TOWARD THE OZARKS BY THE FOLLOWING DAY. THE 00Z ECMWF IS NEARLY A DAY QUICKER WITH THIS SYSTEM WHICH IS EVEN FASTER THAN ITS ENSEMBLE MEAN. THERE IS PLENTY OF UNCERTAINTY HERE BUT THE IDEA OF ARCTIC AIR BEING IN PLACE OVER THE EASTERN U.S. COUPLED WITH STRONG JET ENERGY MOVED THROUGH SEEMS TO BE MORE AGREED UPON THAN PREVIOUSLY. ENSEMBLE LOW PLOTS DO SHOW WAVES OF LOW PRESSURE ANYWHERE FROM THE SOUTHERN U.S. INTO THE GULF STREAM ON FRIDAY. GIVEN SUFFICIENT MODEL CLUSTERING THROUGH DAY 4/TUESDAY...WAS ABLE TO UTILIZE A MULTI-OPERATIONAL MODEL CONSENSUS FAVORING THE 00Z GFS/ECMWF/CMC WITH A BIT MORE WEIGHTING ON THE GFS/ECMWF. BY DAY 5/WEDNESDAY...GRADUALLY WENT TOWARD AN EQUAL BLEND OF OPERATIONAL MODELS RELATIVE TO THE ENSEMBLES. GIVEN GROWING SPREAD BEYOND THIS PERIOD...WENT SOLELY WITH ENSEMBLE MEANS BEING DOMINATED BY THE 00Z GEFS/ECMWF ENSEMBLE MEANS. CONFIDENCE IN THE FORECAST BY DAY 7/DECEMBER 29 BECOMES BELOW AVERAGE GIVEN MANY DETAILS WHICH NEED TO COME TOGETHER FIRST. ...WEATHER HIGHLIGHTS/THREATS... THE TEMPERATURES DURING THE PERIOD WILL BE DOMINATED BY LARGE NEGATIVE ANOMALIES GIVEN THE INFLUENCE OF ARCTIC AIR MASSES ON THE FORECAST. THE LARGEST DEPARTURES FROM CLIMATOLOGY WILL BE LIKELY FROM THE UPPER INTERMOUNTAIN WEST EASTWARD INTO THE NORTHERN PLAINS/UPPER MIDWEST. SUCH READINGS ARE EXPECTED TO BE AROUND 25 TO 30 DEGREES BELOW CLIMATOLOGY AS LOWS PLUNGE INTO THE -20S F WHILE HIGHS STRUGGLE TO GET OUT OF THE -10 TO 0F RANGE. ALL THIS COLD AIR WILL TRANSLATE SOUTH AND EAST ENSURING WELL BELOW NORMAL TEMPERATURES THROUGHOUT THE FORECAST PERIOD. MEANWHILE...THE WARM SPOT THROUGHOUT THE PERIOD WILL BE OVER THE SOUTHWESTERN U.S. INTO THE CENTRAL GREAT BASIN AND FOUR CORNERS REGION AS TEMPERATURES WILL GENERALLY BE 5 TO 15 DEGREES ABOVE AVERAGE. THE BIG STORY EARLY ON SHOULD BE THE HEAVY SNOW OVER NEW ENGLAND GIVEN THE COASTAL STORM IN PLAY CHRISTMAS MORNING. AS IT MOVES AWAY TOWARD THE CANADIAN MARITIMES...EXPECT GUSTY WINDS TO PREVAIL OVER MUCH OF THE GREAT LAKES/NORTHEASTERN U.S. DEEP CYCLONIC FLOW WILL FAVOR LAKE EFFECT SNOW OVER ALL OF THE MAJOR GREAT LAKES. RUBIN-OSTER