EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1001 AM EST THU DEC 21 2017 VALID 12Z SUN DEC 24 2017 - 12Z THU DEC 28 2017 ...ARCTIC AIR TO GRIP MOST OF THE LOWER 48 NEXT WEEK... ...WINTRY WEATHER POSSIBLE CHRISTMAS EVE AND DAY FOR PORTIONS OF THE MID-ATLANTIC AND NORTHEAST... ...15Z MORNING UPDATE... THE MODELS/ENSEMBLES DID NOT IMPROVE THE CLARITY OF THE FORECAST FROM THE PREVIOUS SHIFT. THE FLOW OUT OF THE PACIFIC TOWARD THE WEST COAST (AND/OR ALASKA/HAWAI'I) IS NOTHING SHORT OF A MESS. THE GEFS ENSEMBLES DIVERGE FROM THE ECMWF/CANADIAN ENSEMBLES IN THE SHORT TERM AND TAKE UNTIL ABOUT NEXT THURSDAY TO REALIGN. PREFERENCE LIES WITH THE ECMWF ENSEMBLES OVERALL WITH MUCH LESS (OR NO) WEIGHT TO THE 00Z GFS/CANADIAN AFTER MONDAY. FOR CHRISTMAS DAY, 00Z ECMWF WAS STILL WITHIN THE WESTERN HALF OF THE SPREAD WITH THE SFC LOW PRESSURE NEAR SOUTHEASTERN MASSACHUSETTS BUT HAS BEEN THE MOST CONSISTENT. MULTI-DAY CONSENSUS TREND HAS BEEN A NUDGE EASTWARD EACH 00Z/12Z CYCLE BUT WILL JUST HAVE TO WAIT FOR DECREASED LEAD TIME ONCE THE UNCERTAINTY IN INITIAL CONDITIONS OF THE INSTIGATING SHORTWAVE (OVER NW BRITISH COLUMBIA) DECREASES. BY LATER NEXT WEEK, DEGREE OF WESTERN TROUGHING IS IN QUESTION (NOTE GFS/CANADIAN VS THE ECMWF) OWING TO CONTINUED UNCERTAINTY IN THE PACIFIC, WITH ADDITIONAL UNCERTAINTY IN HOW QUICKLY HEIGHTS IN THE EAST RESPOND. PREVIOUS DISCUSSION IS BELOW. FRACASSO ...OVERVIEW... WELL-ADVERTISED HIGHLY AMPLIFIED PATTERN EXPECTED TO BE IN PLACE BY DAY 3 (SUN), WITH A RIDGE AXIS EXTENDING FROM JUST OFF THE WEST COAST NORTH TO THE ARCTIC, AND A DEEP TROUGH AND UPPER LOW CENTERED SOUTH OF HUDSON BAY. BOTH OF THESE FEATURES SHOW ANOMALIES OF 2-3 STANDARD DEVIATIONS IN THEIR RESPECTIVE DIRECTIONS, INDICATING THE RATHER INTENSE NATURE OF THE AMPLIFICATION. THE FLOW PATTERN WILL ALLOW ARCTIC AIR TO PLUNGE SOUTHWARD INITIALLY INTO THE NORTH CENTRAL U.S., BEFORE THE AIR MASS MODIFIES AND SPREADS EAST NEXT WEEK. THE MOST SIGNIFICANT WEATHER SYSTEM TO AFFECT THE CONUS DURING THE MEDIUM RANGE WILL BE A POTENTIAL NOR'EASTER DEVELOPING NEAR THE SOUTHEAST U.S. COAST ON CHRISTMAS EVE AND MOVING QUICKLY NORTHEASTWARD ALONG THE COASTLINE THROUGH CHRISTMAS DAY. MODEL SPREAD SURROUNDING THIS SYSTEM REMAINS FAIRLY LARGE, AND IS DISCUSSED FURTHER BELOW. BY MID TO LATE NEXT WEEK, MODELS AND ENSEMBLES ARE GENERALLY SUGGESTING A RETURN TO A SOMEWHAT MORE PROGRESSIVE FLOW PATTERN. TELECONNECTIONS ASSOCIATED WITH PERSISTENT POSITIVE HEIGHT ANOMALIES FROM NORTHWESTERN CANADA TO THE ARCTIC SUPPORT CONTINUED COLD TEMPERATURES AND BROAD CENTRAL/EASTERN U.S. TROUGHING, HOWEVER. ...GUIDANCE/PREDICTABILITY ASSESSMENT... THE GFS CONTINUES TO BE MORE PROGRESSIVE THAN CONSENSUS WITH A SHORTWAVE CROSSING THE PLAINS/MS VALLEY ON SUN, WHICH RESULTS IN FASTER HEIGHT FALLS ALONG THE EAST COAST, PUSHING ANY SURFACE FRONTAL WAVE THAT DEVELOPS OUT TO SEA. THE GFS IS EVEN QUITE A BIT MORE PROGRESSIVE WITH THIS FEATURE THAN THE 18Z GEFS MEAN. GIVEN THAT THE CONSENSUS IS ON SOMETHING MORE SIMILAR TO THE ECMWF, THIS REMAINS THE PREFERRED DETERMINISTIC SOLUTION FOR HANDLING OF THIS SURFACE LOW AS IT TRACKS UP THE COASTLINE LATE ON CHRISTMAS EVE AND INTO CHRISTMAS DAY. THE DETERMINISTIC 12Z ECMWF WAS WITHIN A TIGHTLY CLUSTERED CONSENSUS OF ENSEMBLE MEMBERS FROM BOTH THE ECENS AND CMCE ENSEMBLES, ALBEIT ON THE WESTERN EDGE OF CONSENSUS BY 12Z MONDAY. THUS, THE WPC FORECAST LOW POSITION IS SLIGHTLY TO THE EAST OF THE ECMWF, ESSENTIALLY BETWEEN THE ECMWF AND THE ECENS MEAN. THE 12Z CMC WAS ALSO IN THIS VICINITY. IN THE WAKE OF THE EAST COAST LOW PRESSURE SYSTEM, MODELS SHOW GENERAL CONSENSUS THAT THE HUDSON BAY UPPER LOW WILL DRIFT EASTWARD TOWARD ONTARIO/QUEBEC. AN ARCTIC SURFACE FRONT, THE LEADING EDGE OF A FRIGID AIR MASS IS EXPECTED TO CROSS THE MIDWEST ON CHRISTMAS DAY AND REACH THE EASTERN SEABOARD BY TUESDAY. THE GFS/ECMWF SHOW SIMILAR TIMING DIFFERENCES HERE, WITH THE GFS FASTER TO PUSH THE ARCTIC FRONT EAST RELATIVE TO MOST OTHER GUIDANCE. GIVEN THE DESCRIBED CONSIDERATIONS, THE WPC FORECAST WAS GENERALLY WEIGHTED MORE HEAVILY TOWARD THE ECMWF THAN THE GFS FOR DAYS 3-5. DURING THE LATTER PART OF THE MEDIUM RANGE, AS DESCRIBE ABOVE, MODELS SHOW A SOMEWHAT MORE PROGRESSIVE FLOW PATTERN TAKING HOLD, BUT WITH SOME MAINTENANCE OF A RIDGE OFF THE WEST COAST AND INTO THE ARCTIC. NEXT SIGNIFICANT SHORTWAVE LOOKS TO APPROACH THE WEST COAST TUE NIGHT/WED. MODELS HAVE SHOWN POOR CONSENSUS SURROUNDING THE EVOLUTION OF THIS FEATURE ACROSS THE WESTERN U.S., WITH A LARGE DEGREE OF RUN-TO-RUN VARIABILITY AS WELL. THUS, THE FORECAST WAS TRENDED HEAVILY TOWARD THE ECENS/NAEFS MEANS DURING DAYS 6-7. THE EVOLUTION OF THIS FEATURES WILL PLAY A SIGNIFICANT ROLE IN THE POTENTIAL DEVELOPMENT OF A WAVE OF LOW PRESSURE ALONG THE STALLED ARCTIC FRONT ACROSS THE SOUTH CENTRAL U.S. BY THU AND BEYOND. ...WEATHER HIGHLIGHTS/THREATS... SURFACE FRONTAL WAVE DEVELOPMENT ALONG THE SOUTHEAST/MID-ATLANTIC COASTLINE SUN-MON SHOULD SPREAD AN ORGANIZED ROUND OF PRECIP ACROSS AREAS FROM THE CAROLINAS NORTHWARD. WHILE LOW-LEVEL THERMAL FIELDS REMAIN SOMEWHAT UNCERTAIN, LARGELY DEPENDENT ON THE EXACT TRACK OF THE LOW, SOME DEGREE OF WINTER WEATHER IS EXPECTED, ESPECIALLY FOR THE APPALACHIANS AND INTERIOR MID-ATLANTIC/NORTHEAST CHRISTMAS EVE AND DAY. THE CURRENT FORECAST WOULD LIKELY MEAN A RAIN/SNOW LINE SITUATED A BIT WEST OF THE I-95 CORRIDOR, AND A COLD RAIN (WITH PERHAPS SOME SNOW MIXING IN AT TIMES) FOR THE MAJOR CITIES. BOSTON WOULD BE THE MAJOR CITY CLOSEST TO THE RAIN/SNOW LINE IN THE CURRENT FORECAST SCENARIO. MEANWHILE, RAIN AND HIGH ELEVATION SNOW WILL AFFECT THE PACIFIC NORTHWEST SUN-MON AS A WEAKENING PACIFIC FRONT MOVES INLAND. THE ARRIVAL OF A COUPLE ROUNDS OF ADDITIONAL SHORTWAVE ENERGY TUE-WED WILL KEEP GENERALLY UNSETTLED CONDITIONS IN PLACE FOR THE NORTHWEST THROUGH MID-WEEK. TEMPERATURES ACROSS THE CENTRAL U.S. WILL QUICKLY DECREASE SUN-MON AS A SURGE OF ARCTIC AIR SPREADS SOUTHWARD. BY MON-TUE, THE COLDEST ANOMALIES OF 15 TO 30 DEG F BELOW AVERAGE, WILL ARRIVE ACROSS THE NORTHERN PLAINS AND UPPER MIDWEST, WHERE HIGH TEMPERATURES FOR SOME AREAS WILL REMAIN BELOW ZERO, AND LOWS MAY SURPASS -20 DEG F. TEMPERATURES ACROSS THE EASTERN U.S. NEAR OR JUST ABOVE AVERAGE ON SUNDAY WILL DECREASE TO 5 TO 10 DEG F BELOW AVERAGE BY MON, WITH 10-25 DEG F BELOW AVERAGE HIGHS POSSIBLE BY THE MIDDLE OF NEXT WEEK WITH THE ARRIVAL OF THE MODIFIED ARCTIC AIR (LARGEST NEGATIVE DEPARTURES NEAR THE CANADIAN BORDER). RYAN