346  
FXUS02 KWBC 211502  
PMDEPD  
 
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  
 

 
 
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