469  
FXUS02 KWBC 251559  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1058 AM EST MON DEC 25 2017  
 
VALID 12Z THU DEC 28 2017 - 12Z MON JAN 01 2018  
   
..OVERVIEW
 
 
THE LARGE SCALE PATTERN ACROSS NORTH AMERICA WILL REMAIN LARGELY  
PERSISTENT THROUGH THE MEDIUM RANGE, WITH UPPER RIDGING OFF THE  
U.S. WEST COAST, EXTENDING NORTH INTO THE ARCTIC AND TROUGHING  
ACROSS THE MIDWEST/GREAT LAKES/NORTHEAST. A SHORTWAVE MOVING INTO  
THE PAC NW LATE FRI/SAT WILL BRIEFLY WEAKEN THE RIDGE BEFORE IT  
RECOVERS BY EARLY NEXT WEEK. THIS PATTERN WILL SUPPORT CONTINUED  
COLD TEMPERATURES FOR MUCH OF THE CENTRAL AND EASTERN U.S., WITH  
EVEN FLORIDA SUBJECT TO POTENTIAL INTRUSIONS OF COLD AIR MASSES. A  
CLIPPER IS EXPECTED TO CROSS THE GREAT LAKES ON FRI AND THE  
EASTERN U.S. ON SAT. THE ENERGY ASSOCIATED WITH THIS SYSTEM IS  
EXPECTED TO PHASE OFFSHORE WITH AN ADDITIONAL FRONTAL WAVE, WITH  
THE POTENTIAL TO BRING SOME WINTER WEATHER TO PORTIONS OF THE  
EASTERN/NORTHEASTERN STATES. BY EARLY NEXT WEEK THE ARCTIC FRONT  
IS EXPECTED TO BE IN THE GULF OF MEXICO AND NEAR OR PAST SOUTH  
FLORIDA. GUIDANCE INDICATES SOME POTENTIAL FOR A FRONTAL WAVE TO  
DEVELOP EITHER ACROSS THE SOUTHEASTERN U.S. OR IN THE GULF OF  
MEXICO ALONG THIS FRONT, ALTHOUGH CONFIDENCE IN THAT SCENARIO IS  
LOW AT THIS TIME (MORE DETAILS BELOW).  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
MODELS INITIALLY SHOW TIMING AND AMPLITUDE DIFFERENCES WITH A  
SHORTWAVE CROSSING THE NORTHERN PLAINS/MIDWEST THU NIGHT/FRI. THE  
GFS HAS CONSISTENTLY BEEN AMONG THE FASTEST SOLUTIONS WITH THIS  
WAVE, WITH THE ECMWF ON THE SLOW SIDE, BUT CLOSE TO THE  
ECENS/NAEFS MEANS. THE CMC APPEARED TO BE A COMPROMISE, ALBEIT A  
BIT CLOSER TO THE GFS THAN THE ECMWF. THESE DIFFERENCES TRANSLATE  
DOWNSTREAM AND AFFECT THE EVENTUAL PHASING OF THIS ENERGY WITH A  
FRONTAL WAVE OFF THE EAST COAST. IN GENERAL THE ECMWF HAS BEEN A  
BIT MORE CONSISTENT WITH THE HANDLING OF THIS ENERGY AND THE  
PHASING/NEW SURFACE CYCLONE OFF THE NEW ENGLAND COAST BY LATE SAT,  
EVENTUALLY GIVING THE UPPER TROUGH A NEGATIVE TILT AND WRAPPING  
THE SURFACE CYCLONE INTO NEW BRUNSWICK/NOVA SCOTIA. GIVEN THE  
RELATIVE CONSISTENCY OF THE ECMWF SOLUTION ALONG WITH THE SUPPORT  
OF THE ECENS/NAEFS MEANS, AM INCLUDED TO FAVOR SOMETHING IN THAT  
GENERAL DIRECTION FOR THE FORECAST. FARTHER WEST, MODELS ALSO SHOW  
A RANGE OF TIMING FOR THE SHORTWAVE AND ASSOCIATED SURFACE FRONT  
REACHING THE NORTHWESTERN U.S. FRI-SAT, WITH THE GFS  
FASTEST/WEAKEST AND THE ECMWF/CMC SLOWER. ACROSS THIS AREA, A  
BLENDED APPROACH SEEMS BEST. AS A RESULT, THE WPC FORECAST DURING  
DAYS 3-5 WAS BASED LARGELY ON A MULTI-MODEL BLEND INCLUDING THE  
00Z ECMWF/00Z CMC/06Z GFS (WITH MORE WEIGHT PLACE ON THE ECMWF).  
 
DIFFERENCES WITH HOW VARIOUS MODELS HANDLE THE CYCLONE HAVE  
EFFECTS ON THE POTENTIAL OR LACK THEREOF FOR ANOTHER SURFACE WAVE  
TO AFFECT THE EASTERN U.S. BY LATE IN THE FORECAST PERIOD. AS THE  
GFS MOVES THE EASTERN CYCLONE NORTHWARD INTO CANADA MUCH MORE  
QUICKLY DAYS 5-6, THIS SEEMS TO RESULT IN A WEAKENING OF COLD  
ADVECTION AND RISING HEIGHTS/THICKNESSES TO SOME DEGREE ALONG THE  
EAST COAST. THIS WOULD MAKE THE ENVIRONMENT MORE FAVORABLE FOR A  
FRONTAL WAVE THAT MOVES NORTH OFF THE MID-ATLANTIC/NORTHEAST COAST  
BY DAY 7, AS THE GFS SHOWS. GIVEN THAT A SLOWER DAY 5-6 CYCLONE,  
AS SHOWN BY THE ECENS/NAEFS MEANS AND THE ECMWF IS FAVORED, THIS  
SHOULD HELP TO KEEP THE HEIGHT/THICKNESS FIELDS ACROSS THE EAST,  
AND ANY POTENTIAL FRONTAL WAVE BY DAY 7, MORE SUPPRESSED  
SOUTHWARD. THE GFS SOLUTION BY DAY 7 HAS VERY LITTLE IN TERMS OF  
ENSEMBLE SUPPORT AS WELL. AS A RESULT, THE WPC FORECAST BY DAYS  
6-7 IS CONSISTENT WITH CONTINUITY IN SHOWING AN ARCTIC FRONT SOUTH  
OF FLORIDA AND INTO THE GULF OF MEXICO, WITH NO SIGNIFICANT  
FRONTAL WAVE ALONG THE EAST COAST. FARTHER WEST, MODELS ALSO SEEM  
TO SHOW THEIR TYPICAL BIASES WITH ANOTHER SHORTWAVE/FRONTAL SYSTEM  
REACHING THE PAC NW BY NEXT MON, WITH THE GFS ON THE FAST SIDE OF  
THE SPREAD AND THE ECMWF ON THE SLOW SIDE (EVEN SLOWER THAN THE  
ECENS MEAN). GIVEN THE INCREASED SPREAD AND UNCERTAINTY DURING  
DAYS 6-7, THE WPC FORECAST WAS BASED MORE ON A 00Z ECENS/NAEFS  
BLEND, WITH SMALLER COMPONENTS OF THE DETERMINISTIC SOLUTIONS USED  
DURING DAY 3-5. THE GFS WAS EXCLUDED FROM THE FORECAST BY DAY 7  
GIVEN THE PREFERENCE AGAINST ITS EAST COAST LOW SOLUTION BY THAT  
TIME.  
 
   
..WEATHER/THREATS HIGHLIGHTS
 
 
AS MENTIONED ABOVE, THE PATTERN WILL CONTINUE TO SUPPORT THE  
INCURSION OF ARCTIC AIR INTO MUCH OF THE CENTRAL AND EASTERN  
U.S., WITH HIGH TEMPERATURES 15 TO 25 DEG F BELOW AVERAGE THROUGH  
MUCH OF THE PERIOD. WHILE A FEW RECORD LOWS ARE POSSIBLE, THE TYPE  
OF RECORD MOST IN JEOPARDY APPEARS TO BE RECORD LOW MAXIMUM  
TEMPERATURES. THE POTENTIAL FOR AT LEAST SOME WINTER WEATHER WILL  
EXIST ACROSS PORTIONS OF THE SOUTHEAST THU-THU NIGHT AS A WAVE OF  
LOW PRESSURE BEGINS TO DEVELOP ALONG THE FRONT IN THE GULF/WESTERN  
ATLANTIC. AS THIS SYSTEM MOVES NORTH AND EVENTUALLY  
PHASES/DEVELOPS INTO A MATURE CYCLONE ACROSS SOUTHEASTERN CANADA  
BY SUN MORNING, THE POTENTIAL FOR SNOW/WIND WILL INCREASE,  
ESPECIALLY ACROSS NORTHERN/EASTERN NEW ENGLAND. CONFIDENCE IS LOW  
IN ANY ADDITIONAL WINTER THREATS FOR THE EASTERN/SOUTHEASTERN U.S.  
BY EARLY NEXT WEEK, BUT ANY POTENTIAL WEAK WAVE OF LOW PRESSURE  
ALONG THE FRONT IN THE GULF COULD RESULT IN POTENTIAL AREAS OF  
WINTER WEATHER. MEANWHILE, THE PACIFIC NORTHWEST WILL SEE A COUPLE  
ROUNDS OF POTENTIAL PRECIPITATION. THE FIRST WITH SHORTWAVE/FRONT  
ARRIVING FRI NIGHT/SAT, AND ANOTHER BY SUN-MON. BOTH OF THESE WILL  
PRODUCE RAIN AND MOUNTAIN SNOWS FOR THE NORTHWEST AND THE NORTHERN  
ROCKIES.  
 
RYAN  
 

 
 
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