240  
FXUS02 KWBC 241350  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
850 AM EST SUN DEC 24 2017  
 
VALID 12Z WED DEC 27 2017 - 12Z SUN DEC 31 2017  
 
...WIDESPREAD ICE/SNOW THREAT WITHIN ARCTIC AIRMASS OVER THE LOWER  
48 THIS WEEK...  
 
EXPECTED PATTERN/MODEL PREFERENCES  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
SYSTEMS MOVING IN FROM THE MID-LATITUDES OF THE PACIFIC ALONG WITH  
SOME 500 HPA RIDGE STRENGTHENING AND RETROGRESSION IN THE  
SUBTROPICS ARE EXPECTED TO BROADEN/WEAKEN OTHERWISE PERSISTENT  
RIDGING NEAR WESTERN NORTH AMERICA, THOUGH THE RIDGE'S EXISTENCE  
IS SUPPORTED BY THE UPSTREAM GYRE ALOFT WHICH  
STRENGTHENS/RETROGRADES INTO THE BERING SEA WITH TIME. DEEP  
CYCLONIC FLOW ALOFT WILL DOMINATE THE PATTERN FROM THE  
INTERMOUNTAIN WEST TO THE ERN SEABOARD, WITH LITTLE CHANGE  
EXPECTED AS ADDITIONAL SHORTWAVES HELP ASSERT REINFORCING SHOTS OF  
ARCTIC AIR TO THE REGION. DETERMINISTIC MODEL RUNS CONTINUE TO  
OFFER POOR RUN-TO-RUN AND RANDOM DAY-TO-DAY CONTINUITY WITH THE  
OVERALL FLOW IN THE 4-7 DAY TIME FRAME, PARTICULARLY WITH SMALL  
SCALE SHORTWAVES, THOUGH ENSEMBLE MEAN SOLUTIONS REMAIN  
CONSISTENT. DESPITE THE JUMPINESS OF THE DETERMINISTIC GUIDANCE,  
IT SHOWS RATHER GOOD AGREEMENT. THE BIGGEST ISSUES LIE WITH A LOW  
EXPECTED TO FORM OFF THE EAST COAST WHICH TRACKS INTO/NEAR  
ATLANTIC CANADA NEXT SUNDAY AND A SYSTEM MOVING TOWARDS BRITISH  
COLUMBIA FRIDAY INTO SATURDAY. WHILE QUESTIONABLE OFFSHORE THE  
NORTHWEST, THE 00Z CANADIAN SHOWED BETTER CLUSTERING WITH THE 00Z  
ECMWF/06Z GEFS MEAN SOLUTIONS WITH THE LOW OFFSHORE THE NORTHEAST,  
AND WERE FAVORED OVER THE SLOWER ECMWF AND QUICKER GFS.  
 
FOR THE LOWER 48 PRESSURES, FRONTS, 500 HPA HEIGHTS, AND WIND  
GRIDS, THE PREFERENCE WAS FOR AN EVEN BLEND OF THE 00Z ECMWF, 00Z  
UKMET, 00Z CANADIAN, AND 06Z GFS FROM WEDNESDAY INTO FRIDAY.  
THEREAFTER (TO ACCOUNT FOR THE ABOVE ISSUES), SWITCHED OUT THE 00Z  
UKMET AND BLENDED IN INCREASING AMOUNTS OF THE 00Z ECMWF/00Z NAEFS  
MEAN SOLUTIONS TO COMPENSATE WHICH ACCOUNTS FOR UNCERTAINTY,  
PARTICULARLY OFFSHORE THE NORTHWEST AND NORTHEAST, WITH LESS  
WEIGHTING GIVEN TO THE 00Z ECMWF. THE REMAINING GRIDS  
(TEMPERATURES, CLOUDS, WEATHER, PRECIPITATION CHANCES, DEW POINTS)  
WERE DERIVED FROM A MORE ENSEMBLE MEAN HEAVY BLEND, INCLUDING A  
SMALL PERCENTAGE OF THE NATIONAL BLEND OF MODELS. PREFERENCES  
REGARDING QPF AND WINTER WEATHER HAVE YET TO BE DETERMINED FOR DAY  
SHIFT. STAY TUNED.  
 
WEATHER HIGHLIGHTS  
~~~~~~~~~~~~~~~~~~  
THE SPECIFICS AND LONGER TERM RAMIFICATIONS/TRANSITIONS WITHIN  
THIS WINTRY PATTERN INCLUDING ICE/SNOW THREATS REMAINS A QUESTION  
MARK. THE OVERNIGHT WPC MEDIUM RANGE WINTER WEATHER OUTLOOK AND  
WEATHER GRIDS SHOW A FOCUS FOR HEAVIER SNOW PROBABILITIES THROUGH  
MID-LATER WEEK OVER THE NORTHWEST/NORTHERN ROCKIES AND TO THE LEE  
OF THE GREAT LAKES ALONG WITH AN EMERGING WIDESPREAD THREAT THAT  
DEVELOPS UNDERNEATH ACROSS THE MID-LOWER LATITUDES OF THE CENTRAL  
AND EASTERN US WITH POTENTIAL FOR FRIGID HIGH PRESSURE OVERRUNNING  
ICE OVER THE MID-SOUTH AND SNOW FURTHER NORTH WITH SHORTWAVE  
PASSAGES AND ASSOCIATED SURFACE REFLECTIONS. WILL SEE IF ANY  
CHANGES ARE NECESSARY EARLY THIS AFTERNOON.  
 
ROTH/SCHICHTEL  

 
 
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