582  
FXUS02 KWBC 241756  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION...AMENDED  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1256 PM 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. THE DAY 4-7 QPF  
TEMPLATE WAS A THREE WAY SPLIT OF THE 06Z GFS, 12Z GFS, AND 00Z  
ECMWF (THE ECMWF ADDED TO SOMEWHAT ACCOUNT FOR THE GUIDANCE  
SPREAD).  
 
WEATHER HIGHLIGHTS  
~~~~~~~~~~~~~~~~~~  
THE SPECIFICS AND LONGER TERM RAMIFICATIONS/TRANSITIONS WITHIN  
THIS WINTRY PATTERN INCLUDING ICE/SNOW THREATS REMAIN A QUESTION  
MARK. THE 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 ACROSS THE SOUTHEAST AND EAST. THE POTENTIAL FOR FRIGID  
HIGH PRESSURE AND POTENTIALLY ACCUMULATING FREEZING RAIN OVER AR,  
GA, AND THE CAROLINAS THURSDAY INTO FRIDAY MORNING WITH SNOW  
FARTHER INTO THE COLD AIR FROM THE MID-SOUTH AND MID-ATLANTIC  
STATES NORTHEASTWARD THROUGH NEW ENGLAND ASSOCIATED WITH SHORTWAVE  
PASSAGES AND ANY POSSIBLE TROWAL/NORLUN TROUGH-LIKE FEATURES TO  
THE NORTHWEST OF THE LOW DEVELOPMENT OFFSHORE THE EAST COAST.  
 
ROTH/SCHICHTEL  

 
 
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