988  
FXUS02 KWBC 250700  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
200 AM EST MON DEC 25 2017  
 
VALID 12Z THU DEC 28 2017 - 12Z MON JAN 01 2018  
 
...WIDESPREAD ICE/SNOW THREAT WITHIN ARCTIC AIRMASS OVER THE LOWER  
48 THIS WEEK...  
 
OVERALL PATTERN AND GUIDANCE/PREDICTABILITY ASSESSMENT  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
IT REMAINS THE CASE THAT 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 DOMINATES 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...FORECAST  
SPREAD HAS BEEN GRADUALLY DECREASINGING OVER THE LAST COUPLE OF  
DAYS. THE BIGGEST ISSUES CONTINUE TO LIE WITH A LOW EXPECTED TO  
FORM OFF THE EAST COAST HEADING INTO THE NEW YEARS WEEKEND WHICH  
TRACKS INTO/NEAR ATLANTIC CANADA BY AROUND NEXT SUNDAY AND A  
SYSTEM MOVING TOWARDS BRITISH COLUMBIA AND THE PACIFIC NW FRIDAY  
INTO SATURDAY THROUGH A LEAD MID-UPPER LEVEL RIDGE POSITION.  
 
UPDATE...THE 00 UTC ECMWF HAS TRENDED MORE IN LINE WITH RECENT GFS  
RUNS WITH THE ERN PACIFIC FEATURE MOVING INTO BC AND THE PAC NW  
AND DOWNSTREAM ALSO TRENDS TOWARD A MORE SUPPRESSED LOW/QPF/WINTER  
WEATHER THREAT AS WELL. THE EARLIER RELEASED WPC MEDIUM RANGE  
PRODUCT SUITE WAS PRIMARILY DERIVED FROM A COMPOSITE BLEND OF THE  
GEFS/NAEFS/ECMWF ENSEMBLES ALONG WITH CONTINUITY. THAT SOLUTION  
OFFERED LESS AMPLITUDE/MORE PROGRESSION THAN THE 12 UTC  
ECMWF...BUT MAY HAVE TO REVISITED IF THE UPCOMING MODELS AND  
ENSEMBLES CONTINUE THE TRENDS OF THE 00 UTC ECMWF. THE UPCOMING  
QPF AND WINTER WEATHER OUTLOOK PROGS WILL SHOW THIS TREND.  
 
WEATHER/THREATS 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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