103  
FXUS02 KWBC 240653  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
153 AM EST SUN DEC 24 2017  
 
VALID 12Z WED DEC 27 2017 - 12Z SUN DEC 31 2017  
 
...A WIDESPREAD ICE/SNOW THREAT AS ARCTIC AIR GRIPS MUCH OF THE  
LOWER 48 THIS WEEK...  
 
...PATTERN OVERVIEW/IMPACTS AND GUIDANCE/PREDICTABILITY  
ASSESSMENT...  
 
IT REMAINS THE CASE THAT DEEP CYCLONIC FLOW ALOFT WILL DOMINATE  
THE PATTERN FROM THE INTERMOUNTAIN WEST TO THE ERN SEABOARD.  
LITTLE CHANGE IN SUCH A SYNOPTIC REGIME IS EXPECTED AS ADDITIONAL  
SHORTWAVES HELP ASSERT REINFORCING SHOTS OF ARCTIC AIR TO THE  
REGION.  
THIS SIGNIFICANT ARCTIC AIR INFILTRATION ACROSS MUCH OF THE LOWER  
48 IS WELL ADVERTISED BY GUIDANCE. HOWEVER...THE SPECIFICS AND  
LONGER TERM RAMIFICATIONS/TRANSITIONS WITHIN THIS WINTRY PATTERN  
INCLUDING ICE/SNOW THREATS REMAIN LESS CLEAR. DETERMINISTIC MODEL  
RUNS CONTINUE TO OFFER POOR RUN TO RUN CONTINUITY WITH THE OVERALL  
FLOW IN THE 4-7 DAY TIME FRAME...ESPECIALLY WITH THE TIMING AND  
LOCAL WEATHER FOCUS OF NUMEROUS EMBEDDED SMALLER SCALE SYSTEMS.  
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 UNDERNEATH  
ACROSS THE MID-LOWER LATITUDES OF THE CENTRAL AND ERN US WITH  
POTENTIAL FOR FRIGID HIGH PRESSURE OVERRUNNING ICE OVER THE  
MID-SOUTH AND SNOW FURTHER NORTH WITH SHORTWAVE PASSAGES AND  
ASSOCIATED SURFACE REFLECTIONS. THE TIMING/EMPHASIS OF SUPPORTING  
IMPULSES ALOFT ARE QUITE UNCERTAIN AND EVEN THE LATER WEEK LARGER  
SCALE TROUGH AXIS FOCUS HAS SHIFTED FROM THE EAST COAST BACK AND  
FORTH TO THE E-CENTRAL US OR ROCKIES/SWRN US OVER THE PAST FEW  
DAYS WORTH OF CHAOTIC GFS AND ECMWF RUNS. EVEN SO...THERE DOES  
ALSO SEEM TO BE A GROWING SIGNAL FOR POTENTIALLY DEEP EAST COASTAL  
LOW DEVELOPMENT WITH EJECTING LEAD ENERGIES HEADING INTO NEW YEARS  
WEEKEND WHICH WOULD THEN HEAD UP OFF THE COAST. WHILE MOST  
INDIVIDUAL MODEL RUN OR ENSEMBLE MEMBERS OFFER SOME  
PLAUSIBILITY...IT SEEMS PRUDENT TO INSTEAD FAVOR AN ENSMEBLE MEAN  
APPROACH CONSIDERING WIDE FORECAST SPREAD. ACCORDINGLY...THE WPC  
MEDIUM RANGE PRODUCT SUITE WAS PRIMARILTY DERIVED FROM THE  
COMPATABLE 12 UTC GEFS/ECMWF ENSEMBLE MEANS. THE RESULTANT FIELDS  
WERE THEN MANUALLY ADJUSTED TO SHOW SYSTEM DEPTH WASHED OUT BY THE  
BLENDING PROCESS AS SUPPORT/PREDICTABILITY SEEM TO DICTATE. THIS  
WAS MOST EVIDENT WITH THE ERN COASTAL LOW INTO NEXT WEEKEND GIVEN  
DEEPER WPC CONTINUITY TRENDS FROM THE DAYSHIFT AND CONSIDERING  
POTENTIAL FROM A SUBSET OF DEEPENED MODEL/ENSEMBLE MEMBERS.  
 
SCHICHTEL  
 

 
 
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