893  
FXUS02 KWBC 230654  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
154 AM EST SAT DEC 23 2017  
 
VALID 12Z TUE DEC 26 2017 - 12Z SAT DEC 30 2017  
 
...WIDESPREAD ICE/SNOW THREAT AS ARCTIC AIR GRIPS MUCH OF THE  
LOWER 48 NEXT WEEK...  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT AND PATTERN OVERVIEW
 
 
IT REMAINS THE CASE THAT DEEP CYCLONIC FLOW ALOFT WILL DOMINATE  
THE PATTERN FROM THE INTERMOUNTAIN WEST TO THE EASTERN SEABOARD.  
LITTLE CHANGE IN SUCH A SYNOPTIC REGIME IS EXPECTED AS ADDITIONAL  
SHORTWAVES HELP ASSERT REINFORCING SHOTS OF ARCTIC AIR TO THE  
REGION.  
WHILE A SIGNIFICANT SPREAD OF ARCTIC AIR ACROSS MUCH OF THE LOWER  
48 NEXT WEEK SEEMS WELL ADVERTISED BY GUIDANCE THAT IS TRENDING  
EVEN COLDER...THE SPECIFICS AND LONGER TERM  
RAMIFICATIONS/TRANSITIONS WITHIN THIS WINTERY PATTERN INCLUDING  
ICE/SNOW THREATS REMAIN MUCH 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  
NWRN/WRN US TERRAIN AND 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  
DECENT SIGNAL FOR POTENTIAL EAST COASTAL LOW DEVELOPMENT WITH  
EJECTING LEAD ENERGIES HEADING INTO NEW YEARS WEEKEND. OVERALL...A  
BLEND OF REASONABLY COMPATABLE 12 UTC GEFS/NAEFS/ECMWF ENSEMBLE  
MEANS WAS USED TO DERIVE THE WPC MEDIUM RANGE PRODUCT SUITE  
CONSIDERING UNCERTAINTY AND WITH AN EYE TOWARD CONTINUITY. THESE  
ENSEMBLES OFFER A BIT MORE PRECIPITATION AXIS/AMOUNT SUPPRESSION  
THAN THE MORE RECENT 18/00 UTC GEFS RUNS THAT SEEM MARGINALLY LESS  
LIKELY CONSIDERING THE EXTENT OF THE HIGH PRESSURE DOME.  
 
SCHICHTEL  
 

 
 
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