820  
FXUS02 KWBC 231432  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
931 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...  
 
MODEL CHOICES/PATTERN OVERVIEW  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
OVERALL...A BLEND OF THE SIMILAR 00Z ECMWF, 00Z UKMET, 00Z  
CANADIAN, AND 06Z GFS WAS USED FOR THE PRESSURES, 500 HPA HEIGHTS,  
AND WINDS FROM TUESDAY INTO THURSDAY BEFORE SHIFTING TOWARDS THE  
00Z NAEFS MEAN/00Z ECMWF ENSEMBLE MEAN/06Z THEREAFTER WHICH WERE  
IN REASONABLY GOOD AGREEMENT AND MAINTAINED GOOD CONTINUITY. THE  
OTHER GRIDS (DEW POINT, CLOUD, TEMPERATURE, WEATHER, AND  
PRECIPITATION CHANCES) WERE MORE ENSEMBLE MEAN HEAVY TO ACCOUNT  
FOR UNCERTAINTY. QPF AND WINTER WEATHER PROBABILITIES RELATING TO  
DAY SHIFT HAVE YET TO BE DETERMINED. STAY TUNED.  
 
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. THE SPECIFICS AND  
LONGER TERM RAMIFICATIONS/TRANSITIONS WITHIN THIS WINTRY 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 OVERNIGHT 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.  
 
ROTH/SCHICHTEL  

 
 
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