222  
FXUS02 KWBC 231847  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION...AMENDED  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
146 PM 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  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
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.  
 
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 GFS 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. THE DAYS 4-7 QPF WERE BASED ON A BLEND OF THE  
06Z GFS, 12Z GFS, AND 00Z ECMWF, WITH THE GREATEST UNCERTAINTY IN  
THE OHIO VALLEY, MID-SOUTH, AND NORTHEAST.  
 
POTENTIAL IMPACTS  
~~~~~~~~~~~~~~~~~  
A SIGNIFICANT AMOUNT OF ARCTIC AIR ACROSS MUCH OF THE LOWER 48  
NEXT WEEK IS 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 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 DECENT SIGNAL FOR POTENTIAL EAST  
COASTAL LOW DEVELOPMENT WITH EJECTING LEAD ENERGIES HEADING INTO  
NEW YEARS WEEKEND WHICH WOULD THEN HEAD UP THE COAST TOWARDS  
ATLANTIC CANADA. PROBABILITIES OF 10-30% OF 0.25" LIQUID  
EQUIVALENT AS SNOW/SLEET HAVE BEEN DEPICTED TO THE NORTHWEST OF  
ITS TRACK.  
 
ROTH/SCHICHTEL  

 
 
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