748  
FXUS02 KWBC 130645  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
144 AM EST MON DEC 13 2021  
 
VALID 12Z THU DEC 16 2021 - 12Z MON DEC 20 2021  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
MODEL AND ENSEMBLE FORECAST CLUSTERING REMAINS BETTER THAN NORMAL  
FOR THE MAJORITY OF THE SYSTEMS OF CONCERN THROUGH MEDIUM RANGE  
TIME SCALES. THE WPC MEDIUM RANGE PRODUCT SUITE WAS PRIMARILY  
DERIVED FROM A COMPOSITE OF THE 18 UTC GFS AND 12 UTC  
ECMWF/UKMET/CANADIAN ALONG WITH THE 01 UTC NATIONAL BLEND OF  
MODELS DAYS 3-5 (THU-SAT). ADDED INTO THE MIX THE COMPATABLE 18  
UTC GEFS MEAN AND 12 UTC ECMWF ENSEMBLE MEAN DAYS 6/7 AMID  
GRADUALLY GROWING SYSTEM VARIANCE TO MAINTAIN MAX WPC PRODUCT  
CONTINUITY. 00 UTC GUIDANCE SEEMS CONSISTENT.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
A POTENT UPPER TROUGH WILL LIFT NORTHEASTWARD THROUGH THE  
NORTH-CENTRAL U.S. THU AND CYCLOGENESIS WILL LEAD TO AN EXPANSIVE  
AREA OF STRONG WINDS TOGETHER ALONG WITH A SWATH OF SNOW ON THE  
NORTH SIDE OF THE LOW TRACK OVER THE UPPER MIDWEST AND UPPER GREAT  
LAKES. SOUTHEAST OF THE DEEP LOW TRACK, UPPER RIDGING WILL SUPPORT  
SOME RECORD WARM TEMPERATURES AHEAD OF THE COLD FRONT, WITH  
POSITIVE ANOMALIES UPWARDS TO 15-25F SHIFTING FROM THE  
EAST-CENTRAL U.S. TO THE EAST THU-SAT. THE TRAILING PORTION OF THE  
FRONT WILL MEANWHILE STALL ACROSS THE SOUTH, WITH AN AXIS OF  
ENHANCED RAINFALL AND POTENTIAL OF LOCAL RUNOFF ISSUES FROM THE  
SOUTHERN PLAINS THROUGH THE MID-SOUTH/OH VALLEY WITH RETURN FLOW.  
A DEVELOPING FRONTAL WAVE MAY ALSO BRING A SWATH OF WINTRY  
PRECIPITATION FROM THE OH VALLEY STATES THROUGH NORTHERN NEW  
ENGLAND INTO THE WEEKEND AS THE ENHANCED RAINFALL AXIS DROPS  
SOUTHWARD ACROSS THE SOUTH AS COOLING HIGH PRESSURE BUILDS ACROSS  
THE PLAINS AND TO THE EAST.  
 
UPSTREAM, A MOISTURE SURGE WITH MODERATE PRECIPITATION WILL WORK  
INTO CALIFORNIA AND ACROSS THE NORTHWEST THU AHEAD OF AN  
APPROACHING PACIFIC SYSTEM. THIS SYSTEM WILL GRADUALLY WEAKEN INTO  
FRIDAY. MOISTURE FROM ANOTHER PACIFIC SYSTEM MAY THEN MODESTLY  
IMPACT THE PACIFIC NORTHWEST FRI INTO THE WEEKEND.  
 
SCHICHTEL  
 
ADDITIONAL 3-7 DAY HAZARD INFORMATION CAN BE FOUND ON THE WPC  
MEDIUM RANGE HAZARDS OUTLOOK CHART AT:  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP  
 
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDICES ARE AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 

 
 
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