495  
FXUS02 KWBC 140711  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
210 AM EST TUE DEC 14 2021  
 
VALID 12Z FRI DEC 17 2021 - 12Z TUE DEC 21 2021  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
MODEL AND ENSEMBLE FORECAST SPREAD REMAINS LESS THAN NORMAL FOR  
THE MAJORITY OF THE SYSTEMS OF CONCERN THROUGH THE WEEKEND,  
BOLSTERING FORECAST CONFIDENCE. IN THIS PERIOD, THE WPC MEDIUM  
RANGE PRODUCT SUITE WAS PRIMARILY DERIVED FROM A COMPOSITE OF BEST  
CLUSTERED GUIDANCE FROM THE 18 UTC GFS, THE 12 UTC ECMWF AND THE  
01 UTC NATIONAL BLEND OF MODELS. ENSEMBLE SUPPORT IS ALSO STRONG  
AND WPC PRODUCT CONTINUITY IS WELL MAINTAINED. THESE MODELS  
SOLUTIONS DIVERGE MORE SIGNIFICANTLY EARLY NEXT WEEK FOR DAYS 6/7  
WITH THE TIMING/AMPLITUDE OF EMBEDDED SYSTEMS, BUT MOST NOTABLY  
WITH THE MULTI-STREAM FLOW OVER THE EAST/WESTERN ATLANTIC WHERE  
THE GFS DEVELOPS A MAJOR COASTAL LOW WHILE THE ECMWF SUPPRESSES  
DEVELOPMENT. 18 UTC GEFS AND 12 UTC ECMWF ENSEMBLES OFFER MEMBER  
CLUSTERS IN BOTH CAMPS. GIVEN GROWING UNCERTAINTY, OPTED TO BLEND  
THE MORE COMPATIBLE GEFS/ECMWF ENSEMBLE MEANS AND SHOW MODEST  
POTENTIAL FOR A COASTAL SYSTEM AND ASSOCIATED PRECIPITATION  
POTENTIAL TO SPREAD/DEVELOP ACROSS THE REGION. 00 UTC GUIDANCE  
REMAINS SPLIT ON DEVELOPMENT POTENTIAL AT THESE LONGER TIME  
FRAMES, BUT THE ECMWF HAS TRENDED IN SUPPORT OF A DEEP LOW, JUST  
DELAYED. THE FLOW OFFERS THAT POTENTIAL.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
A STRONG SURFACE LOW WILL LIFT INTO CANADA THU. WELL TO THE SOUTH,  
UPPER RIDGING SHOULD SUPPORT SOME RECORD WARM TEMPERATURES AHEAD  
OF THE ATTENDANT COLD FRONT, WITH POSITIVE ANOMALIES UPWARDS TO  
15-25F SHIFTING SOUTHWARD OVER THE EAST TO THE SOUTH/SOUTHEAST  
INTO FRI/SAT. THE TRAILING PORTION OF THE FRONT WILL MEANWHILE  
STALL AND REGROUP FROM THE SOUTH, WITH AN AXIS OF ENHANCED  
RAINFALL AND POTENTIAL FOR LOCAL RUNOFF AND FLOODING/FLASH  
FLOODING ISSUES FROM THE SOUTHERN PLAINS THROUGH THE  
MID-SOUTH/OHIO VALLEY WITH RETURN FLOW FROM THE GULF OF MEXICO. A  
DEVELOPING FRONTAL LOW WILL BRING A SWATH OF WINTRY PRECIPITATION  
FROM THE GREAT LAKE STATES THROUGH NORTHERN NEW ENGLAND INTO THE  
WEEKEND AS THE TRAILING ENHANCED RAINFALL AXIS DROPS SOUTHWARD  
ACROSS THE GULF COAST AND SOUTHEAST AS COOLING HIGH PRESSURE  
BUILDS ACROSS THE PLAINS AND TO THE EAST. THERE IS THEN POTENTIAL  
FOR AN EARLY NEXT WEEK EAST COAST SYSTEM THAT IF REALIZED WOULD  
SPREAD RAINS UP THE EAST COAST, PRESENT A MARITIME THREAT, AND  
EVEN OFFER SOME NORTHEAST SNOW POTENTIAL.  
 
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 IMPACT THE  
PACIFIC NORTHWEST WITH MODERATE TO POSSIBLY LOCALLY HEAVY  
PRECIPITATION AMOUNTS FRI INTO THE WEEKEND, WITH ADDITIONAL  
ENERGIES DIGGING SOUTHWARD OVER THE EASTERN PACIFIC EARLY NEXT  
WEEK THAT WOULD EVENTUALLY AFFECT CA.  
 
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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