327  
FXUS02 KWBC 141943  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
241 PM EST TUE DEC 14 2021  
 
VALID 12Z FRI DEC 17 2021 - 12Z TUE DEC 21 2021  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE LATEST GUIDANCE AND THE ENSEMBLE MEANS ARE IN FAIRLY GOOD  
AGREEMENT INITIALLY AND THROUGH MOST OF THE EXTENDED PERIOD, THUS  
LEADING TO AN ABOVE AVERAGE CONFIDENCE WITH THIS FORECAST. THE WPC  
MEDIUM RANGE PRODUCT SUITE WAS PRIMARILY COMPRISED OF A BLEND OF  
THE 12Z CMC/ECWMF/GFS/UKMET AND THE NAEFS, GEFS AND EC ENSEMBLE  
MEANS. THE INCLUSION OF THE MEANS INCREASED DURING THE MIDDLE AND  
LATER PERIODS. THE ENSEMBLE SUPPORT WAS ALSO STRONG AND WPC  
PRODUCT CONTINUITY WAS 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.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
A STRONG SURFACE LOW PRESSURE SYSTEM WILL BE LIFTING NORTH THROUGH  
EASTERN CANADA/CANADIAN MARITIMES THURSDAY AND FRIDAY, WITH ITS  
ASSOCIATED TRAILING FRONTAL BOUNDARY DRAPED ACROSS THE  
MID-ATLANTIC TO THE DESERT SOUTHWEST. SHOWERS AND THUNDERSTORMS  
ALONG THIS SLOW MOVING BOUNDARY IS EXPECTED TO BE MODERATE TO  
HEAVY AT TIMES, WHICH MAY LEAD TO EXCESSIVE RAINFALL AND LOCALIZED  
FLOODING CONCERNS FROM THE SOUTHERN PLAINS THROUGH THE  
MID-SOUTH/OHIO VALLEY. 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 FRIDAY AND  
SATURDAY.  
 
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. SOME OF  
THE GUIDANCE IS SUGGESTING THAT BY EARLY NEXT WEEK THIS FEATURE  
MAY SPIN UP AND TRACK UP THE EAST COAST; POTENTIALLY YIELDING  
MARITIME THREATS, SPREADING RAIN NEAR THE MID-ATLANTIC AND  
POSSIBLY SNOW FOR THE NORTHEAST.  
 
MEANWHILE, MOISTURE SURGING ONSHORE THE WEST COAST IS CAUSING  
MODERATE PRECIPITATION ACROSS THE NORTHWEST AND INTO CALIFORNIA  
AHEAD OF AN APPROACHING PACIFIC SYSTEM; WHICH IS EXPECTED TO  
GRADUALLY WEAKEN INTO FRIDAY. MOISTURE FROM ANOTHER PACIFIC SYSTEM  
MAY THEN IMPACT THE PACIFIC NORTHWEST WITH MODERATE TO POSSIBLY  
LOCALLY HEAVY PRECIPITATION AMOUNTS INTO THE WEEKEND, WITH  
ADDITIONAL ENERGIES DIGGING SOUTHWARD OVER THE EASTERN PACIFIC  
EARLY NEXT WEEK THAT WOULD EVENTUALLY AFFECT CALIFORNIA.  
 
CAMPBELL  
 
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  
 
HAZARDS:  
- HEAVY PRECIPITATION ACROSS PORTIONS OF THE PACIFIC NORTHWEST AND  
THE NORTHERN GREAT BASIN, SAT,  
DEC 18.  
- HEAVY RAIN ACROSS PORTIONS OF THE LOWER MISSISSIPPI VALLEY, THE  
CENTRAL APPALACHIANS, THE  
TENNESSEE VALLEY, THE MIDDLE MISSISSIPPI VALLEY, THE SOUTHERN  
PLAINS, AND THE OHIO VALLEY, FRI-SAT,  
DEC 17-DEC 18.  
- HEAVY RAIN ACROSS PORTIONS OF THE SOUTHERN PLAINS, SAT-SUN, DEC  
18-DEC 19.  
- HEAVY SNOW ACROSS PORTIONS OF THE NORTHERN ROCKIES AND THE  
NORTHERN GREAT BASIN, SAT-SUN, DEC  
18-DEC 19.  
 
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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