010  
FXUS02 KWBC 161946  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
245 PM EST THU DEC 16 2021  
 
VALID 12Z SUN DEC 19 2021 - 12Z THU DEC 23 2021  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE PERIOD BEGINS FRIDAY WITH A VIGOROUS SHORTWAVE MOVING THROUGH  
THE NORTHEAST U.S. WITH SURFACE LOW PRESSURE EXITING THE COAST.  
MEANWHILE, A SOUTHERN STREAM SHORTWAVE WILL MOVE THROUGH THE  
SOUTHERN PLAINS THIS WEEKEND, EVENTUALLY INTO THE SOUTHEAST, WITH  
A POTENTIALLY WELL ORGANIZED COASTAL LOW DEVELOPING IN THE GULF OF  
MEXICO AND SLIDING ACROSS FLORIDA ON TUESDAY AND INTO THE WESTERN  
ATLANTIC BY WEDNESDAY. ELSEWHERE, WEAKER SHORTWAVES MOVE THROUGH  
THE FLOW ACROSS THE FAR NORTHERN TIER OF THE U.S. WITH UPPER LEVEL  
TROUGHING BECOMING ESTABLISHED OFF/OVER THE WEST COAST EARLY NEXT  
WEEK.  
 
OVERALL, THE GUIDANCE IS IN GOOD AGREEMENT WITH THE SYNOPTIC  
PATTERN THROUGH MUCH OF THE PERIOD. THE UKMET SEEMED A BIT FASTER  
WITH SYSTEMS THAN THE REST OF THE GUIDANCE, SO IT WAS NOT INCLUDED  
IN TODAYS BLEND. OTHERWISE, A GENERAL MODEL BLEND BETWEEN THE 06Z  
GFS AND THE 00Z ECMWF AND CMC WORKED WELL FOR DAYS 3-5 WITHIN A  
PATTERN WITH ABOVE AVERAGE FORECAST CONFIDENCE. BY DAYS 6-7, SOME  
BIGGER QUESTIONS BEGIN TO EMERGE REGARDING DETAILS OF AN ORGANIZED  
SURFACE LOW NEAR THE SOUTHEAST EARLY NEXT WEEK AND ITS PROXIMITY  
TO THE COAST. THERE ARE ALSO DIFFERENCES OUT WEST WITH EVOLUTION  
OF A SYSTEM IN THE EASTERN PACIFIC AND INTERACTIONS WITH THE WEST.  
GENERALLY, THESE ARE TYPICAL DIFFERENCES FOR THAT LATE IN THE  
PERIOD AND SO A ROUGHLY 50/50 BLEND OF THE ENSEMBLE MEANS WITH THE  
DETERMINISTICS SEEMED TO OFFER A GOOD STARTING POINT CONSISTENT  
WITH THE PREVIOUS WPC FORECAST.  
 
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
NORTHERN STREAM UPPER TROUGH PASSAGE AND AN EXITING FRONTAL LOW  
MAY SUPPORT SOME LINGERING SNOWS SUN OVER NORTHERN NEW ENGLAND AND  
OFF THE LOWER GREAT LAKES. LOCALLY ENHANCED RAIN SHOULD FOCUS WITH  
WAVES ALONG A TRAILING/STALLING FRONT SUNK THROUGH THE GULF COAST  
AND SOUTHEAST INTO EARLY NEXT WEEK. THE HEAVIEST RAINS SHOULD STAY  
OFF THE GULF COAST ON MONDAY, BUT A PERIOD OF MODERATE TO HEAVY  
RAIN WILL LIKELY IMPACT PORTIONS OF FLORIDA BY TUESDAY. DEPENDING  
ON PROXIMITY OF THE SURFACE LOW TO THE COAST, LOCALLY HEAVY  
RAINFALL MAY CLIP PORTIONS OF THE SOUTHEAST COAST AS THE LOW MOVES  
OUT INTO THE WESTERN ATLANTIC.  
 
AN UPPER TROUGH AND SURFACE SYSTEM PASSAGE WILL SUPPORT LINGERING  
TERRAIN ENHANCED PRECIPITATION OVER THE NORTHWEST ON SUNDAY.  
DETAILS REMAIN LESS CERTAIN UPSTREAM OVER THE EASTERN PACIFIC, BUT  
THERE REMAINS A GOOD GUIDANCE SIGNAL THAT AN AMPLIFIED UPPER  
TROUGH DIGGING EARLY NEXT WEEK WOULD OFFER POTENTIAL TO FOCUS  
INGREDIENTS FOR MODERATE TO HEAVY PRECIPITATION FOR NORTH-CENTRAL  
CA CONSIDERING SLOW SYSTEM APPROACH, A DEVELOPING MOISTURE PLUME,  
AND FAVORED SOUTHERN FACING TERRAIN ENHANCEMENT. SOME HEAVIER  
MODEL QPF SOLUTIONS WOULD FAVOR LOCAL RUNOFF/FLOODING ISSUES. THE  
MAIN PRECIPITATION FOCUS, INCLUDING HEAVY SHASTA AND SIERRA SNOWS,  
IS SET TO SPREAD DOWN ACROSS CENTRAL-SOUTHERN CA INTO MIDWEEK,  
WITH ADDITIONAL ACTIVITY SPREADING ACROSS THE GREAT BASIN TERRAIN  
TO THE ROCKIES WITH INLAND SHORTWAVE SYSTEM PROGRESSION. THERE IS  
ALSO THEN AN EMERGING GUIDANCE SIGNAL FOR UPPER TROUGH ENERGY TO  
DIG FROM ALASKA TO RENEW PRECIPITATION INTO THE PACIFIC NORTHWEST  
BY NEXT WED/THU.  
 
UPPER RIDGING ACROSS THE NATIONS MIDSECTION SHOULD SPREAD ABOVE  
NORMAL TEMPERATURES (THOUGH NOT NEARLY AS EXTREME AS CURRENT  
VALUES) FROM THE NORTHERN/CENTRAL PLAINS SOUTHWARD TOWARDS THE  
GULF COAST WITH DAYTIME HIGHS POTENTIALLY 15 TO 20F ABOVE NORMAL  
FOR THE SOUTHERN HIGH PLAINS BY NEXT THURSDAY. ELSEWHERE,  
TEMPERATURES THROUGH THE MEDIUM RANGE SHOULD BE NEAR SEASONAL  
AVERAGES FOR MUCH OF THE EAST, WITH SLIGHTLY BELOW NORMAL TEMPS  
ASSOCIATED WITH WEAK SHORTWAVES THROUGH THE NORTHERN TIER AND ALSO  
THE SHORTWAVE THROUGH THE SOUTHEAST EARLY IN THE PERIOD.  
 
SANTORELLI/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  
 
HAZARDS:  
- HEAVY RAIN ACROSS PORTIONS OF CALIFORNIA, THE CENTRAL GREAT  
BASIN, AND THE PACIFIC NORTHWEST, MON-TUE, DEC 20-DEC 21.  
- HEAVY SNOW ACROSS PORTIONS OF THE CENTRAL GREAT BASIN AND  
CALIFORNIA, MON-WED, DEC 20-DEC 22.  
- HEAVY SNOW ACROSS PORTIONS OF THE CENTRAL GREAT BASIN AND  
CALIFORNIA, TUE-WED, DEC 21-DEC 22.  
 
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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