866  
FXUS02 KWBC 170710  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
209 AM EST SUN DEC 17 2023  
 
VALID 12Z WED DEC 20 2023 - 12Z SUN DEC 24 2023  
 
...EMERGING PRECIPITATION THREAT FOR SOUTHERN CALIFORNIA AND THE  
SOUTHWEST...  
 
   
..OVERVIEW
 
 
A CLOSED UPPER TROUGH/LOW WILL DIG SOUTHWARD OFF CALIFORNIA  
MID-LATE WEEK BEFORE EJECTING INLAND ACROSS SOUTHERN  
CALIFORNIA/NORTHERN MEXICO AND THE SOUTHWEST NEXT WEEKEND. THIS  
WILL BRING PERIODS OF ENHANCED PRECIPITATION INTO SOUTHERN  
CALIFORNIA, THE SOUTHWEST. MEANWHILE, A DEEP UPPER LOW/TROUGH OVER  
THE EAST COAST MIDWEEK WILL CUT OFF ANOTHER LOW OFF THE EAST COAST  
INTO LATER WEEK, ACTING MAINLY AS A MARITIME THREAT. IN BETWEEN  
THE WESTERN AND EASTERN FEATURES, WARMING UPPER RIDING WILL  
POSITION OVER THE CENTRAL INTO EAST-CENTRAL U.S..  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE WPC MEDIUM RANGE PRODUCT SUITE WAS PRIMARILY DERIVED FOR  
WEDNESDAY INTO FRIDAY FROM A COMPOSITE BLEND OF BEST CLUSTERED  
GUIDANCE FROM THE 18 UTC GFS AND 12 UTC ECMWF/UKMET ALONG WITH  
COMPATIBLE INPUT FROM THE 01 UTC NATIONAL BLEND OF MODELS (NBM)  
AND WPC CONTINUITY IN AN EFFORT TO PROVIDE SUFFICIENT DETAIL  
CONSISTENT WITH A PATTERN WITH SEEMINGLY ABOVE NORMAL  
PREDICTABILITY. THE 12 UTC CANADIAN SOLUTION OFFERS AN OUTLIER  
SOLUTION WHOSE CLOSER HELD ATLANTIC CLOSED LOW PROXIMITY ALLOWS  
FOR MUCH WETTER PATTERN FOR NEW ENGLAND MID-LATE WEEK, BUT THE  
NEWER 00 UTC RUN HAS TRENDED STRONGLY TOWARD THE GUIDANCE  
COMPOSITE. PREFER A COMPOSITE OF THE GFS/ECMWF AND GEFS/ECMWF  
ENSEMBLE MEANS BY NEXT WEEKEND FOR THE BEST DETAIL AS FEASIBLE IN  
A PERIOD WITH SLOWLY GROWING FORECAST SPREAD.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
LOCALLY ENHANCED RAIN CHANCES WILL SHIFT SOUTHWARD ACROSS  
CALIFORNIA THIS WEEK AS A PLUME OF DEEPENED MOISTURE IS CHANNELED  
AROUND A CLOSED UPPER TROUGH/SURFACE SYSTEM DUG OVER THE EASTERN  
PACIFIC. RAIN AMOUNTS AND RATES MAY BE SUFFICIENT FOR SOME LOCAL  
FLOODING CONCERNS, ESPECIALLY GIVEN SOME INSTABILITY COULD REACH  
COASTAL AREAS. THE WPC EXCESSIVE RAINFALL OUTLOOK (ERO) OFFERS  
MARGINAL RISKS FOR WEDNESDAY INTO THURSDAY FROM CENTRAL TO  
SOUTHERN COASTAL CALIFORNIA. ORGANIZED RAINS AND TERRAIN/MOUNTAIN  
FOCUSING SNOWS SET TO WORK INLAND THROUGH THE SIERRA AND WITH SOME  
THREAT FROM THE SOUTHWEST ACROSS THE SOUTH-CENTRAL GREAT  
BASIN/ROCKIES FOR THE HOLIDAY WEEKEND WITH SLOW SYSTEM APPROACH.  
MEANWHILE, SYSTEM/ENERGY DOWNSTREAM PROPAGATION MAY ALSO GENERATE  
AN EMERGING WET PATTERN FROM THE SOUTH-CENTRAL PLAINS WITH SYSTEM  
GENESIS AND WITH MORE UNCERTAINTY TO THE EAST GIVEN RETURN  
MOISTURE INFLOW SIGNAL IN GUIDANCE. THIS COULD INCLUDE SOME  
NORTHERN TIER WINTRY PRECIPITATION. MEANWHILE UPSTREAM, UPPER  
SHORTWAVE TROUGH AND SURFACE SYSTEM PROGRESSION INTO THE NORTHWEST  
SHOULD SUPPORT PERIODS OF ENHANCED PRECIPITATION FRIDAY 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 FORECAST, EXCESSIVE RAINFALL OUTLOOK,  
WINTER WEATHER OUTLOOK PROBABILITIES, HEAT INDICES AND KEY  
MESSAGES 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/#PAGE=ERO  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=OVW  
 

 
 

 
 
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