071  
FXUS02 KWBC 090659  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
159 AM EST THU DEC 09 2021  
 
VALID 12Z SUN DEC 12 2021 - 12Z THU DEC 16 2021  
 
...AN ATMOSPHERIC RIVER WILL SPREAD SOUTHWARD ALONG THE WEST COAST  
EARLY NEXT WEEK BRINGING COASTAL RAIN AND HEAVY MOUNTAIN SNOW...  
   
..OVERVIEW
 
 
THE MEDIUM RANGE PERIOD WILL BEGIN SUNDAY WITH UPPER TROUGHING  
ACROSS THE EAST THAT WILL MOVE QUICKLY INTO THE ATLANTIC, WITH  
UPPER RIDGING BEHIND IT EXPANDING FROM THE CENTRAL U.S. INTO THE  
EASTERN U.S. AS THE WORKWEEK STARTS, CAUSING UNSEASONABLY WARM  
TEMPERATURES. MEANWHILE, A SLOWLY DIGGING AND RELOADING TROUGH IN  
THE EASTERN PACIFIC WILL DIRECT A PERSISTENT AND MOIST INFLOW  
(ATMOSPHERIC RIVER) TO THE WEST COAST, WITH COASTAL RAIN AND HEAVY  
MOUNTAIN SNOWS THAT SHOULD SPREAD SOUTH AND EASTWARD WITH TIME AS  
THE TROUGH MOVES. BY WEDNESDAY INTO THURSDAY, AN UPPER LOW MAY  
SPIN UP FROM THE TROUGH ALONG WITH A SURFACE LOW, WHICH COULD  
BRING ANOTHER ROUND OF PRECIPITATION TO THE NORTH-CENTRAL U.S. BUT  
WITH LOW CONFIDENCE IN THE DETAILS.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE 12/18Z (YESTERDAY) CYCLE OF MODEL GUIDANCE CONTINUES TO SHOW  
VERY GOOD AGREEMENT ESPECIALLY THROUGH THE FIRST PART OF THE  
PERIOD WITH THE OVERALL PATTERN DESCRIBED ABOVE. THERE REMAIN SOME  
MINOR MODEL DIFFERENCES INITIALLY WITH THE EASTERN PACIFIC TROUGH  
REGARDING THE DISTRIBUTION OF ENERGY AND THE POTENTIAL  
FOR/PLACEMENT OF A CLOSED LOW TO DEVELOP WITHIN IT. BY AROUND  
TUESDAY, THERE ARE SOME SLIGHT MODEL DIFFERENCES WITH THE TIMING  
OF THE TROUGH AXIS SHIFTING EASTWARD. THE DETERMINISTIC GUIDANCE  
WAS GENERALLY WITHIN THE CENTER OF THE GUIDANCE ENVELOPE, WHILE  
GEFS ENSEMBLE MEMBERS GENERALLY TAKE A FASTER/MORE EASTERN TRACK  
COMPARED TO EC ENSEMBLE MEMBERS. BUT OVERALL GUIDANCE AGREES IN A  
TRACK OF THE TROUGH AXIS SOUTHWARD AND THEN EASTWARD, BRINGING  
PERSISTENT MOIST SOUTHWESTERLY FLOW INTO THE WEST COAST EARLY IN  
THE WEEK AND SPREADING INLAND BY TUESDAY. GREATER DIFFERENCES  
ARISE BY WEDNESDAY-THURSDAY WITH THIS TROUGH AS IT EJECTS FARTHER  
EASTWARD AND WITH UPSTREAM SHORTWAVE ENERGY DROPPING SOUTH THROUGH  
THE EASTERN PACIFIC. WHILE THERE ARE SOME VARIATIONS IN STRENGTH  
AND TRACK OF THESE FEATURES, THE 12/18Z DETERMINISTIC MODELS HAVE  
GOOD CONSENSUS SPINNING UP A CLOSED MID-UPPER LOW IN THE  
NORTH-CENTRAL U.S. BY THURSDAY (THOUGH SOME INCOMING 00Z GUIDANCE  
SHOWS A COMPACT SHORTWAVE THAT IS NOT CLOSED). A SURFACE LOW  
ASSOCIATED WITH THIS FEATURE VARIES SOMEWHAT IN DEPTH AND TRACK AS  
WELL, BUT AT LEAST THERE IS CONSENSUS FOR THE EXISTENCE OF THESE  
FEATURES BY THE END OF THE MEDIUM RANGE PERIOD. MEANWHILE, MODELS  
AGREE WITH THE BROAD RIDGING ACROSS CENTRAL AND EASTERN PARTS OF  
THE U.S. THROUGH MUCH OF THE WEEK.  
 
THE WPC FORECAST UTILIZED A MULTI-MODEL DETERMINISTIC BLEND OF THE  
12/18Z GUIDANCE EARLY IN THE PERIOD, AND WAS ABLE TO HOLD ONTO A  
MAJORITY DETERMINISTIC MODELS BLEND THROUGH THE PERIOD GIVEN  
REASONABLY GOOD AGREEMENT, THOUGH INCREASED RELIANCE ON THE  
ENSEMBLE MEANS ESPECIALLY BY DAY 7. THIS MAINTAINED EXCELLENT  
CONTINUITY WITH THE PREVIOUS WPC FORECAST FOR DAYS 3-6 AS WELL.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
MOIST ONSHORE FLOW WITH A WEAK ATMOSPHERIC RIVER WILL CAUSE  
WIDESPREAD PRECIPITATION IN THE WESTERN U.S. THROUGH THE FIRST  
HALF OF NEXT WEEK. PRECIPITATION IS FORECAST FOR THE PACIFIC  
NORTHWEST, NORTHERN ROCKIES, AND ESPECIALLY NORTHERN CALIFORNIA ON  
SUNDAY, WHICH WILL SPREAD SOUTHWARD INTO SOUTHERN CALIFORNIA  
MONDAY AND EAST INTO THE GREAT BASIN AND FOUR CORNERS STATES ON  
TUESDAY. ENHANCED TOTALS ARE LIKELY ALONG FAVORED TERRAIN, WITH  
HEAVY MOUNTAIN SNOW ESPECIALLY IN THE SIERRA NEVADA AND PORTIONS  
OF THE ROCKIES. COASTAL RAIN IS EXPECTED IN CALIFORNIA, AND SOME  
LOCALLY HIGH RAINFALL RATES CAUSING FLOODING ISSUES, ESPECIALLY  
FOR SENSITIVE BURN SCAR AREAS, CANNOT BE RULED OUT. TEMPERATURES  
WILL BE NEAR TO SOMEWHAT BELOW NORMAL IN THE WEST UNDERNEATH THE  
UPPER TROUGH, WITH TEMPERATURES 10-20F BELOW AVERAGE FOR PARTS OF  
CALIFORNIA AND NEVADA.  
 
FARTHER EAST, THE MAIN STORY IS INCREASING WARMTH IN THE CENTRAL  
AND EASTERN U.S. THROUGH THE PERIOD GIVING THE STRONG RIDGING  
ALOFT. MILD TO WARM TEMPERATURES ABOUT 10-20F ABOVE AVERAGE BEGIN  
IN THE CENTRAL U.S. SUNDAY, WHILE TEMPERATURES SHOULD ONLY  
INCREASE FROM THERE, REACHING WIDESPREAD 20+ AND SOME LOCATIONS  
WITH 30+ TEMPERATURE ANOMALIES BY WEDNESDAY--CURRENTLY FORECAST IN  
CENTRAL PARTS OF THE PLAINS/MISSISSIPPI VALLEY WHERE HIGHS COULD  
REACH INTO THE 70S. RECORD WARM MINIMUM AND MAXIMUM TEMPERATURES  
ARE POSSIBLE/LIKELY FROM THE SOUTHERN PLAINS TO GREAT LAKES  
REGION. TEMPERATURES 10-20F ABOVE AVERAGE ARE FORECAST TO EXPAND  
INTO THE EASTERN SEABOARD AS WELL. ANY WIDESPREAD PRECIPITATION IS  
NOT LIKELY IN THE CENTRAL/EASTERN U.S. UNTIL NEXT WEDNESDAY AS A  
LOW PRESSURE SYSTEM COMES INTO THE MIDWEST, WITH SNOW POSSIBLE IN  
THE NORTHERN LATITUDES AND RAIN POSSIBLE ALONG ITS TRAILING FRONT,  
BUT SPECIFIC PRECIPITATION TOTALS WILL HAVE TO BE IRONED OUT IN  
FUTURE FORECAST UPDATES.  
 
TATE  
 
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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