435  
FXUS02 KWBC 200700  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
159 AM EST MON DEC 20 2021  
 
VALID 12Z THU DEC 23 2021 - 12Z MON DEC 27 2021  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE LARGE SCALE PATTERN CONTINUES TO SHOW BETTER THAN AVERAGE  
AGREEMENT AND PREDICTABILITY. SHORTWAVES TRACKING AROUND A VERY  
STRONG CENTRAL PACIFIC INTO BERING SEA/ALASKA UPPER RIDGE WILL  
PERIODICALLY DIVE INTO AND REINFORCE AN AMPLIFIED TROUGH WHOSE  
AXIS SHOULD BE NEAR THE WEST COAST, POSSIBLY DRIFTING SLIGHTLY  
INLAND LATER IN THE PERIOD. FAST FLOW DOWNSTREAM--BETWEEN LOW  
HEIGHTS OVER CANADA AND A RIDGE TRACKING OVER MEXICO AND THE  
SOUTHERN GULF OF MEXICO--WILL CARRY ALONG EJECTING FEATURES THAT  
SHOULD SUPPORT A COUPLE SURFACE SYSTEMS DURING THE PERIOD. EXPECT  
THE WEST COAST UPPER TROUGH TO SUPPORT A BROAD AREA OF  
PRECIPITATION OVER THE WEST FOR MULTIPLE DAYS WITH AREAS FROM  
WESTERN OREGON INTO THE SIERRA NEVADA LIKELY TO SEE THE HIGHEST  
FIVE-DAY TOTALS. ALSO OF NOTE, A STRONG TEMPERATURE CONTRAST WILL  
SET UP BETWEEN MUCH ABOVE NORMAL TEMPERATURES OVER AND EAST OF THE  
CENTRAL/SOUTHERN PLAINS AND WELL BELOW NORMAL TEMPERATURES OVER  
THE EXTREME NORTHERN PLAINS. EVEN WITH THE ABOVE AVERAGE  
CONFIDENCE IN THE LARGE SCALE PATTERN, EMBEDDED DETAILS HAVE LOWER  
PREDICTABILITY SO SOME SPECIFICS FOR THE LOWER 48 FORECAST REMAIN  
UNCERTAIN.  
 
THE FIRST MAJOR FEATURE OF NOTE WILL BE A WEST COAST SHORTWAVE  
THAT EJECTS EASTWARD AFTER EARLY THURSDAY AND LIKELY PRODUCES A  
PLAINS THROUGH SOUTHEASTERN CANADA/NORTHEAST U.S. SYSTEM AROUND  
FRIDAY AND CHRISTMAS DAY/SATURDAY. OVER RECENT DAYS THERE HAS  
BEEN A FAIR AMOUNT OF VARIABILITY FOR THE CHARACTER OF THIS ENERGY  
NEAR THE WEST COAST AND HOW QUICKLY IT WILL CONTINUE ONWARD. AN  
AVERAGE OF 12Z/18Z GUIDANCE EVENTUALLY TRENDING TOWARD A MIX OF  
MODELS AND MEANS PROVIDED A GOOD INTERMEDIATE SOLUTION EARLY IN  
THE PERIOD AND HELD CLOSE TO CONTINUITY LATER AS THE 18Z GFS/12Z  
ECMWF ENDED UP TRENDING SLOWER THAN IN THE PREVIOUS CYCLE WHILE  
THE ENSEMBLE MEANS AND CMC CONTINUE TO REFLECT A FASTER SOLUTION  
PER CONTINUITY. OVERALL THE NEW 00Z MODEL RUNS HAVE BECOME LESS  
AMPLIFIED WITH THE WEST COAST SHORTWAVE AS OF EARLY THURSDAY AND  
DIFFER FOR DETAILS OF SEPARATE OFFSHORE ENERGY, AFFECTING  
COVERAGE/TIMING OF PRECIPITATION OVER SOUTHERN PARTS OF THE WEST.  
THIS CHANGE HAS ALLOWED THE GFS AND ECMWF TO RETURN TO A FASTER  
SURFACE PROGRESSION OVER EASTERN NORTH AMERICA THOUGH.  
 
THE AFOREMENTIONED STRATEGY ALSO WORKED WELL FOR RESOLVING LOWER  
CONFIDENCE DETAILS WITHIN AND DOWNSTREAM FROM THE PERSISTENT WEST  
COAST TROUGH AFTER EJECTION OF THE FIRST SHORTWAVE. THERE IS A  
REASONABLE SIGNAL IN THE MODELS AND ENSEMBLE MEANS THAT ANOTHER  
GENERAL AREA OF LOW PRESSURE SHOULD EVOLVE OVER THE CENTRAL U.S.  
AROUND SUNDAY-MONDAY.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
A BROAD AREA OF THE WEST WILL SEE LOWER ELEVATION RAIN AND  
MID-HIGH ELEVATION SNOW WITH A PERSISTENT FLOW OF MOISTURE  
DIRECTED ONSHORE AHEAD OF SHORTWAVES DIGGING INTO THE WEST COAST  
MEAN TROUGH ALOFT, WITH MOISTURE AND TERRAIN ENHANCEMENT EXTENDING  
AS FAR EAST AS THE ROCKIES. THERE WILL BE PERIODS WHEN THE  
PRECIPITATION WILL BE MODERATE OR HEAVY WHICH MAY INCREASE SOIL  
SATURATION AND SENSITIVITY TO LOCAL RUNOFF; HOWEVER UNCERTAINTY ON  
TIMING AND SHORTWAVE INTERACTIONS WILL TEMPER CONFIDENCE IN  
RESOLVING THE EXACT LOCATION AND INTENSITY OF LOWER ELEVATION  
RAINFALL WITH RESPECT TO LOCAL RUNOFF/FLOODING ISSUES OVER  
CALIFORNIA. THE OREGON CASCADES AND PARTS OF NORTHERN/CENTRAL  
CALIFORNIA ARE LIKELY TO HAVE SOME OF THE HIGHEST ACCUMULATIONS,  
ESPECIALLY HEAVY SNOW IN THE SIERRA NEVADA. THESE SNOWS MAY HAVE  
HIGH LIQUID EQUIVALENTS IN THE FAVORED UPSLOPE AREAS OF THE SIERRA  
NEVADA RANGE. FAVORED TERRAIN OVER THE GREAT BASIN AND  
NORTHERN/CENTRAL ROCKIES WILL LIKELY SEE MEANINGFUL PRECIPITATION  
AS WELL. THE TIMING AND AMOUNTS OF PRECIPITATION OVER SOUTHERN  
PARTS OF THE WEST HAVE BECOME MORE OF A QUESTION MARK WITH LATEST  
ADJUSTMENTS IN THE MODELS. COLDER AIR PUSHING INTO NORTHWEST MAY  
LEAD TO VERY LOW SNOW LEVELS BY NEXT SUNDAY-MONDAY WHILE OTHER  
PARTS OF THE WEST MAY ALSO SEE DECLINING SNOW LEVELS AS THE UPPER  
TROUGH AXIS NUDGES EASTWARD.  
 
A LATE WEEK INTO WEEKEND SYSTEM SHOULD SPREAD PRECIPITATION ACROSS  
THE NORTHEASTERN QUADRANT OF THE COUNTRY. BEST POTENTIAL FOR SOME  
SNOW WILL BE ACROSS THE EXTREME NORTHERN TIER INCLUDING THE UPPER  
GREAT LAKES AND PARTS OF NEW ENGLAND. CONFIDENCE IS ONLY MODERATE  
AT BEST FOR SYSTEM DETAILS GIVEN MODEL VARIABILITY THUS FAR.  
ANOTHER AREA OF PRECIPITATION (SOME SNOW IN NORTHERN AREAS) MAY  
DEVELOP OVER THE NORTHERN AND EAST-CENTRAL U.S. TOWARD EARLY NEXT  
WEEK WITH ANOTHER AREA OF LOW PRESSURE.  
 
THE CENTRAL AND SOUTHERN PLAINS WILL SEE MUCH ABOVE NORMAL  
TEMPERATURES FOR MOST OF THE PERIOD WITH PLUS 15-25F ANOMALIES.  
HIGHS COULD EVEN REACH OR EXCEED 30F ABOVE NORMAL AT SOME  
LOCATIONS ON FRIDAY. FLOW AHEAD OF THE LATE WEEK SYSTEM WILL PUSH  
SOME OF THIS WARMTH INTO THE EAST AS WELL. DAILY RECORDS FOR  
HIGHS/WARM LOWS WILL BE POSSIBLE WHERE TEMPERATURES ARE MOST  
ANOMALOUS. IN CONTRAST, TEMPERATURES THAT ARE 15-30F OR MORE  
BELOW NORMAL SHOULD SETTLE INTO PARTS OF MONTANA AND NORTH DAKOTA.  
THE WEST WILL TREND GRADUALLY COLDER WITH TIME, LEADING TO SOME  
HIGHS 10-15F BELOW NORMAL BY THE WEEKEND AND EARLY NEXT WEEK.  
 
RAUSCH/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  
 
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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