768  
FXUS02 KWBC 201916  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
215 PM EST MON DEC 20 2021  
 
VALID 12Z THU DEC 23 2021 - 12Z MON DEC 27 2021  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
DURING THE EXTENDED PERIOD THERE WILL BE MULTIPLE SHORTWAVES  
TRACKING AROUND THE VERY STRONG RIDGE OVER THE CENTRAL PACIFIC  
OCEAN INTO THE BERING SEA/ALASKA WHICH WILL DIVE INTO AND  
REINFORCE AN AMPLIFIED TROUGH WHOSE AXIS SHOULD BE NEAR THE WEST  
COAST, POSSIBLY DRIFTING SLIGHTLY INLAND LATER IN THE PERIOD.  
THESE EJECTED IMPULSES ARE DEPICTED TO BE CARRIED DOWNSTREAM THAT  
WILL SUPPORT THE DEVELOPMENT OF A PAIR OF LOW PRESSURE SYSTEMS.  
WITH A BROAD TROUGH NEAR THE WEST COAST, A LARGE AREA OF  
PRECIPITATION WILL BE MAINTAINED FOR SEVERAL DAYS. ALSO OF NOTE, A  
STRONG TEMPERATURE CONTRAST WILL SET UP BETWEEN WELL ABOVE NORMAL  
TEMPERATURES PROGGED FOR THE CENTRAL AND SOUTHERN PLAINS WHILE  
WELL BELOW NORMAL TEMPERATURES SETTLES IN OVER THE EXTREME  
NORTHERN PLAINS.  
 
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. THERE HAS BEEN A NOTABLE  
VARIABILITY WITH THE TIMING/STRENGTH/PROGRESSION ETC THE PAST FEW  
MODELS RUNS BUT IS FAVORING A LESS AMPLIFIED SOLUTION. 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.TO MAINTAIN A SINCE OF CONTINUITY, THE WPC  
MODEL PREFERENCES INCLUDED THE 00Z/06Z GFS, 00Z ECWMF/CMC/UKMET  
WITH INCREASING WEIGHTING OF THE EC ENSEMBLE, THE NAEFS AND GEFS  
MEANS BY THE LATTER PERIODS.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
A PERSISTENT PLUME OF MOISTURE WILL BE DIRECTED ONSHORE THE WEST  
COAST AHEAD OF MULTIPLE SHORTWAVES DIGGING INTO THE WEST COAST  
MEAN TROUGH ALOFT WITH MOISTURE AND TERRAIN ENHANCEMENT EXTENDING  
AS FAR EAST AS THE ROCKIES AND TO SOUTHERN CALIFORNIA/GREAT BASIN.  
MODERATE TO HEAVY PRECIPITATION CAN BE EXPECTED OVER THE WESTERN  
HALF OF THE PACIFIC NORTHWEST AND PORTIONS OF CALIFORNIA, WHICH  
WILL INCREASE THE 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. ABUNDANT LIFT AND MOISTURE INTO THE  
FAVORED UPSLOPE AREAS OF THE OREGON CASCADES AND THE SIERRA NEVADA  
WILL LIKELY RESULT IN THE HIGHEST ACCUMULATIONS AND LIQUID  
EQUIVALENTS. MEANINGFUL PRECIPITATION WILL LIKELY BE OBSERVED  
ACROSS THE HIGHEST TERRAIN OF THE GREAT BASIN AND THE CENTRAL AND  
NORTHERN ROCKIES. 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 FILTERING INTO  
THE 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.  
 
ACROSS THE NORTHERN TIER, AN INITIAL SYSTEM IS EXPECTED TO SPREAD  
SPREAD PRECIPITATION ACROSS THE NORTHEASTERN SECTION OF THE  
COUNTRY. ANOTHER LOW PRESSURE SYSTEM WILL PASS THROUGH THE  
NORTHERN STATES BY NEXT WEEK USHERING PRECIPITATION WITH SOME  
SPOTS OF SNOW TO NORTHERN STATES AND ALSO THE EAST-CENTRAL U.S.  
TEMPERATURES WILL BE COLDER THAN SEASONAL AVERAGE, WITH MANY  
LOCATIONS IN THE DAKOTAS AND MONTANA AVERAGING 15 TO 30 DEGREES F  
BELOW NORMAL FOR LATE DECEMBER. ADDITIONALLY, THE WEST WILL BEGIN  
TO TREND COOLER BY THE END OF THE EXTENDED FORECAST WITH DAILY  
HIGHS AROUND 10 TO 15 DEGREES F COOLER.  
 
FOR THE CENTRAL AND SOUTHERN PLAINS THERE WON'T BE A WHITE  
CHRISTMAS BECAUSE TEMPERATURES WILL BE QUITE TOASTY, WITH DAILY  
MAXIMUMS AROUND 15 TO 25 DEGREES F ABOVE THE MEAN FOR LATE  
DECEMBER. A FEW LOCATIONS MAY ACTUALLY SURPASS 30 DEGREES F ABOVE  
AVERAGE FRIDAY  
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. DAILY RECORDS FOR HIGHS/WARM LOWS WILL BE  
POSSIBLE WHERE TEMPERATURES ARE MOST ANOMALOUS AND THIS WARMTH  
WILL SPREAD INTO THE EAST U.S.  
 
CAMPBELL/RAUSCH  
 
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  
 

 
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page