116  
FXUS02 KWBC 240711  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
210 AM EST FRI DEC 24 2021  
 
VALID 12Z MON DEC 27 2021 - 12Z FRI DEC 31 2021  
 
...POTENTIAL FOR HEAVY MOUNTAIN SNOWS IN THE WEST CONTINUES  
THROUGH EARLY NEXT WEEK...  
 
...BITTERLY COLD TEMPERATURES ARE FORECAST ACROSS PORTIONS OF THE  
NORTHWEST TO NORTHERN PLAINS NEXT WEEK...  
 
   
..OVERVIEW
 
 
A RATHER STAGNANT PATTERN IS FORECAST FOR THE MEDIUM RANGE PERIOD  
LED BY A PERSISTENT AND HIGHLY AMPLIFIED NORTHERN PACIFIC  
RIDGE/UPPER HIGH. DOWNSTREAM, DEEP TROUGHING IS EXPECTED OVER THE  
WESTERN U.S. AS A LIKELY CLOSED MID-UPPER LOW DRIFTS EASTWARD OVER  
SOUTHWESTERN AND SOUTH-CENTRAL CANADA, WHILE SHORTWAVES EJECT FROM  
THE UPPER TROUGH AND ARE CARRIED ALONG IN FAST WEST-SOUTHWESTERLY  
MEAN FLOW, WHICH WILL SUPPORT A SERIES OF LOW PRESSURE SYSTEMS  
DEVELOPING AND TRACKING OVER THE CENTRAL AND NORTHEASTERN U.S..  
MEANWHILE, ANOTHER UPPER HIGH MOVING FROM THE GULF OF MEXICO EARLY  
WEEK INTO THE BAHAMAS BY MIDWEEK WILL SPREAD RIDGING AND ABOVE  
NORMAL TEMPERATURES TO THE SOUTHEASTERN QUADRANT OF THE COUNTRY  
SOUTH OF THE STORM TRACK.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE LATEST MODEL GUIDANCE REMAINS IN MORE OR LESS GOOD AGREEMENT  
WITH THE LARGE SCALE PATTERN THROUGH MUCH OF NEXT WEEK. THERE ARE  
SOME UNCERTAINTIES IN THE DETAILS, PARTICULARLY PERTAINING TO A  
COUPLE OF SHORT WAVES ROTATING OUT OF THE WESTERN U.S. TROUGH. THE  
12Z/YESTERDAY UKMET WAS MUCH WEAKER WITH AN INITIAL SHORTWAVE ON  
MONDAY-TUESDAY INTO THE UPPER GREAT LAKES AND EVENTUALLY THE  
NORTHEAST THAN THE CONSENSUS SO IT WAS NOT INCLUDED IN THE WPC  
BLEND. MID WEEK, ENERGY SHOULD DROP DOWN THE WEST COAST WITH SOME  
GUIDANCE (SUCH AS THE 12Z AND 18Z YESTERDAY ECMWF AND GFS RUNS)  
INDICATING POTENTIAL FOR A CLOSED LOW OVER OR OFF THE CENTRAL  
CALIFORNIA COAST BY LATER NEXT WEEK. THERE IS PLENTY OF  
VARIABILITY IN THIS FORECAST WITH REGARDS TO TIMING AND EVOLUTION  
SO THE FORECAST REMAINS HIGHLY UNCERTAIN AT THIS TIME.  
 
FOR THE WPC BLEND, THE GUIDANCE PRESENTED ENOUGH AGREEMENT FOR  
DAYS 3-5 TO WARRANT A PURELY DETERMINISTIC MODEL BLEND BETWEEN THE  
GFS, ECMWF, AND CMC. AFTER THAT, THE ENSEMBLE MEANS WERE  
INTRODUCED TO HELP MITIGATE THE SMALLER SCALE DETAILS DIFFERENCES  
WHICH WILL TAKE SEVERAL DAYS TO RESOLVE STILL. THIS APPROACH  
PROVIDED GOOD CONTINUITY WITH YESTERDAYS WPC FORECAST AS WELL.  
 
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
ROUNDS OF PRECIPITATION SHOULD CONTINUE INTO THE WEST AS THE  
MEDIUM RANGE PERIOD BEGINS MONDAY AS SHORTWAVE ENERGY ROUNDING THE  
PERSISTENT UPPER TROUGH PERIODICALLY ENHANCES MOISTURE INFLOW. THE  
HIGHEST SNOW TOTALS ARE LIKELY FOR THE TYPICAL TERRAIN ENHANCED  
AREAS OF THE SIERRA NEVADA, AND PORTIONS OF THE ROCKIES MONDAY AND  
TUESDAY. THE BEST MOISTURE/PRECIPITATION FOCUS SHOULD SHIFT  
SOUTHWARD WITH TIME INTO SOUTHERN CALIFORNIA AND THE SOUTHWEST ON  
WEDNESDAY INTO THURSDAY.  
 
GENERALLY PROGRESSIVE LOW PRESSURE SYSTEMS WILL SPREAD PERIODIC  
PRECIPITATION ACROSS THE NORTH-CENTRAL AND NORTHEASTERN U.S.  
THROUGH NEXT WEEK. THE BEST SNOW CHANCES SHOULD BE CONFINED TO THE  
GREAT LAKES AND INTERIOR NORTHEAST, WITH RAIN TO THE SOUTH. THE  
ASSOCIATED TRAILING COLD FRONT SLOWLY SINKING SOUTHEASTWARD COULD  
PROVIDE A FOCUS FOR ENHANCED RAINFALL ACROSS THE OHIO VALLEY  
AROUND TUESDAY-WEDNESDAY AND TOWARD THE TENNESSEE VALLEY AND  
SOUTHERN APPALACHIANS/SOUTHEAST BY THURSDAY-FRIDAY OF NEXT WEEK,  
BUT WITH UNCERTAINTY REMAINING REGARDING THE AMOUNTS AND AXIS OF  
THE HEAVIEST RAIN.  
 
MILD TO WARM WEATHER IS EXPECTED ACROSS THE SOUTH-CENTRAL U.S. AND  
NORTHEASTWARD THROUGH NEXT WEEK. THE SOUTHERN PLAINS TO  
MISSISSIPPI/TENNESSEE VALLEYS WILL BE AT THE CENTER OF THE  
GREATEST WARM TEMPERATURE ANOMALIES, ESPECIALLY MONDAY-WEDNESDAY  
WHERE MANY PLACES COULD BE 20 TO 30 DEGREES ABOVE NORMAL. DAILY  
RECORD HIGH MINIMUM AND MAXIMUM TEMPERATURES ARE LIKELY. ANOMALOUS  
TO RECORD WARMTH SHOULD PRESS SLIGHTLY EASTWARD AS THE WEEK  
PROGRESSES, WITH WELL ABOVE AVERAGE TEMPERATURES EXPECTED TO  
SPREAD INTO PARTS OF THE OHIO VALLEY PARTS OF THE EAST LATER NEXT  
WEEK. MEANWHILE, BITTERLY COLD TEMPERATURES ARE LIKELY IN PARTS OF  
THE NORTHWESTERN AND NORTH-CENTRAL U.S., WITH DAYTIME HIGHS (AND A  
LESSER EXTENT OVERNIGHT LOWS) NEARLY 20-40 DEGREES BELOW AVERAGE.  
THESE VALUES COMBINED WITH GUSTY WINDS COULD RESULT IN DANGEROUSLY  
COLD WIND CHILLS ESPECIALLY FOR PARTS OF THE NORTHERN HIGH PLAINS  
WHERE WIND CHILL VALUES COULD DIP WELL BELOW 0F. TEMPERATURES ARE  
ALSO EXPECTED TO TREND COLDER IN THE REST OF THE WESTERN U.S.,  
WITH EXPANDING COVERAGE OF HIGHS 10-25F BELOW NORMAL NEXT WEEK.  
INCREASING NUMBERS OF LOCATIONS ALONG AND JUST INLAND FROM THE  
WEST COAST COULD SEE DAILY RECORDS FOR LOW MAXIMA MONDAY THROUGH  
WEDNESDAY.  
 
SANTORELLI/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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