963  
FXUS02 KWBC 250550  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1250 AM EST SAT DEC 25 2021  
 
VALID 12Z TUE DEC 28 2021 - 12Z SAT JAN 01 2022  
 
...BITTERLY COLD TEMPERATURES ARE FORECAST ACROSS PORTIONS OF THE  
NORTHWEST TO NORTHERN PLAINS EARLY NEXT WEEK...  
 
   
..OVERVIEW
 
 
THE UPPER PATTERN REMAINS RELATIVELY UNCHANGED THROUGHOUT MUCH OF  
THE MEDIUM RANGE PERIOD LED BY A PERSISTENT AND HIGHLY AMPLIFIED  
NORTHERN PACIFIC RIDGE/UPPER HIGH. DOWNSTREAM, TROUGHING IS  
EXPECTED OVER THE WESTERN U.S. AS A MID-UPPER LOW DRIFTS EASTWARD  
OVER WEST-CENTRAL CANADA, AND SHORTWAVES EJECT FROM THE UPPER  
TROUGH AND ARE CARRIED ALONG IN FAST WEST-SOUTHWESTERLY MEAN FLOW.  
THIS SHOULD SUPPORT A SERIES OF LOW PRESSURE SYSTEMS TRACKING OVER  
THE CENTRAL AND NORTHEASTERN U.S.. MEANWHILE, ANOTHER UPPER HIGH  
OVER THE BAHAMAS WILL ALLOW FOR RIDGING AND ABOVE NORMAL  
TEMPERATURES ACROSS THE SOUTHEASTERN QUADRANT OF THE COUNTRY TO  
THE SOUTH OF THE STORM TRACK. BY THE LATTER HALF OF THE WEEK,  
MODEL GUIDANCE IS INDICATING THE POTENTIAL FOR A SOUTHERN STREAM  
UPPER LOW DROPPING THROUGH THE EASTERN PACIFIC JUST OFF THE  
CALIFORNIA COAST.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE LATEST MODELS AND ENSEMBLES CONTINUE TO SHOW GOOD AGREEMENT  
WITH THE LARGE SCALE PATTERN THROUGH MUCH OF THE PERIOD, BUT WITH  
SOME LINGERING UNCERTAINTIES IN THE SMALLER SCALE DETAILS. A  
SERIES OF SHORTWAVES ROTATING OUT OF THE WESTERN U.S. TROUGH  
TOWARDS THE GREAT LAKES/NORTHEAST HAVE COME INTO A LITTLE BIT  
BETTER AGREEMENT BUT THERE ARE SOME TIMING/PLACEMENT DIFFERENCES  
WILL MAY TAKE UNTIL THE SHORT RANGE TO RESOLVE. THIS WOULD IMPACT  
THE EXACT AXIS OF HEAVY RAINFALL ACROSS THE OHIO VALLEY MID NEXT  
WEEK. THE OTHER NOTABLE AREA OF CONCERN IS REGARDING THE NEXT  
SHORTWAVE DROPPING DOWN THE WEST COAST, WITH POTENTIAL FOR A  
CLOSED LOW OFF THE CALIFORNIA COAST BY NEXT THURSDAY. THE  
18Z/YESTERDAY GFS FOLLOWED SUIT WITH ITS PREDECESSOR RUNS IN BEING  
WEAKER AND MUCH FASTER THAN THE BETTER CONSENSUS IT BRINGS THE  
ENERGY INTO THE SOUTHWEST BY EARLY FRIDAY WHILE THE REST OF THE  
DETERMINISTIC MODELS (AND ENSEMBLE MEANS) SHOW A WELL DEVELOPED  
CLOSED LOW WELL OFF OF SOUTHERN CALIFORNIA AT THAT TIME.  
 
FOR THE WPC BLEND, THE AVAILABLE GUIDANCE PRESENTED ENOUGH  
AGREEMENT FOR DAYS 3-4 TO WARRANT A PURELY DETERMINISTIC MODEL  
BLEND FOR DAYS 3-4/TUESDAY-WEDNESDAY. AFTER THAT, THE ENSEMBLE  
MEANS WERE INTRODUCED AND GRADUALLY INCREASED AS THE PERIOD  
PROGRESSED TO HELP MITIGATE THE SMALLER SCALE DETAIL DIFFERENCES.  
THE GFS WAS NOT INCLUDED IN THE BLEND AFTER DAY 4, DUE TO THE  
ISSUES WITH THE CLOSED LOW OFF SOUTHERN CALIFORNIA. THIS APPROACH  
ALSO PROVIDED GOOD CONTINUITY WITH THE PREVIOUS WPC FORECAST  
OVERALL.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
PRECIPITATION ACROSS THE WEST SHOULD BE WANING BY THE START OF THE  
PERIOD ON TUESDAY, ALTHOUGH SOME MODEST SNOW ACCUMULATIONS MAY  
CONTINUE IN THE HIGHER TERRAIN, ESPECIALLY FOR PARTS OF ARIZONA  
AND THE COLORADO ROCKIES. ACTIVITY SHOULD QUIET DOWN ON WEDNESDAY,  
BUT ANOTHER ROUND OF HEAVY MOUNTAIN SNOWS ARE POSSIBLE FOR  
PORTIONS OF THE PACIFIC NORTHWEST/NORTHERN ROCKIES AHEAD OF THE  
SHORTWAVE DROPPING DOWN THE WEST COAST ON THURSDAY. THE CLOSED LOW  
DEVELOPMENT OFF THE CALIFORNIA COAST MAY HELP DIRECT INCREASED  
MOISTURE INTO PARTS OF SOUTHERN CALIFORNIA AS WELL LATER NEXT WEEK.  
 
GENERALLY PROGRESSIVE LOW PRESSURE SYSTEMS WILL SPREAD PERIODIC  
PRECIPITATION ACROSS PARTS OF THE CENTRAL AND EASTERN 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.  
 
BITTERLY COLD TEMPERATURES WILL BE ONGOING BY TUESDAY IN PARTS OF  
THE NORTHWESTERN AND NORTH-CENTRAL U.S., WITH DAYTIME HIGHS (AND A  
LESSER EXTENT OVERNIGHT LOWS) 20 TO NEARLY 40 DEGREES BELOW  
AVERAGE. SOME AREAS OF NORTH DAKOTA AND MONTANA MAY STAY BELOW 0F  
FOR MORE THAN 48 HOURS TUESDAY-WEDNESDAY BEFORE MODERATING  
SOMEWHAT BY THURSDAY. COLD TEMPERATURES 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 FOR MANY. TEMPERATURES ARE ALSO EXPECTED TO TREND COLDER  
IN THE REST OF THE WESTERN U.S., WITH EXPANDING COVERAGE OF HIGHS  
10-25F BELOW NORMAL EARLY TO MID NEXT WEEK. INCREASING NUMBERS OF  
LOCATIONS ALONG AND JUST INLAND FROM THE WEST COAST COULD SEE  
DAILY RECORDS FOR LOW MAXIMUM AND MINIMUM TEMPERATURES TUESDAY AND  
WEDNESDAY. MEANWHILE, MILD TO WARM WEATHER IS EXPECTED ACROSS THE  
SOUTH-CENTRAL U.S. AND NORTHEASTWARD THROUGH NEXT WEEK WHERE MANY  
PLACES FROM THE SOUTHERN PLAINS TO THE OHIO VALLEY/MID-ATLANTIC  
COULD BE 20 TO 30 DEGREES ABOVE NORMAL TUESDAY INTO WEDNESDAY.  
DAILY RECORD HIGH MINIMUM AND MAXIMUM TEMPERATURES ARE LIKELY. AS  
THE PATTERN DEAMPLIFIES JUST SLIGHTLY OVER THE COUNTRY LATER NEXT  
WEEK, TEMPERATURES ACROSS BOTH THE WEST AND EAST SHOULD MODERATE,  
BUT STILL MAINTAIN A GENERAL COOL IN THE WEST, WARM IN THE EAST,  
JUST LESS EXTREME.  
 
SANTORELLI  
 
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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