645  
FXUS02 KWBC 260641  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
140 AM EST SUN DEC 26 2021  
 
VALID 12Z WED DEC 29 2021 - 12Z SUN JAN 02 2022  
 
...BITTERLY COLD TEMPERATURES TO CONTINUE THROUGH MUCH OF THE  
PERIOD ACROSS PORTIONS OF THE NORTHERN PLAINS...  
 
   
..OVERVIEW
 
 
AN AMPLIFIED RIDGE/UPPER HIGH SHOULD REMAIN ANCHORED OVER THE  
NORTHERN PACIFIC INTO NEXT WEEKEND, RESULTING IN AMPLIFIED AND  
RENEWED TROUGHING DOWNSTREAM ACROSS THE WESTERN U.S. INCLUDING A  
SOUTHERN STREAM CUTOFF UPPER LOW WHICH DROPS DOWN THE CALIFORNIA  
COAST LATER THIS WEEK. NORTHERN STREAM SHORTWAVES THROUGH THE  
WESTERN TROUGH AND ROUNDING THE BASE OF A LARGE VORTEX ACROSS  
CENTRAL CANADA, WILL ALLOW FOR A SERIES OF LOW PRESSURE SYSTEMS TO  
TRACK FROM THE CENTRAL U.S. TO THE NORTHEAST. MEANWHILE, ANOTHER  
UPPER HIGH OVER THE CARIBBEAN WILL ALLOW FOR RIDGING ACROSS THE  
SOUTHEASTERN QUADRANT OF THE COUNTRY TO THE SOUTH OF THE STORM  
TRACK. THE GUIDANCE INDICATES THE ANOMALOUSLY STRONG HIGH OVER THE  
NORTHERN PACIFIC MAY FINALLY BEGIN TO WEAKEN NEXT WEEKEND WHICH  
SHOULD ALLOW FOR SOME RIDGING TO MOVE INTO THE NORTHWEST AND SHIFT  
THE GENERAL TROUGH AXIS EASTWARD.  
   
..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.  
RELATIVELY MINOR MODEL VARIATIONS WERE SEEN WITH A SERIES OF  
SHORTWAVES ROTATING OUT OF THE WESTERN U.S. TROUGH TOWARDS THE  
GREAT LAKES/NORTHEAST AND THE ASSOCIATED SURFACE LOWS  
WEDNESDAY-FRIDAY. MODELS ARE COMING INTO A LITTLE BIT BETTER  
AGREEMENT ON A CLOSED CUTOFF LOW OF THE CALIFORNIA COAST  
THURSDAY-FRIDAY. THE 18Z GFS IS A BIT FARTHER WEST OF THE BETTER  
CONSENSUS WHILE THE UKMET IS THE QUICKEST TO BRING IT INTO THE  
SOUTHWEST NEXT WEEKEND. AT THIS POINT, PREFER A MIDDLE GROUND  
SOLUTION CLOSER TO THE CMC/ECMWF AND ENSEMBLE MEANS. AS THE  
SOUTHERN STREAM LOW/ENERGY WORKS INTO THE SOUTHWEST, RIDGING  
SHOULD ONCE AGAIN BUILD INTO THE NORTHWEST AS THE ANOMALOUS NORTH  
PACIFIC RIDGE FINALLY WEAKENS. THE MODELS SHOW SOME UNCERTAINTIES  
REGARDING TIMING AND STRENGTH OF THE RIDGE BY DAYS 6-7 RELATED TO  
QUESTIONS UPSTREAM WITH TROUGHING DROPPING DOWN FROM ALASKA.  
 
THE EARLY PART OF THE MEDIUM RANGE FORECAST WAS BASED ON A BLEND  
OF PURELY DETERMINISTIC MODELS, MINUS THE UKMET WHICH WAS MUCH TOO  
QUICK WITH THE SOUTHERN STREAM ENERGY INTO THE SOUTHWEST. AFTER  
DAY 4, THE ENSEMBLE MEANS WERE INTRODUCED INTO THE BLEND AND  
GRADUALLY INCREASED TO HELP MITIGATE THE LATE PERIOD DIFFERENCES.  
THIS PROVIDED GOOD CONTINUITY THROUGH DAY 6 WITH YESTERDAYS WPC  
FORECAST.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
PRECIPITATION ACROSS THE WEST SHOULD BE RATHER QUIET COMPARED TO  
THE SHORT RANGE PERIOD, ALTHOUGH SOME MODEST SNOW ACCUMULATIONS  
MAY CONTINUE IN THE HIGHER TERRAIN, ESPECIALLY FOR PARTS OF  
ARIZONA AND THE COLORADO ROCKIES ON WEDNESDAY. BY THURSDAY THOUGH,  
ANOTHER ROUND OF HEAVY MOUNTAIN SNOWS ARE POSSIBLE FOR PORTIONS OF  
THE PACIFIC NORTHWEST/NORTHERN ROCKIES PARTICULARLY AHEAD OF THE  
SHORTWAVE DROPPING DOWN THE WEST COAST. THE CLOSED LOW DEVELOPMENT  
OFF THE CALIFORNIA COAST MAY HELP DIRECT INCREASED MOISTURE INTO  
PARTS OF SOUTHERN CALIFORNIA AND THE SOUTHWEST AS WELL  
WEDNESDAY-FRIDAY, WITH SNOW LIKELY IN THE HIGHER TERRAIN.  
 
A COUPLE OF GENERALLY PROGRESSIVE COLD FRONTS WILL MOVE THROUGH  
THE CENTRAL AND EASTERN U.S., ALLOWING FOR PERIODS OF WIDESPREAD  
AND MODERATE PRECIPITATION ACROSS THE TENNESSEE VALLEY AND  
APPALACHIANS. THE FIRST ROUND ON WEDNESDAY INTO THURSDAY MAY BRING  
A THREAT FOR LOCALLY HEAVY TO EXCESSIVE RAINFALL, PARTICULARLY FOR  
PARTS OF THE SOUTHERN APPALACHIANS, BUT WITH LINGERING UNCERTAINTY  
REGARDING AMOUNTS AND AXIS OF HEAVIES RAIN. PRECIPITATION CHANCES  
INCREASE AGAIN IN ROUGHLY THE SAME REGION BY THE WEEKEND AS THE  
NEXT SYSTEM MAKES ITS WAY EASTWARD. SOME LIGHT SNOW MAY FALL  
ACROSS NORTHERN AREAS FROM THE GREAT LAKES INTO THE NORTHEAST WITH  
THE SECOND FRONT.  
 
BITTERLY COLD TEMPERATURES SHOULD CONTINUE THROUGH AT LEAST  
SATURDAY IN PARTS OF ESPECIALLY THE NORTHERN PLAINS WITH DAYTIME  
HIGHS (AND A LESSER EXTENT OVERNIGHT LOWS) 20 TO 30 DEGREES BELOW  
AVERAGE. SOME AREAS OF NORTHERN NORTH DAKOTA AND MONTANA MAY STAY  
BELOW 0F EVEN INTO NEXT WEEKEND, DESPITE TEMPERATURES MODERATING  
SLIGHTLY AS THE WEEK PROGRESSES. THE ADDITION OF GUSTY WINDS COULD  
RESULT IN DANGEROUSLY COLD WIND CHILLS, ESPECIALLY FOR NORTHERN  
PARTS OF THE HIGH PLAINS WHERE WIND CHILL VALUES COULD DIP WELL  
BELOW 0F FOR MANY. TEMPERATURES ARE ALSO EXPECTED TO BE BELOW  
NORMAL ACROSS THE REST OF THE WESTERN U.S., WITH THE COLDEST  
ANOMALIES ON WEDNESDAY WHERE DAYTIME HIGHS COULD STILL BE 10-20F  
BELOW NORMAL. SOME RECORDS ARE STILL POSSIBLE INTO WEDNESDAY  
ACROSS PARTS OF THE WEST. MEANWHILE, MILD TO WARM WEATHER IS  
EXPECTED ACROSS THE SOUTH-CENTRAL U.S. AND NORTHEASTWARD THROUGH  
NEXT WEEK WHERE MANY PLACES FROM THE SOUTHERN HALF OF THE PLAINS  
TO THE OHIO VALLEY/MID-ATLANTIC COULD BE 15-25 DEGREES ABOVE  
NORMAL ON 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  
PATTERN OF COOL IN THE WEST/NORTH-CENTRAL U.S. AND WARM IN THE  
SOUTH-CENTRAL/EASTERN U.S., JUST LESS EXTREME. A POTENTIAL PATTERN  
SHIFT MAY FINALLY BRING VALUES BACK CLOSER TO NORMAL BY LATE NEXT  
WEEKEND.  
 
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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