773  
FXUS02 KWBC 261858  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
157 PM 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 THROUGH MUCH OF THE WEEK, EVENTUALLY GETTING  
SUPPRESSED NEXT WEEKEND BY POTENT UPPER-LEVEL ENERGY DROPPING  
SOUTH FROM HIGH LATITUDES. DOWNSTREAM, THIS PATTERN WILL RESULT IN  
GENERAL TROUGHING (AND CHILLY WEATHER) ACROSS THE WESTERN U.S.,  
INCLUDING A SOUTHERN STREAM LIKELY CLOSED LOW THAT 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 ACROSS THE CENTRAL AND EASTERN CONUS. THE MEAN  
TROUGH AND A COLD FRONT AHEAD OF IT ARE FORECAST TO FINALLY PRESS  
EASTWARD INTO THE CENTRAL U.S. NEXT WEEKEND AS THE PATTERN CHANGES  
IN THE WAKE OF THE NORTHERN PACIFIC HIGH WEAKENING.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE LATEST MODELS AND ENSEMBLES CONTINUE TO SHOW GOOD AGREEMENT  
WITH THE LARGE SCALE PATTERN DESCRIBED ABOVE THROUGH MUCH OF THE  
PERIOD, BUT WITH SOME TYPICAL UNCERTAINTIES IN THE SMALLER SCALE  
DETAILS. RELATIVELY MINOR MODEL VARIATIONS WERE SEEN WITH A SERIES  
OF SHORTWAVES AND THE ASSOCIATED SURFACE LOWS TRACKING ACROSS THE  
CENTRAL U.S. TO NORTHEAST WEDNESDAY-FRIDAY. FOR THE MOST PART,  
00/06Z MODEL GUIDANCE CONTINUES TO INDICATE FAIRLY GOOD CONSENSUS  
FOR THE SHORTWAVE DROPPING SOUTHWARD NEAR THE WEST COAST WEDNESDAY  
AND DEVELOPING INTO A CLOSED/CUTOFF LOW AT LEAST FOR SOME PERIOD  
OF TIME THURSDAY-FRIDAY (THOUGH TIMING OF THE CUTOFF DIFFERS  
SOMEWHAT FROM MODEL TO MODEL). THE GREATER CONCERNS ARE FOR THE  
TRACK AND TIMING OF THE LOW--THE 00Z UKMET REMAINED A FAST OUTLIER  
WITH THE EASTWARD TURN OF THE UPPER LOW AND ITS ABSORPTION BACK  
INTO THE MAIN FLOW COMPARED TO CONSENSUS, WITH THE 00Z GFS  
SLIGHTLY SLOW. PREFERRED A BLEND OF THE MORE AGREEABLE 06Z GFS AND  
00Z ECMWF AND CMC WITH THIS FEATURE. BY THE WEEKEND, GUIDANCE ALL  
HAS THE TROUGH AXIS PUSHING INTO THE CENTRAL U.S. AS RIDGING  
FINALLY BUILDS INTO THE WEST, THOUGH WITH SOME TIMING AND  
AMPLITUDE DIFFERENCES THAT ARE WELL WITHIN NORMAL SPREAD FOR A  
DAYS 6-7 FORECAST.  
 
THE WPC FORECAST FOR THE EARLY PART OF THE MEDIUM RANGE PERIOD WAS  
BASED ON A BLEND OF THE 06Z GFS AND 00Z ECMWF/UKMET/CMC GUIDANCE,  
QUICKLY PHASING OUT THE UKMET FOR THE REASONS ABOVE. SINCE  
DETERMINISTIC GUIDANCE WAS IN FAIRLY GOOD AGREEMENT THROUGH THE  
FORECAST PERIOD, WAS ABLE TO KEEP THE BLEND FOR THE 500 MB  
HEIGHTS/SURFACE PRESSURES ABOUT 70% DETERMINISTIC MODELS THROUGH  
THE PERIOD, WITH THE OTHER 30% GEFS AND EC ENSEMBLE MEANS. FAVORED  
THE GEFS MEAN SLIGHTLY MORE GIVEN IT SHOWED MORE INDICATION OF THE  
POTENTIAL FOR A CLOSED LOW OFFSHORE OF CALIFORNIA LATE THIS WEEK.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
A COUPLE OF GENERALLY PROGRESSIVE COLD FRONTS WILL MOVE THROUGH  
THE CENTRAL AND EASTERN U.S., ALLOWING FOR PERIODS OF WIDESPREAD  
RAIN AND THUNDERSTORMS ACROSS THE TENNESSEE VALLEY AND  
APPALACHIANS INTO THE SOUTHEAST. THE FIRST ROUND ON WEDNESDAY INTO  
THURSDAY MAY BRING A THREAT FOR LOCALLY HEAVY TO EXCESSIVE  
RAINFALL FOR THE TENNESSEE VALLEY AND SOUTHERN APPALACHIANS AND  
POTENTIAL SEVERE WEATHER ACROSS THE DEEP SOUTH, BUT WITH LINGERING  
UNCERTAINTY REGARDING PLACEMENT AND RAINFALL AMOUNTS.  
PRECIPITATION CHANCES INCREASE AGAIN IN ROUGHLY THE SAME REGION BY  
THE WEEKEND AS THE NEXT COLD FRONT MAKES ITS WAY EASTWARD. THIS  
SECOND SYSTEM MAY BRING WINTRY WEATHER ACROSS NORTHERN AREAS FROM  
THE MIDWEST/GREAT LAKES INTO THE NORTHEAST OVER THE WEEKEND.  
 
AFTER A BREAK FROM HEAVY PRECIPITATION ACROSS MUCH OF THE WEST ON  
WEDNESDAY (WITH THE EXCEPTION OF SOUTHERN CALIFORNIA), ANOTHER  
ROUND OF HEAVY MOUNTAIN SNOWS IS POSSIBLE FOR PORTIONS OF THE  
PACIFIC NORTHWEST/NORTHERN ROCKIES AHEAD OF THE SHORTWAVE DROPPING  
DOWN THE WEST COAST. AS THIS FEATURE DEVELOPS INTO THE LIKELY  
CLOSED LOW, IT WILL DIRECT INCREASED MOISTURE INTO PARTS OF  
SOUTHERN CALIFORNIA AND INTO THE FOUR CORNERS REGION THROUGH  
FRIDAY OR SATURDAY, WITH SNOW LIKELY IN THE HIGHER TERRAIN.  
 
BITTERLY COLD TEMPERATURES SHOULD CONTINUE THROUGH AT LEAST  
SATURDAY PARTICULARLY FOR THE NORTHERN PLAINS WITH DAYTIME HIGHS  
(AND A LESSER EXTENT OVERNIGHT LOWS) 20 TO 35 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 BY ABOUT 5-15 DEGREES ACROSS THE REST OF THE WESTERN U.S.,  
WITH RECORD COOL HIGHS STILL POSSIBLE WEDNESDAY. TEMPERATURES  
COULD COOL FURTHER OVER THE INTERMOUNTAIN WEST BY SATURDAY AS A  
HIGH PRESSURE SYSTEM BUILDS. 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 THROUGH SATURDAY. DAILY RECORD HIGH MINIMUM  
AND MAXIMUM TEMPERATURES ARE LIKELY. A POTENTIAL PATTERN SHIFT MAY  
FINALLY BRING VALUES BACK CLOSER TO NORMAL, POSSIBLY BELOW AVERAGE  
IN THE SOUTH-CENTRAL U.S., BY SUNDAY.  
 
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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