889  
FXUS02 KWBC 270619  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
119 AM EST MON DEC 27 2021  
 
VALID 12Z THU DEC 30 2021 - 12Z MON JAN 03 2022  
 
...BITTERLY COLD TEMPERATURES TO CONTINUE THURSDAY-SATURDAY ACROSS  
PORTIONS OF THE NORTHERN PLAINS...  
 
   
..OVERVIEW
 
 
A BLOCKY AND AMPLIFIED RIDGE/UPPER HIGH ANCHORED OVER THE NORTHERN  
PACIFIC SHOULD FINALLY BEGIN TO BREAK DOWN AND WEAKEN NEXT  
WEEKEND. BEFORE THAT THOUGH, RENEWED TROUGHING (AND CHILLY  
WEATHER) WILL CONTINUE ACROSS THE WESTERN U.S., INCLUDING A  
SOUTHERN STREAM CLOSED LOW NEAR SOUTHERN CALIFORNIA  
THURSDAY-FRIDAY. AN INITIAL NORTHERN STREAM SHORTWAVE WILL SEND  
ONE SYSTEM ACROSS THE GREAT LAKES/NORTHEAST LATE THIS WEEK, BUT BY  
THE WEEKEND A STRONGER SYSTEM (AND NORTHERN PACIFIC RIDGE  
WEAKENING) SHOULD FINALLY ALLOW FOR A PATTERN SHIFT AS AMPLIFIED  
TROUGHING SHIFTS INTO THE CENTRAL U.S. AND THE EAST.  
   
..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  
LINGERING UNCERTAINTIES IN THE SMALLER SCALE DETAILS. THE 12/18Z  
GUIDANCE CONTINUES TO INDICATE GOOD CONSENSUS FOR CLOSED LOW  
DEVELOPMENT OFF THE CALIFORNIA COAST, WITH JUST SOME MAINLY MINOR  
QUESTIONS REMAINING IN TERMS OF TIMING AND HOW QUICKLY THE ENERGY  
EJECTS INTO THE SOUTHWEST AND MERGES WITH A STRONGER SHORTWAVE  
FROM THE NORTH. AFTER THIS, AGAIN, SOME TIMING AND STRENGTH  
VARIABILITY WITH THE AMPLIFIED TROUGH AS IT SLIDES INTO THE  
CENTRAL AND EVENTUALLY EASTERN U.S., AND THEN BIGGER DIFFERENCES  
ON SURFACE LOW PLACEMENT ACROSS THE NORTHEAST NEXT SUNDAY AND  
MONDAY. GIVEN THE DAY 6-7 TIME FRAME THOUGH, THESE DIFFERENCES ARE  
STILL WELL WITHIN THE NORMAL SPREAD.  
 
WPC RELIED ON THE DETERMINISTIC MODELS FOR DAYS 3-4, ELIMINATING  
THE UKMET BY DAY 5 AS ITS STILL A BIT FASTER WITH THE CLOSED LOW  
AND SHORTWAVE INTO THE SOUTHWEST MID PERIOD. GIVEN GROWING  
UNCERTAINTIES LATE PERIOD, FAVORED SOME INCORPORATION OF THE  
ENSEMBLE MEANS, BUT STILL ABLE TO MAINTAIN AT LEAST 50% OF THE  
DETERMINISTIC GFS AND ECMWF EVEN AT DAY 7. THIS ALSO MAINTAINED  
GOOD CONSISTENCY WITH YESTERDAYS WPC FORECAST.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
A COLD FRONT INTO THE EAST ON THURSDAY WILL ALLOW FOR SOME  
LINGERING SHOWERS ACROSS MAINLY THE SOUTHEAST. PRECIPITATION  
CHANCES INCREASE AGAIN IN ROUGHLY THE SAME REGION BY THE WEEKEND  
AS THE NEXT COLD FRONT MAKES ITS WAY EASTWARD. ANOTHER WELL  
DEFINED COLD FRONT DIVING SOUTH AND EAST THIS WEEKEND SHOULD  
RESULT IN ANOTHER ROUND OF WIDESPREAD MODERATE TO LOCALLY HEAVY  
RAINFALL FOR THE TENNESSEE VALLEY/SOUTHERN APPALACHIANS, AND INTO  
THE LOWER OHIO VALLEY ON SATURDAY. EXPECT RAINFALL WITH THIS  
SYSTEM TO GRADUALLY PRESS EAST ON SUNDAY, WITH THE BEST CHANCE FOR  
ORGANIZED RAINS ALONG AND SOUTH OF THE FRONT ACROSS PARTS OF THE  
SOUTHEAST. EXPECT SOME WINTRY WEATHER IN THE NORTH SIDE OF THE  
PRECIPITATION SHIELD FROM THE MIDWEST TO THE GREAT LAKES AND THE  
NORTHEAST AS WELL.  
 
ANOTHER ROUND OF HEAVY MOUNTAIN SNOWS IS POSSIBLE FOR PORTIONS OF  
THE PACIFIC NORTHWEST/NORTHERN ROCKIES ON THURSDAY AHEAD OF A  
SHORTWAVE INTO THE NORTHWEST. FARTHER SOUTH, THE LIKELY CLOSED LOW  
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, ESPECIALLY THE COLORADO ROCKIES  
AS THE NORTHERN STREAM AND SOUTHERN STREAM SYSTEMS MERGE. THE NEXT  
FRONTAL SYSTEM TOWARDS THE WEST SHOULD BRING YET ANOTHER ROUND OF  
HEAVY MOUNTAIN SNOWS TO THE PACIFIC NORTHWEST BY THIS WEEKEND.  
 
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.  
THROUGH SUNDAY. 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 THROUGH  
SATURDAY. MORE 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 AND THE EAST BY NEXT MONDAY.  
 
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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