624  
FXUS02 KWBC 230701  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
200 AM EST THU DEC 23 2021  
 
VALID 12Z SUN DEC 26 2021 - 12Z THU DEC 30 2021  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
MODELS AND ENSEMBLES CONTINUE TO INDICATE A HIGHLY AMPLIFIED  
PACIFIC-INTO-ALASKA UPPER RIDGE TO PERSIST INTO THE MIDDLE OF NEXT  
WEEK AND SUPPORT A DEEP UPPER TROUGH THAT DRIFTS EAST FROM THE  
WEST COAST THROUGH THE INTERIOR WEST, ANCHORED BY A PIECE OF THE  
POLAR VORTEX THAT DIPS TOWARD SOUTHERN CANADA. THE TROUGH WILL  
MAINTAIN A LENGTHY PERIOD OF CHILLY AND WET/SNOWY CONDITIONS OVER  
THE WEST WITH THE HIGHEST PRECIPITATION AMOUNT FROM THE OREGON  
CASCADES DOWN THROUGH THE SIERRA NEVADA. INDIVIDUAL SHORTWAVES  
EJECTING FROM THE UPPER TROUGH AND CARRIED ALONG IN FAST  
WEST-SOUTHWESTERLY MEAN FLOW WILL SUPPORT A SERIES OF  
WAVES/FRONTAL SYSTEMS TO DEVELOP OVER THE CENTRAL PLAINS AND THEN  
TRACK QUICKLY TOWARD THE GREAT LAKES. PREDICTABILITY FOR SYSTEM  
DETAILS HAS BEEN AND PROBABLY WILL CONTINUE TO BE LOW. THE POLAR  
VORTEX EXTENSION IN CANADA WILL LIKELY SUPPORT INTRUSION OF ARCTIC  
INTO THE NORTHERN PLAINS. THIS WILL BE CONTRASTED WITH WELL ABOVE  
NORMAL READINGS OVER THE DEEP SOUTH INTO THE EASTERN U.S. TO THE  
SOUTH OF THE GENERAL STORM TRACK.  
 
THE WPC MEDIUM-RANGE FORECASTS WERE COMPOSED BASED ON A BLEND OF  
40% FROM THE 18Z GFS/GEFS, 40% FROM THE 12Z ECMWF/EC MEAN, AND 20%  
FROM THE CMC CMC MEAN. AN EVEN BLEND OF THE DETERMINISTIC AND  
ENSEMBLE SOLUTIONS WAS USED FOR DAYS 3 TO 5 WHERE MODEL SPREAD WAS  
AT MANAGEABLE LEVELS. BY DAY 6, THE DETERMINISTIC SOLUTIONS BEGIN  
TO DIVERGE WHILE THE CORRESPONDING ENSEMBLE MEANS REMAIN  
COMPATIBLE WITH ONE ANOTHER. A HIGHER PERCENTAGE OF THE ENSEMBLE  
MEANS WAS THEREFORE INCORPORATED FOR DAYS 6 AND 7.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
SHORTWAVE TROUGHS/ENERGY ROUNDING A PERSISTENT UPPER TROUGH THAT  
DRIFTS INLAND FROM THE WEST COAST SATURDAY THROUGH MONDAY WILL  
PERIODICALLY ENHANCE THE FLOW OF MOISTURE INTO THE WEST. AREAS OF  
FOCUSED/TERRAIN-ENHANCED PRECIPITATION WILL EXTEND FROM THE COAST  
INTO THE ROCKIES. THE BEST POTENTIAL FOR EPISODES OF MODERATE TO  
HEAVY PRECIPITATION AND HIGHEST PRECIPITATION TOTALS WILL EXTEND  
FROM THE OREGON CASCADES AND COASTAL RANGES DOWN THROUGH CENTRAL  
CALIFORNIA AND THE SIERRA NEVADA. AT LOWER ELEVATIONS OF OR/CA,  
THE CONTINUED RAINFALL WILL INCREASE THE SOIL SATURATION AND  
SENSITIVITY TO LOCAL RUNOFF. MEANINGFUL PRECIPITATION WILL ALSO  
BE LIKELY ACROSS THE HIGHEST TERRAIN OF THE GREAT BASIN AND THE  
CENTRAL/NORTHERN ROCKIES. COLDER AIR FILTERING INTO THE PACIFIC  
NORTHWEST WILL BRING SNOW LEVELS FOR SEATTLE AND PORTLAND TO SEA  
LEVEL BY SATURDAY WHILE OTHER PARTS OF THE WEST WILL ALSO SEE  
LOWERING SNOW LEVELS AS THE UPPER TROUGH AXIS NUDGES INLAND.  
 
GENERALLY FAST-MOVING WAVES WILL PERIODICALLY SPREAD PRECIPITATION  
ACROSS THE NORTHERN TIER STATES AND OVER PARTS OF THE EAST. THE  
MAJORITY OF WINTRY WEATHER SHOULD BE CONFINED TO AREAS FROM THE  
NORTHERN PLAINS THROUGH THE GREAT LAKES WITH SOME FURTHER CHANGES  
FOR NEW ENGLAND. THE FIRST ROUND OF WINTRY WEATHER SHOULD BE FROM  
THE HOLIDAY WEEKEND INTO MONDAY. A SECOND ROUND SHOULD FOLLOW ON  
TUESDAY BUT MODEL SOLUTION DIVERGES REGARDING THE ASSOCIATED LOW  
PRESSURE SYSTEM TRACKING TOWARD THE GREAT LAKES. PRECIPITATION  
COVERAGE AND AMOUNTS REMAIN MORE UNCERTAIN THAN DESIRED SO  
CONTINUE TO MONITOR FORECASTS AS DETAILS GAIN BETTER CLARITY.  
FARTHER SOUTH, THERE CONTINUES TO BE MEANINGFUL MODEL SIGNAL FOR  
AN AXIS OF ENHANCED RAINFALL ACROSS THE OHIO VALLEY NEAR/NORTH OF  
A MEANDERING FRONT FOR EARLY NEXT WEEK.  
 
THE SOUTHERN GREAT PLAINS WILL BE AT THE CENTER OF THE GREATEST  
WARM TEMPERATURE ANOMALIES THIS WEEKEND WITH DECENT COVERAGE OF  
MAX/MIN READINGS 20-30F OR SO ABOVE NORMAL. SUCH TEMPERATURES  
WILL AT RECORD LEVELS ACROSS MUCH OF TEXAS ON SUNDAY. THIS RECORD  
SETTING WARMTH WILL SHIFT EAST MONDAY/TUESDAY WITH THE MOST LIKELY  
MAX TEMPERATURE RECORDS FOR THE CENTRAL GULF COAST WITH  
TEMPERATURE ANOMALIES OF PLUS 20F OR GREATER EXTENDING NORTHEAST  
FROM THERE ACROSS THE OHIO/TENNESSEE VALLEYS. ANOMALIES SHOULD  
BECOME SOMEWHAT MORE SUPPRESSED AND LESS EXTREME BY NEXT WEDNESDAY  
AS THE MEAN FRONTAL BOUNDARY EAST OF THE ROCKIES PUSHES A LITTLE  
FARTHER SOUTH. MEANWHILE THE CORE OF COLDEST NORTHERN TIER AIR  
WILL BE OVER MONTANA AND NORTH DAKOTA THROUGH MONDAY TEMPERATURES  
WILL LIKELY BE 20-40F BELOW NORMAL. THE LEADING EDGE OF THIS COLD  
AIRMASS WILL MAKE A GRADUAL SOUTHEASTWARD PROGRESS ACROSS THE  
CENTRAL PLAINS EARLY NEXT WEEK. WESTERN U.S. TEMPERATURES WILL  
TREND COLDER WITH THE NORTHWEST SEEING EXPANDING COVERAGE OF HIGHS  
10-25F BELOW NORMAL NEXT MONDAY-WEDNESDAY. INCREASING NUMBERS OF  
LOCATIONS ALONG AND JUST INLAND FROM THE WEST COAST LOOK SEE DAILY  
RECORDS FOR LOW MAXIMA SUNDAY THROUGH TUESDAY. THE WARMTH IN THE  
SOUTH WITH BE IN STARK CONTRAST WITH THE FRIGID SUB-ZERO READINGS  
IN THE NORTHERN PLAINS.  
 
KONG/JACKSON  
 
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  
 
HAZARDS:  
- HEAVY PRECIPITATION ACROSS PORTIONS OF CALIFORNIA AND THE  
PACIFIC NORTHWEST, SUN, DEC 26.  
- HEAVY SNOW ACROSS PORTIONS OF CALIFORNIA AND THE PACIFIC  
NORTHWEST, SAT-MON, DEC 25-DEC 27.  
- HEAVY SNOW ACROSS PORTIONS OF THE CENTRAL GREAT BASIN,  
CALIFORNIA, AND THE SOUTHWEST, SAT-TUE, DEC 25-DEC 28.  
- HEAVY SNOW ACROSS PORTIONS OF THE GREAT LAKES AND THE UPPER  
MISSISSIPPI VALLEY, SUN, DEC 26.  
- FLOODING OCCURRING OR IMMINENT ACROSS PORTIONS OF THE PACIFIC  
NORTHWEST.  
- HIGH WINDS ACROSS PORTIONS OF THE SOUTHERN ROCKIES, THE CENTRAL  
ROCKIES, THE SOUTHERN PLAINS, AND THE CENTRAL PLAINS, SUN, DEC 26.  
- MUCH BELOW NORMAL TEMPERATURES ACROSS PORTIONS OF THE PACIFIC  
NORTHWEST, THE NORTHERN ROCKIES, AND THE NORTHERN GREAT BASIN,  
MON-WED, DEC 27-DEC 29.  
- MUCH BELOW NORMAL TEMPERATURES ACROSS PORTIONS OF THE NORTHERN  
PLAINS AND THE NORTHERN ROCKIES, SAT-WED, DEC 25-DEC 29.  
- MUCH BELOW NORMAL TEMPERATURES ACROSS PORTIONS OF THE UPPER  
MISSISSIPPI VALLEY AND THE NORTHERN PLAINS, TUE-WED, DEC 28-DEC 29.  
 
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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