521  
FXUS02 KWBC 221801  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
100 PM EST WED DEC 22 2021  
 
VALID 12Z SAT DEC 25 2021 - 12Z WED DEC 29 2021  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
A VERY AMPLIFIED PACIFIC-INTO-ALASKA UPPER RIDGE PERSISTS INTO THE  
MIDDLE OF NEXT WEEK AND SUPPORTS A DEEP UPPER TROUGH THAT DRIFTS  
EAST FROM THE WEST COAST THROUGH THE INTERIOR WEST, ANCHORED BY A  
MEAN LOW OVER NORTHWESTERN CANADA. THE TROUGH WILL MAINTAIN A  
LENGTHY PERIOD OF CHILLY AND WET/SNOWY CONDITIONS OVER THE WEST  
WITH THE HIGHEST FIVE-DAY LIQUID EQUIVALENTS LIKELY FROM THE OR  
CASCADES 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 THE  
EAST FROM THE CO ROCKIES, BRINGING SNOW TO THE NORTHERN TIER AND  
RAIN FOR THE CENTRAL AND SOUTHERN SECTIONS OF THE CONUS.  
PREDICTABILITY FOR SYSTEM DETAILS HAS BEEN AND PROBABLY WILL  
CONTINUE TO BE LOW. ASSOCIATED FRONTS WILL CREATE A PRONOUNCED  
TEMPERATURE CONTRAST, BETWEEN WELL ABOVE NORMAL READINGS OVER THE  
CENTRAL AND SOUTHERN SECTIONS EAST O THE ROCKIES AND MUCH BELOW  
NORMAL TEMPERATURES SLOWLY EXPANDING OVER THE NORTHERN PLAINS.  
 
THE FORECAST OVER THE EAST SATURDAY NIGHT THROUGH SUNDAY REMAINS  
FAIRLY UNCERTAIN AS A WAVE SHIFTS EAST FROM THE GREAT LAKES TO OFF  
THE NORTHEAST. THE 06Z AND 12Z GFS REMAIN THE STRONGEST WITH THIS  
SYSTEM AND WAS THUS LIMITED IN THE MODEL BLEND FOR DAYS 3/4.  
 
BEHIND THIS LEADING SYSTEM, THE NEXT WELL-DEFINED SHORTWAVE  
DIGGING INTO THE MEAN TROUGH SHOULD EXTEND NEARLY THE LENGTH OF  
THE WEST COAST BY SATURDAY EVENING AND EJECT EAST OVER THE ROCKIES  
THROUGH SUNDAY, BRINGING A SYSTEM INTO THE UPPER MIDWEST/GREAT  
LAKES MONDAY. THE 06Z/12Z GFS RUNS REMAIN SOMEWHAT WEAKER AND MORE  
SUPPRESSED THAN MOST OTHER GUIDANCE. THEN THE THIRD WAVE TRACKS  
EAST FROM THE ROCKIES MONDAY NIGHT POTENTIALLY REACHING THE GREAT  
LAKES TUESDAY NIGHT. UNCERTAINTY REIGNS WITH THE FOLLOWING WAVE  
FROM THE GULF OF ALASKA TUESDAY/WEDNESDAY WITH THE 06Z/12Z GFS  
TRACKING IT OFF THE WEST COAST, THE 00Z CMC STALLING IT NEAR  
VANCOUVER ISLAND AND THE 00Z ECMWF OVER THE INTERIOR NORTHWEST.  
 
THE 00Z UKMET REMAINED SLOWER WITH THE INITIAL WAVE ON DAY 3, BUT  
WAS NOT MUCH DIFFERENT THAN THE 00Z CMC THERE, SO SMALL  
PERCENTAGES OF BOTH WERE USED ON DAYS 3/4 BEFORE GETTING REMOVED  
IN FAVOR OF MORE 06Z GEFS AND 00Z ECENS MEANS. THE DAYS 6/7  
FORECAST IS RATHER UNCERTAIN WITH THE TROUGH EXPANDING OVER THE  
WEST FROM TWO AMPLIFYING SHORTWAVES, SO THE TIMING AND STRENGTH OF  
THESE FEATURES WILL COME INTO BETTER FOCUS THROUGH THE NEXT FEW  
FORECAST CYCLES.  
   
..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 FIVE-DAY TOTALS WILL EXTEND FROM  
THE OR CASCADES AND COASTAL RANGES 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 EASTWARD.  
 
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. PRECIPITATION COVERAGE AND AMOUNTS REMAIN MORE  
UNCERTAIN THAN DESIRED SO CONTINUE TO MONITOR FORECASTS AS DETAILS  
GAIN BETTER CLARITY. DEPENDING ON POSITION OF THE MEAN FRONTAL  
BOUNDARY OVER THE EAST AND WAVE DETAILS, RAINFALL COULD TREND  
HEAVIER OVER THE OHIO/TENNESSEE VALLEYS INTO THE APPALACHIANS BY  
TUESDAY-WEDNESDAY.  
 
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  
BREAK DAILY HIGH MAX RECORDS ACROSS MUCH OF TEXAS SUNDAY. THIS  
RECORD SETTING WARM AIR SHIFTS 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.  
 
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  
 
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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