367  
FXUS02 KWBC 220700  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
159 AM EST WED DEC 22 2021  
 
VALID 12Z SAT DEC 25 2021 - 12Z WED DEC 29 2021  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
TODAY'S GUIDANCE CONTINUES TO SHOW A VERY AMPLIFIED PACIFIC INTO  
ALASKA UPPER RIDGE SUPPORTING A DEEP UPPER TROUGH OVER THE WEST  
COAST/INTERIOR WEST, ANCHORED BY A MEAN LOW OVER WESTERN CANADA.  
THE TROUGH WILL MAINTAIN A LENGTHY PERIOD OF CHILLY AND WET/SNOWY  
CONDITIONS OVER WEST WITH THE HIGHEST FIVE-DAY LIQUID EQUIVALENTS  
LIKELY TO BE BETWEEN THE CASCADES AND 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 OF THE ROCKIES, BRINGING SNOW TO  
NORTHERN AREAS AND RAIN FARTHER SOUTH. PREDICTABILITY FOR SYSTEM  
DETAILS HAS BEEN AND PROBABLY WILL CONTINUE TO BE A LOT LOWER THAN  
THAT FOR THE LARGE SCALE MEAN PATTERN. ASSOCIATED FRONTS WILL  
SEPARATE A PRONOUNCED TEMPERATURE CONTRAST, BETWEEN WELL ABOVE  
NORMAL READINGS OVER AND EAST OF THE SOUTHERN HALF OR SO OF THE  
PLAINS AND MUCH BELOW NORMAL TEMPERATURES SETTLING OVER THE FAR  
NORTHERN PLAINS.  
 
THE FORECAST OVER THE EAST DURING CHRISTMAS WEEKEND IS MORE  
UNCERTAIN THAN TYPICALLY WOULD BE THE CASE FOR THE DAYS 3-4 TIME  
FRAME. STARTING WITH YESTERDAY'S 00Z ECMWF, GUIDANCE HAS RECENTLY  
CHANGED SOME DETAILS FOR WEST COAST SHORTWAVE EVOLUTION AROUND  
THURSDAY-FRIDAY. THIS HAS LED TO A DAMPENING OF A WAVE REACHING  
THE GREAT LAKES BY THE START OF THE PERIOD WHILE ENHANCING A  
TRAILING WAVE TO THE SOUTHWEST. STILL THERE SIGNIFICANT  
DIFFERENCES FOR THE SECOND WAVE EVEN IN THE NEW 00Z GUIDANCE,  
RANGING BETWEEN THE GFS THAT REMAINS QUITE STRONG LIKE ITS 18Z RUN  
(AND FASTER 12Z RUN) AND THE WEAK/SUPPRESSED CMC.  
 
BEHIND THIS LEADING SYSTEM, THE NEXT WELL-DEFINED SHORTWAVE  
DIGGING INTO THE MEAN TROUGH SHOULD BE NEAR THE WEST COAST EARLY  
CHRISTMAS DAY/SATURDAY AND EJECT THROUGH/BEYOND THE WEST  
THEREAFTER, BRINGING A SYSTEM INTO THE UPPER MIDWEST/GREAT LAKES  
AROUND MONDAY. LATEST GFS RUNS ARE SOMEWHAT WEAKER AND MORE  
SUPPRESSED THAN MOST OTHER GUIDANCE WHILE THE 00Z GEFS MEAN OFFERS  
A SLIGHT NUDGE IN THE CONSENSUS DIRECTION. THEN ANOTHER FEATURE  
TRAILS BY ABOUT TWO DAYS, BRINGING A WAVE INTO THE EAST BY NEXT  
WEDNESDAY, WHILE ADDITIONAL ENERGY (WITH A LITTLE MORE SPREAD FOR  
SHAPE, AND POSSIBLY SUPPORTING A SURFACE SYSTEM) SHOULD ARRIVE  
NEAR THE WEST COAST AROUND MIDWEEK AS WELL.  
 
THE 12Z UKMET WAS OUT OF SYNC WITH AN AVERAGE OF OTHER GUIDANCE  
OVER THE EAST EARLY IN THE PERIOD SO THE FIRST PART OF THE  
FORECAST CONSISTED OF A BLEND OF OTHER MODELS AND A SMALL 18Z  
GEFS/12Z ECMWF MEAN COMPONENT. THEN ENSEMBLE WEIGHT GRADUALLY  
INCREASED TO 30-50 PERCENT LATE IN THE PERIOD. THE END RESULT  
REFLECTED VARYING IDEAS OF CONTINUITY AND GUIDANCE TRENDS WHILE  
STAYING ON THE CONSERVATIVE SIDE WHERE GUIDANCE SPREAD WAS THE  
GREATEST.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
THE FORECAST REMAINS LITTLE CHANGED, WITH SHORTWAVE TROUGHS/ENERGY  
ROUNDING A PERSISTENT UPPER TROUGH ALIGNED NEAR OR INLAND FROM THE  
WEST COAST PERIODICALLY ENHANCING 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 CASCADES AND COASTAL AREAS OF THE PACIFIC  
NORTHWEST INTO NORTHERN CALIFORNIA AND THE SIERRA NEVADA. AT  
LOWER ELEVATIONS 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 TO SEA  
LEVEL BY SATURDAY NIGHT WHILE OTHER PARTS OF THE WEST WILL ALSO  
SEE DECLINING 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 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 HALF OF THE GREAT PLAINS WILL BE AT THE CENTER OF THE  
GREATEST WARM TEMPERATURE ANOMALIES DURING THE PERIOD WITH DECENT  
COVERAGE OF MAX/MIN READINGS 20-30F OR SO ABOVE NORMAL. SUCH  
TEMPERATURES LOOK TO CHALLENGE DAILY HIGH MAX RECORDS ACROSS MUCH  
OF TEXAS FROM THE WEEKEND INTO EARLY NEXT WEEK. AT TIMES PLUS 20F  
OR GREATER ANOMALIES MAY EXTEND AS FAR EAST/NORTHEAST AS PARTS OF  
THE OHIO/TENNESSEE VALLEYS AHEAD OF EACH FRONTAL SYSTEM.  
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 WHERE  
TEMPERATURES WILL LIKELY BE 20-40F BELOW NORMAL. THE LEADING EDGE  
OF THIS AIRMASS WILL MAKE VERY GRADUAL SOUTHEASTWARD PROGRESS  
DURING THE PERIOD. WESTERN U.S. TEMPERATURES WILL TREND COLDER  
WITH THE NORTHWEST SEEING EXPANDING COVERAGE OF HIGHS 10-25F BELOW  
NORMAL NEXT MONDAY-WEDNESDAY. SOME LOCATIONS IN THE WEST COULD  
SEE DAILY RECORDS FOR COLD HIGHS.  
 
RAUSCH  
 
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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