149  
FXUS02 KWBC 212040  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
339 PM EST TUE DEC 21 2021  
 
VALID 12Z FRI DEC 24 2021 - 12Z TUE DEC 28 2021  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
AN IMPRESSIVELY STRONG AND PERSISTENT RIDGE OVER THE EAST-CENTRAL  
NORTH PACIFIC AND INTO ALASKA WILL MAINTAIN AMPLIFIED TROUGHING  
ALONG OR A LITTLE INLAND FROM THE WEST COAST AND A DEEP CYCLONIC  
GYRE OVER CANADA'S NW TERRITORIES THURSDAY THROUGH THIS WEEKEND  
AND POSSIBLY WELL INTO OR THROUGH NEXT WEEK. THIS TROUGHING AND  
DEEP LOW WILL SUPPORT AN EXTENDED PERIOD OF PRECIPITATION AND  
MOSTLY BELOW NORMAL HIGH TEMPERATURES OVER A BROAD PORTION OF THE  
WEST. THE TROUGH'S SHAPE WILL VARY FROM DAY TO DAY DEPENDING ON  
INDIVIDUAL SHORTWAVES DIGGING INTO THE TROUGH WITH GOOD CONFIDENCE  
ON A POTENT REINFORCING SHORTWAVE TROUGH PUSHING DOWN THE COAST  
FRIDAY NIGHT BEFORE SWINGING INLAND INTO CA SATURDAY WITH THE NEXT  
NOTABLE WAVE LIKELY TUESDAY/TUESDAY NIGHT. SHORTWAVE TROUGHS AND  
IMPULSES EJECTING EAST FROM THE TROUGH WILL PERIODICALLY TRACK  
EAST ACROSS THE ROCKIES AND THE NORTHERN TIER OF THE CONUS.  
ASSOCIATED FRONTS WILL SEPARATE A PRONOUNCED TEMPERATURE CONTRAST,  
BETWEEN WELL ABOVE NORMAL READINGS OVER AND EAST OF THE SOUTHERN  
TWO-THIRDS OF THE PLAINS AND MUCH BELOW NORMAL TEMPERATURES  
BECOMING ESTABLISHED OVER THE FAR NORTHERN PLAINS.  
 
THE PRIMARY FORECAST PROBLEMS INVOLVE THE SPECIFICS OF INDIVIDUAL  
SHORTWAVES WITHIN THE WESTERN TROUGH AND THEN THEIR TRACK AND  
INTENSITY OVER THE NORTHEASTERN U.S. WITH A CONFLICT OF LARGE  
SCALE PATTERN TELECONNECTIONS -- THE PACIFIC/WESTERN U.S. PATTERN  
FAVORS ABOVE NORMAL UPPER HEIGHTS OVER THE EAST WHILE THE  
RIDGING/POSITIVE HEIGHT ANOMALIES NORTHEAST OF HUDSON BAY FAVORING  
MODERATELY BELOW NORMAL EASTERN U.S. HEIGHTS. OVERALL THE 00Z CMC  
IS NOTABLY DIFFERENT THAN THE GFS/ECM/UKMET (NAMELY WITH A MORE  
SOUTHERN CANADIAN COLD CORE LOW/GYRE), SO IT WAS NOT INCLUDED IN  
THE MODEL BLEND.  
 
UNCERTAINTY STILL IS NOTED WITH TIMING AND STRENGTH OF TWO WAVES  
SHIFTING EAST ACROSS THE GREAT LAKES FRIDAY NIGHT AND SATURDAY  
NIGHT. THE 06Z GFS WAS MUCH MORE POTENT WITH THE SECOND WAVE THAN  
ANY OTHER 00Z GUIDANCE AND THE 12Z GFS HAS BACKED OFF TO A MORE  
OPEN AND THUS FASTER SOLUTION (WHERE THE FEATURE QUICKLY SHIFTS  
OFFSHORE OVER THE MID-ATLANTIC SATURDAY NIGHT) WHICH IS IN BETTER  
AGREEMENT WITH THE LATEST CONSENSUS.  
 
THE AMPLIFYING TROUGH OFF THE WEST COAST FRIDAY NIGHT ROUNDS A  
LOBE OF LOW PRESSURE THAT ANCHORS OVER THE SALISH SEA FRIDAY  
THROUGH SUNDAY PER THE LATEST NON-CMC GUIDANCE CONSENSUS. THIS  
TROUGH THEN SWINGS EAST ACROSS THE WEST AND THEN SUPPORT ONE OR  
MORE SURFACE LOWS/FRONTS FROM THE PLAINS EASTWARD SUNDAY ONWARD.  
HOWEVER, THE DETAILS IN THIS TROUGH DEVELOPMENT/PROGRESSION IN  
DETERMINISTIC GUIDANCE IS DIFFERENT ENOUGH TO WARRANT SOME  
INCLUSION OF 06Z GEFS AND 00Z ECENS MEANS EVEN FOR DAY 3. BY  
MONDAY-TUESDAY INDIVIDUAL MODEL RUNS AND ENSEMBLE MEMBERS  
INCREASINGLY DIVERGE FOR DETAILS AT THE SURFACE AND ALOFT EAST OF  
THE ROCKIES, PERHAPS REFLECTING THE COMPETING TELECONNECTION  
TENDENCIES OVER THE EAST. THESE DIFFERENCES, AS WELL AS DECREASING  
CONFIDENCE FOR DETAILS WITHIN THE WESTERN TROUGH LATE IN THE  
PERIOD, FAVOR TRENDING THE INITIAL 00Z/06Z OPERATIONAL/ENSEMBLE  
MODEL BLEND USED THROUGH DAY 5 TO LEAN MORE TOWARD THE ECENS AND  
GEFS MEANS FOR DAYS 6/7.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
SHORTWAVE TROUGHS/ENERGY ROUNDING A PERSISTENT UPPER TROUGH  
ALIGNED NEAR THE WEST COAST WILL PERIODICALLY ENHANCE THE FLOW OF  
MOISTURE INTO THE WEST, WITH AREAS OF FOCUSED/TERRAIN-ENHANCED  
PRECIPITATION EXTENDING FROM THE COAST INTO THE ROCKIES. EXPECT  
EPISODES OF MODERATE TO HEAVY PRECIPITATION FROM THE CASCADES AND  
COASTAL AREAS OF THE PACIFIC NORTHWEST INTO CALIFORNIA, WHICH WILL  
INCREASE THE SOIL SATURATION AND SENSITIVITY TO LOCAL RUNOFF. THE  
HIGHEST PRECIPITATION/LIQUID EQUIVALENTS SHOULD EXTEND FROM THE  
OREGON CASCADES INTO NORTHWEST CALIFORNIA AND THE SIERRA NEVADA.  
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.  
 
IT IS LOOKING LIKELY THAT TWO AREAS OF PRECIPITATION IN THE  
NORTHEASTERN QUADRANT OF THE CONUS WILL ACCOMPANY A PAIR OF  
SHORTWAVE TROUGHS SATURDAY/SUNDAY WITH THE FIRST PUSHING OFF NEW  
ENGLAND SATURDAY/CHRISTMAS MORNING AND THE NEXT CROSSING FROM THE  
CENTRAL PLAINS SATURDAY MORNING AND THE NORTHERN MID-ATLANTIC  
SATURDAY NIGHT. WINTRY WEATHER IS LIKELY ON THE NORTH SIDE OF BOTH  
THESE WAVES FROM THE UPPER GREAT LAKES THROUGH NEW ENGLAND. THE  
NEXT WAVE LOOKS TO TAKE A MORE NORTHERN ROUTE, TRACKING FROM THE  
CENTRAL PLAINS TO THE LAKE SUPERIOR AND INTO CANADA SUNDAY/MONDAY.  
 
 
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 ABOVE NORMAL. SUCH  
TEMPERATURES LOOK TO CHALLENGE DAILY HIGH MAX RECORDS ACROSS MUCH  
OF TEXAS SATURDAY THROUGH MONDAY AND MAYBE TUESDAY. THIS WARMTH  
WILL EXPAND EAST WITH THE NORTHERN EXTENT DETERMINED BY  
SYSTEM/FRONTAL PROGRESSION OVER THE NORTHERN TIER. ANOMALIES  
SHOULD BECOME SOMEWHAT MORE SUPPRESSED AND LESS EXTREME BY NEXT  
TUESDAY AS THE MEAN TROUGHING CONTINUES TO BROADEN AND SHIFT  
EASTWARD TOWARD THE ROCKIES. MEANWHILE TEMPERATURES OF 20 TO 40F  
BELOW NORMAL WILL PUSH ACROSS MT/ND THIS WEEKEND, EXPANDING TO  
SD/MN BY MONDAY WITH CLOSER TO NORMAL WINTER TEMPERATURES REACHING  
THE CENTRAL PLAINS AND UPPER GREAT LAKES BY 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  
 
HAZARDS:  
- HEAVY PRECIPITATION ACROSS PORTIONS OF CALIFORNIA AND THE  
PACIFIC NORTHWEST, FRI-MON, DEC 24-DEC 27.  
- HEAVY RAIN ACROSS PORTIONS OF CALIFORNIA, FRI, DEC 24.  
- HEAVY SNOW ACROSS PORTIONS OF CALIFORNIA AND THE PACIFIC  
NORTHWEST, FRI-MON, DEC 24-DEC 27.  
- HEAVY SNOW ACROSS PORTIONS OF THE CENTRAL GREAT BASIN AND  
CALIFORNIA, SAT-MON, DEC 25-DEC 27.  
- HEAVY SNOW ACROSS PORTIONS OF THE SOUTHERN ROCKIES, THE CENTRAL  
ROCKIES, THE CENTRAL GREAT BASIN, AND THE SOUTHWEST, FRI, DEC 24.  
- FLOODING OCCURRING OR IMMINENT ACROSS PORTIONS OF THE PACIFIC  
NORTHWEST.  
- HIGH WINDS ACROSS PORTIONS OF THE CENTRAL PLAINS, THE CENTRAL  
ROCKIES, THE NORTHERN PLAINS, THE SOUTHERN ROCKIES, THE SOUTHERN  
PLAINS, AND THE SOUTHWEST, FRI, DEC 24.  
- MUCH BELOW NORMAL TEMPERATURES ACROSS PORTIONS OF THE PACIFIC  
NORTHWEST, THE NORTHERN ROCKIES, AND THE NORTHERN GREAT BASIN,  
SUN-TUE, DEC 26-DEC 28.  
- MUCH BELOW NORMAL TEMPERATURES ACROSS PORTIONS OF THE NORTHERN  
PLAINS AND THE NORTHERN ROCKIES, SAT-TUE, DEC 25-DEC 28.  
 
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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