551  
FXUS02 KWBC 231915  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
215 PM EST THU DEC 23 2021  
 
VALID 12Z SUN DEC 26 2021 - 12Z THU DEC 30 2021  
 
...POTENTIAL FOR HEAVY MOUNTAIN SNOW AND LIGHT/MODERATE LOWER  
ELEVATION PACIFIC NORTHWEST SNOW CONTINUES THROUGH EARLY NEXT WEEK  
ACROSS MUCH OF THE WEST...  
 
...BITTERLY COLD TEMPERATURES ARE FORECAST ACROSS PORTIONS OF THE  
NORTHWEST TO NORTHERN PLAINS NEXT WEEK...  
 
   
..OVERVIEW
 
 
A RATHER STAGNANT PATTERN IS FORECAST FOR THE MEDIUM RANGE PERIOD  
LED BY A PERSISTENT AND HIGHLY AMPLIFIED NORTHERN PACIFIC  
RIDGE/UPPER HIGH. DOWNSTREAM, DEEP TROUGHING IS EXPECTED OVER THE  
WESTERN U.S. AS A LIKELY CLOSED MID-UPPER LOW DRIFTS EASTWARD OVER  
SOUTHWESTERN AND SOUTH-CENTRAL CANADA, WHILE SHORTWAVES EJECT FROM  
THE UPPER TROUGH AND ARE CARRIED ALONG IN FAST WEST-SOUTHWESTERLY  
MEAN FLOW, WHICH WILL SUPPORT A SERIES OF LOW PRESSURE SYSTEMS  
DEVELOPING AND TRACKING OVER THE CENTRAL AND NORTHEASTERN U.S.  
MEANWHILE, ANOTHER UPPER HIGH MOVING FROM THE GULF OF MEXICO EARLY  
WEEK INTO THE BAHAMAS BY MIDWEEK WILL SPREAD RIDGING AND ABOVE  
NORMAL TEMPERATURES TO THE SOUTHEASTERN QUADRANT OF THE COUNTRY  
SOUTH OF THE STORM TRACK.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
MODEL GUIDANCE REMAINS IN GOOD AGREEMENT WITH THE LARGE-SCALE  
PATTERN DESCRIBED ABOVE, ESPECIALLY THROUGH THE MIDDLE OF NEXT  
WEEK. LESS CERTAIN ASPECTS OF THE FORECAST INCLUDE THE ROUNDS OF  
SHORTWAVE ENERGY PROPAGATING EASTWARD FROM THE MAIN TROUGH. THERE  
ARE SOME VARIATIONS IN THE 00/06Z MODEL GUIDANCE WITH A NORTHEAST  
SHORTWAVE ALREADY BY EARLY SUNDAY, WITH THE 00Z GFS AMONG THE  
STRONGEST WITH THE ENERGY, WHICH ALSO BRINGS ITS ASSOCIATED  
SURFACE LOW CLOSER TO THE NORTHEAST COAST COMPARED TO THE  
CONSENSUS THAT IS FARTHER EAST IN THE ATLANTIC. THAT SOLUTION WAS  
NOT FAVORED AND THE EARLY PART OF THE MEDIUM RANGE FORECAST WAS  
BASED ON A BLEND FAVORING THE 06Z GFS AND 00Z ECMWF, ALONG WITH  
SMALLER PROPORTIONS OF THE 00Z UKMET AND CMC. GOOD MODEL AGREEMENT  
WITH THIS BLEND ALSO PERSISTED WITH FRONTAL TIMING IN THE CENTRAL  
U.S. FOR EARLY WEEK, DESPITE SOME VARIABILITY IN LOCATION OF  
SURFACE LOWS ALONG THE FRONT.  
 
BY TUESDAY THROUGH THURSDAY, MOST GUIDANCE INDICATES A MID-UPPER  
LOW CLOSING OFF OVER SOUTHWESTERN/SOUTH-CENTRAL CANADA AS A PIECE  
OF THE POLAR VORTEX LIKELY DROPS SOUTHWARD, AND THIS WAS INDICATED  
BY THE LATEST WPC FORECAST. SHORTWAVES SPINNING AROUND THE LOW  
HAVE LESS CERTAINTY WITH STRENGTH AND TIMING ONCE AGAIN. THERE IS  
A GOOD BIT OF RUN-TO-RUN VARIABILITY IN MODEL GUIDANCE, BUT FOR  
THE 00Z/06Z CYCLE, THE 06Z GFS APPEARED TO BE THE GREATEST OUTLIER  
IN DETERMINISTIC GUIDANCE, ESSENTIALLY DIVIDING THE UPPER LOW BY  
EARLY THURSDAY WITH ONE CLOSED LOW ALONG THE OR/CA BORDER. THUS BY  
THE LATTER PART OF THE FORECAST PERIOD, TRANSITIONED TO USING A  
PROPORTION OF THE 00Z GFS RATHER THAN THE 06Z WHILE STILL FAVORING  
THE 00Z ECMWF, AND ENDED THE FORECAST PERIOD WITH 50% GEFS AND EC  
ENSEMBLE MEANS COMBINED WITH 50% DETERMINISTIC MODELS TO TEMPER  
THE INDIVIDUAL FORECAST DIFFERENCES.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
ROUNDS OF PRECIPITATION ARE FORECAST FOR THE WEST AS SHORTWAVE  
ENERGY ROUNDING THE PERSISTENT UPPER TROUGH PERIODICALLY ENHANCES  
MOISTURE INFLOW. THE HIGHEST SNOW TOTALS ARE LIKELY FOR THE  
TYPICAL TERRAIN ENHANCED AREAS OF THE CASCADES, SIERRA NEVADA, AND  
PORTIONS OF THE ROCKIES, WHILE COASTAL OREGON AND CALIFORNIA COULD  
SEE LOCALLY HEAVY PRECIPITATION AS WELL. SNOW LEVELS COULD LOWER  
ENOUGH UNDER THE COLD UPPER TROUGH TO BRING SNOW TO CITIES LIKE  
SEATTLE AND PORTLAND CONTINUING FROM THE SHORT RANGE PERIOD INTO  
SUNDAY, WHILE OTHER PARTS OF THE WEST COULD ALSO SEE LOWERING SNOW  
LEVELS AS THE UPPER TROUGH AXIS NUDGES INLAND.  
 
GENERALLY PROGRESSIVE LOW PRESSURE SYSTEMS WILL SPREAD PERIODIC  
PRECIPITATION ACROSS THE NORTH-CENTRAL AND NORTHEASTERN U.S.  
THROUGH NEXT WEEK. SNOW CHANCES SHOULD BE CONFINED TO THE MIDWEST  
TO GREAT LAKES AND INTERIOR NORTHEAST, WITH A ROUND OF HEAVIER  
SNOW POSSIBLE FOR THE UPPER MIDWEST SUNDAY INTO MONDAY. ADDITIONAL  
CHANCES FOR WINTRY WEATHER ARE POSSIBLE LATER NEXT WEEK AS WELL,  
BUT PRECIPITATION TYPE, COVERAGE, AND AMOUNTS ARE STILL UNCERTAIN.  
THE ASSOCIATED TRAILING COLD FRONT SLOWLY SINKING SOUTHEASTWARD  
COULD PROVIDE A FOCUS FOR ENHANCED RAINFALL ACROSS THE OHIO VALLEY  
AROUND TUESDAY-WEDNESDAY AND TOWARD THE TENNESSEE VALLEY AND  
SOUTHERN APPALACHIANS BY THURSDAY OF NEXT WEEK, BUT WITH  
UNCERTAINTY REMAINING REGARDING THE AMOUNTS AND AXIS OF THE  
HEAVIEST RAIN.  
 
MILD TO WARM WEATHER IS EXPECTED ACROSS THE SOUTH-CENTRAL U.S.  
NORTHEASTWARD THROUGH NEXT WEEK. THE SOUTHERN GREAT PLAINS WILL BE  
AT THE CENTER OF THE GREATEST WARM TEMPERATURE ANOMALIES ON SUNDAY  
(20-35F ABOVE NORMAL), WITH RECORD DAILY HIGH MINIMUM AND MAXIMUM  
TEMPERATURES LIKELY. ANOMALOUS TO RECORD WARMTH MAY PRESS SLIGHTLY  
EASTWARD AS THE WEEK PROGRESSES, WITH TEMPERATURES 20+ DEGREES  
ABOVE AVERAGE EXPECTED TO SPREAD INTO PARTS OF THE MISSISSIPPI AND  
TENNESSEE VALLEYS. HIGHS IN THE 70S WILL BE WIDESPREAD IN THE GULF  
COAST STATES, WITH SOME TEMPERATURES EXCEEDING 80F. MEANWHILE,  
FRIGID TEMPERATURES ARE LIKELY IN PARTS OF THE NORTHWESTERN AND  
NORTH-CENTRAL U.S., SPREADING FROM MONTANA SUNDAY INTO MORE OF THE  
NORTHWEST AND NORTHERN PLAINS THROUGH MIDWEEK. WITH TEMPERATURES  
20-40 DEGREES BELOW AVERAGE, EVEN HIGH TEMPERATURES COULD REMAIN  
BELOW 0F FOR MONTANA AND NORTH DAKOTA AROUND TUESDAY AND  
WEDNESDAY. TEMPERATURES ARE ALSO EXPECTED TO TREND COLDER IN THE  
REST OF THE WESTERN U.S., WITH EXPANDING COVERAGE OF HIGHS 10-25F  
BELOW NORMAL NEXT MONDAY-THURSDAY. INCREASING NUMBERS OF LOCATIONS  
ALONG AND JUST INLAND FROM THE WEST COAST COULD SEE DAILY RECORDS  
FOR LOW MAXIMA SUNDAY THROUGH WEDNESDAY.  
 
TATE  
 
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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