503  
FXUS02 KWBC 241859  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
158 PM EST FRI DEC 24 2021  
 
VALID 12Z MON DEC 27 2021 - 12Z FRI DEC 31 2021  
 
...POTENTIAL FOR HEAVY MOUNTAIN SNOW IN THE WEST CONTINUES THROUGH  
EARLY NEXT WEEK...  
 
...BITTERLY COLD TEMPERATURES ARE FORECAST ACROSS PORTIONS OF THE  
NORTHWEST TO NORTHERN PLAINS NEXT WEEK...  
 
   
..OVERVIEW
 
 
A RATHER STAGNANT PATTERN IS FORECAST FOR AT LEAST THE EARLY PART  
OF THE MEDIUM RANGE PERIOD LED BY A PERSISTENT AND HIGHLY  
AMPLIFIED NORTHERN PACIFIC RIDGE/UPPER HIGH. DOWNSTREAM, TROUGHING  
IS EXPECTED OVER THE WESTERN U.S. AS A MID-UPPER LOW DRIFTS  
EASTWARD OVER WEST-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. BY THE LATTER HALF OF THE WEEK, MODEL  
GUIDANCE IS INDICATING THE POTENTIAL FOR A SOUTHERN STREAM UPPER  
LOW DROPPING THROUGH THE EASTERN PACIFIC JUST WEST OF THE  
CALIFORNIA COAST.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
MODEL GUIDANCE SHOWS RELATIVELY GOOD AGREEMENT WITH THE LARGE  
SCALE PATTERN OF WEST TROUGHING AND SOUTHEAST RIDGING THROUGH MUCH  
OF NEXT WEEK, BUT WITH SOME DIFFERENCES IN SOME SMALLER SCALE  
DETAILS, PARTICULARLY PERTAINING TO A COUPLE OF SHORTWAVES  
ROTATING OUT OF THE WESTERN U.S. TROUGH. THERE IS GOOD CONSENSUS  
WITH AN INITIAL SHORTWAVE AND SURFACE LOW MOVING ACROSS THE GREAT  
LAKES MONDAY, AND MODEL AGREEMENT IS INCREASING SOMEWHAT FOR  
ANOTHER SHORTWAVE AND LOW THERE BY EARLY WEDNESDAY, WITH A  
SOMEWHAT FASTER MOVING TRAILING COLD FRONT COMPARED TO PREVIOUS  
FORECASTS. THIS IS IN CONJUNCTION WITH A MODEL TREND TOWARD LESS  
ENERGY DROPPING SOUTH WITH A LESS CONSOLIDATED/NOT CLOSED UPPER  
LOW OVER SOUTHWESTERN/SOUTH-CENTRAL CANADA COMPARED TO GUIDANCE  
FROM A DAY OR SO AGO. THIS ASPECT OF THE FORECAST REMAINS  
UNCERTAIN GIVEN THE RUN-TO-RUN VARIABILITY. THE OTHER MAIN AREA OF  
MODEL DIFFERENCES IN THE 00/06Z CYCLE IS WITH THE POSSIBLE  
SOUTHERN STREAM LOW DEVELOPING AS ENERGY DROPS SOUTH ALONG THE  
EASTERN PACIFIC AND WEST COAST. GENERALLY, MODEL GUIDANCE HAS  
TRENDED TOWARD THE EXISTENCE OF THIS FEATURE, THOUGH WITH  
DIFFERENCES IN THE TIMING OF ITS CLOSING OFF AND WITH ITS TRACK.  
THE 00Z GFS SEEMED TO BE AN EXCEPTION AT THIS POINT IN SHOWING A  
WEAK AND FAST TROUGH THURSDAY-FRIDAY, WITH ITS 06Z AND 12Z RUNS AS  
WELL AS RUNS FROM YESTERDAY INDICATING A CLOSED LOW.  
 
FOR THE WPC BLEND, THE 00Z/06Z GUIDANCE PRESENTED ENOUGH AGREEMENT  
FOR DAYS 3-4 TO WARRANT A PURELY DETERMINISTIC MODEL BLEND BETWEEN  
THE 06Z GFS, 00Z ECMWF, AND 00Z CMC. AFTER THAT, THE ENSEMBLE  
MEANS WERE INTRODUCED AND INCREASED AS THE PERIOD PROGRESSED TO  
HELP MITIGATE THE SMALLER SCALE DETAIL DIFFERENCES, WHICH COULD  
TAKE SEVERAL DAYS TO RESOLVE STILL. FAVORED THE 00Z EC ENSEMBLE  
MEAN DUE TO ITS BETTER REPRESENTATION OF A SOUTHERN STREAM  
TROUGH/LOW OFF OF CALIFORNIA COMPARED TO THE GEFS MEANS. THIS  
APPROACH PROVIDED GOOD CONTINUITY WITH THE PREVIOUS WPC FORECAST  
OVERALL.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
PRECIPITATION IS LIKELY TO CONTINUE IN THE WEST AS THE MEDIUM  
RANGE PERIOD BEGINS MONDAY AS SHORTWAVE ENERGY ROUNDS THE  
PERSISTENT UPPER TROUGH AND ENHANCES MOISTURE INFLOW. MULTIPLE  
FEET OF SNOW ARE FORECAST FOR THE SIERRA NEVADA CONTINUING FROM  
SUNDAY INTO MONDAY, WHILE OTHER TERRAIN AREAS OF CALIFORNIA COULD  
ALSO SEE LOCAL ENHANCEMENT. ADDITIONAL PRECIPITATION SHOULD SPREAD  
INTO THE INTERMOUNTAIN WEST AS WELL ESPECIALLY ON MONDAY AND  
TUESDAY, WITH THE MOGOLLON RIM AND PORTIONS OF THE ROCKIES WITH  
POSSIBLY HEAVY TOTALS. SOUTHERN CALIFORNIA COULD SEE MORE  
PRECIPITATION FROM MIDWEEK ONWARD AS AN UPPER LOW MAY FORM  
OFFSHORE, DIRECTING ANOTHER ROUND OF MOISTURE INTO THE AREA.  
 
GENERALLY PROGRESSIVE LOW PRESSURE SYSTEMS WILL SPREAD PERIODIC  
PRECIPITATION ACROSS PARTS OF THE CENTRAL AND EASTERN U.S. THROUGH  
NEXT WEEK. THE BEST SNOW CHANCES SHOULD BE CONFINED TO THE GREAT  
LAKES AND INTERIOR NORTHEAST, WITH RAIN TO THE SOUTH. 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/SOUTHEAST BY THURSDAY-FRIDAY OF NEXT WEEK,  
BUT WITH UNCERTAINTY REMAINING REGARDING THE AMOUNTS AND AXIS OF  
THE HEAVIEST RAIN.  
 
BITTERLY COLD TEMPERATURES ARE LIKELY IN PARTS OF THE NORTHWESTERN  
AND NORTH-CENTRAL U.S., WITH DAYTIME HIGHS (AND A LESSER EXTENT  
OVERNIGHT LOWS) 20 TO NEARLY 40 DEGREES BELOW AVERAGE. SOME AREAS  
OF NORTH DAKOTA AND MONTANA MAY STAY BELOW 0F FOR MORE THAN 48  
HOURS MONDAY-WEDNESDAY BEFORE MODERATING SOMEWHAT BY THURSDAY.  
COLD TEMPERATURES COMBINED WITH GUSTY WINDS COULD RESULT IN  
DANGEROUSLY COLD WIND CHILLS, ESPECIALLY FOR PARTS OF THE NORTHERN  
HIGH PLAINS WHERE WIND CHILL VALUES COULD DIP WELL BELOW 0F, EVEN  
WHERE TEMPERATURES ARE ABOVE ZERO. 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 WEEK. INCREASING  
NUMBERS OF LOCATIONS ALONG AND JUST INLAND FROM THE WEST COAST  
COULD SEE DAILY RECORDS FOR LOW MAXIMUM AND MINIMUM TEMPERATURES  
MONDAY THROUGH WEDNESDAY. MEANWHILE, MILD TO WARM WEATHER IS  
EXPECTED ACROSS THE SOUTH-CENTRAL U.S. AND NORTHEASTWARD THROUGH  
NEXT WEEK. THE SOUTHERN PLAINS TO MISSISSIPPI/TENNESSEE VALLEYS  
WILL BE AT THE CENTER OF THE GREATEST WARM TEMPERATURE ANOMALIES,  
ESPECIALLY MONDAY-WEDNESDAY WHERE MANY PLACES COULD BE 20 TO 30  
DEGREES ABOVE NORMAL. DAILY RECORD HIGH MINIMUM AND MAXIMUM  
TEMPERATURES ARE LIKELY. ANOMALOUS TO RECORD WARMTH SHOULD PRESS  
SLIGHTLY EASTWARD AS THE WEEK PROGRESSES, WITH WELL ABOVE AVERAGE  
TEMPERATURES EXPECTED TO SPREAD INTO PARTS OF THE OHIO VALLEY  
PARTS OF THE EAST LATER NEXT WEEK.  
 
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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