783  
FXUS02 KWBC 251858  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
157 PM EST SAT DEC 25 2021  
 
VALID 12Z TUE DEC 28 2021 - 12Z SAT JAN 01 2022  
 
...BITTERLY COLD TEMPERATURES ARE FORECAST ACROSS PORTIONS OF THE  
NORTHWEST TO NORTHERN PLAINS FOR MUCH OF NEXT WEEK...  
 
   
..OVERVIEW
 
 
AS A HIGHLY AMPLIFIED RIDGE/UPPER HIGH PERSISTS OVER THE NORTHERN  
PACIFIC THROUGH MUCH OF NEXT WEEK, TROUGHING IS GENERALLY EXPECTED  
DOWNSTREAM ACROSS THE WESTERN U.S. DURING THE PERIOD WHILE UPPER  
LOWS MEANDER OVER CANADA. SOME ENCROACHMENT OF THE TROUGHING INTO  
THE NORTH-CENTRAL AND NORTHEASTERN TIER OF THE U.S. IS ALSO LIKELY  
AS SHORTWAVES ROUND THE TROUGH, WHICH WILL ALSO CAUSE A SERIES OF  
LOW PRESSURE SYSTEMS TO TRACK ACROSS THE CENTRAL U.S. TO  
NORTHEAST. MEANWHILE, ANOTHER UPPER HIGH OVER THE CARIBBEAN TO THE  
BAHAMAS WILL ALLOW FOR RIDGING AND ABOVE NORMAL TEMPERATURES  
ACROSS THE SOUTHEASTERN QUADRANT OF THE COUNTRY TO THE 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 OFF THE CALIFORNIA COAST.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE LATEST MODELS AND ENSEMBLES CONTINUE TO SHOW GOOD AGREEMENT  
WITH THE LARGE SCALE PATTERN THROUGH MUCH OF THE PERIOD, BUT WITH  
SOME LINGERING UNCERTAINTIES IN THE SMALLER SCALE DETAILS  
ESPECIALLY BY THE LATTER PART OF THE WEEK. RELATIVELY MINOR MODEL  
VARIATIONS WERE SEEN WITH A SERIES OF SHORTWAVES ROTATING OUT OF  
THE WESTERN U.S. TROUGH TOWARDS THE GREAT LAKES/NORTHEAST AND THE  
ASSOCIATED SURFACE LOWS TUESDAY-WEDNESDAY, BUT SOME  
STRENGTH/PLACEMENT DIFFERENCES MAY TAKE UNTIL THE SHORT RANGE TO  
RESOLVE, AND COULD IMPACT THE FRONTAL PLACEMENT AND EXACT AXIS OF  
HEAVY RAINFALL ACROSS THE OHIO VALLEY MIDWEEK. ANOTHER NOTABLE  
AREA OF CONCERN IS REGARDING THE NEXT SHORTWAVE DROPPING DOWN THE  
WEST COAST, WITH A CLOSED LOW LIKELY TO DEVELOP OFF THE CALIFORNIA  
COAST BY LATE WEDNESDAY OR THURSDAY. DETERMINISTIC MODELS FROM  
00/06Z AND THE INCOMING 12Z CYCLE STILL SUPPORT THE IDEA OF THIS  
DEVELOPMENT, BUT WITH DIFFERENCES PARTICULARLY WITH THE TIMING OF  
ITS FORMATION, EASTERN TURN TOWARD THE COAST, AND ABSORPTION BACK  
INTO THE MEAN FLOW BY LATE WEEK. THE 00Z ECMWF WAS A BIT OF AN  
EARLY OUTLIER IN CLOSING OFF THE LOW BY EARLY WEDNESDAY, WHILE THE  
06Z GFS WAS A QUICKER OUTLIER WITH THE TURN EASTWARD INTO THE  
CALIFORNIA COAST COMPARED TO OTHER GUIDANCE/CONSENSUS AND ITS  
PREVIOUS (AND NEWER 12Z) RUNS.  
 
THE EARLY PART OF THE MEDIUM RANGE FORECAST WAS BASED ON A BLEND  
OF PURELY DETERMINISTIC MODELS--THE 00Z ECMWF/CMC/UKMET AND THE  
00Z/06Z GFS. AS THE PERIOD PROGRESSED, ELIMINATED THE 06Z GFS FROM  
THE BLEND AND ALSO INTRODUCED AND GRADUALLY INCREASED THE EC AND  
GEFS ENSEMBLE MEANS TO HELP MITIGATE THE SMALLER SCALE DETAIL  
DIFFERENCES. THIS PROVIDED GOOD CONTINUITY WITH THE PREVIOUS WPC  
FORECAST.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
PRECIPITATION ACROSS THE WEST SHOULD BE WANING BY THE START OF THE  
PERIOD ON TUESDAY, ALTHOUGH SOME MODEST SNOW ACCUMULATIONS MAY  
CONTINUE IN THE HIGHER TERRAIN, ESPECIALLY FOR PARTS OF ARIZONA  
AND THE COLORADO ROCKIES. ACTIVITY SHOULD QUIET DOWN IN THE WEST  
ON WEDNESDAY, BUT ANOTHER ROUND OF HEAVY MOUNTAIN SNOWS ARE  
POSSIBLE FOR PORTIONS OF THE PACIFIC NORTHWEST/NORTHERN ROCKIES  
PARTICULARLY ON THURSDAY AHEAD OF THE SHORTWAVE DROPPING DOWN THE  
WEST COAST. THE CLOSED LOW DEVELOPMENT OFF THE CALIFORNIA COAST  
MAY HELP DIRECT INCREASED MOISTURE INTO PARTS OF SOUTHERN  
CALIFORNIA AND POSSIBLY THE SOUTHWEST AS WELL LATER NEXT WEEK.  
 
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 WILL BE ONGOING BY TUESDAY 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 TUESDAY-WEDNESDAY, POSSIBLY EVEN INTO THE  
LATTER HALF OF THE WEEK, DESPITE TEMPERATURES MODERATING SLIGHTLY  
AS THE WEEK PROGRESSES. COLD TEMPERATURES COMBINED WITH GUSTY  
WINDS COULD RESULT IN DANGEROUSLY COLD WIND CHILLS, ESPECIALLY FOR  
NORTHERN PARTS OF THE HIGH PLAINS WHERE WIND CHILL VALUES COULD  
DIP WELL BELOW 0F FOR MANY. TEMPERATURES ARE ALSO EXPECTED TO  
TREND COLDER IN THE REST OF THE WESTERN U.S., WITH EXPANDING  
COVERAGE OF HIGHS 10-25F BELOW NORMAL EARLY TO MID NEXT WEEK.  
INCREASING NUMBERS OF LOCATIONS ALONG AND JUST INLAND FROM THE  
WEST COAST COULD SEE DAILY RECORDS FOR LOW MAXIMUM AND MINIMUM  
TEMPERATURES TUESDAY AND WEDNESDAY. MEANWHILE, MILD TO WARM  
WEATHER IS EXPECTED ACROSS THE SOUTH-CENTRAL U.S. AND  
NORTHEASTWARD THROUGH NEXT WEEK WHERE MANY PLACES FROM THE  
SOUTHERN HALF OF THE PLAINS TO THE OHIO VALLEY/MID-ATLANTIC COULD  
BE 20 TO 30 DEGREES ABOVE NORMAL TUESDAY INTO WEDNESDAY. DAILY  
RECORD HIGH MINIMUM AND MAXIMUM TEMPERATURES ARE LIKELY. AS THE  
PATTERN DEAMPLIFIES JUST SLIGHTLY OVER THE COUNTRY LATER NEXT  
WEEK, TEMPERATURES ACROSS BOTH THE WEST AND EAST SHOULD MODERATE,  
BUT STILL MAINTAIN A GENERAL PATTERN OF COOL IN THE  
WEST/NORTH-CENTRAL U.S. AND WARM IN THE SOUTH-CENTRAL/EASTERN  
U.S., JUST LESS EXTREME.  
 
TATE/SANTORELLI  
 
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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