687  
FXUS02 KWBC 271856  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
155 PM EST MON DEC 27 2021  
 
VALID 12Z THU DEC 30 2021 - 12Z MON JAN 03 2022  
 
...BITTERLY COLD TEMPERATURES TO CONTINUE THURSDAY-SATURDAY ACROSS  
PORTIONS OF THE NORTHERN PLAINS...  
 
   
..OVERVIEW
 
 
A BLOCKY AND AMPLIFIED RIDGE/UPPER HIGH ANCHORED OVER THE NORTHERN  
PACIFIC THROUGH LATE THIS WEEK WILL CONTINUE TO SUPPORT WESTERN  
U.S. UPPER TROUGH REINFORCEMENT AND CHILLY WEATHER. A SEPARATE  
PIECE OF ENERGY WITHIN THE LARGER SCALE TROUGH SHOULD TRACK A  
LITTLE OFFSHORE SOUTHERN CALIFORNIA THURSDAY-FRIDAY. EXPECT THE  
PATTERN TO CHANGE AFTER FRIDAY AS A NEW UPPER RIDGE BUILDS FARTHER  
WEST, LEADING TO A DEEPENING TROUGH OVER THE NORTHEAST PACIFIC AND  
RENEWED MOISTURE FEED INTO THE NORTHWEST U.S. MEANWHILE THIS  
PATTERN SHIFT WILL KICK OUT THE LATE WEEK WESTERN TROUGH THAT WILL  
ULTIMATELY SUPPORT A WIDESPREAD AREA OF PRECIPITATION ACROSS THE  
EASTERN HALF OF THE COUNTRY IN ASSOCIATION WITH ORGANIZED LOW  
PRESSURE/FRONTS. A MUCH WEAKER WAVY FRONTAL SYSTEM WILL DEPART  
FROM THE EAST LATE THIS WEEK.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE SPREAD AMONG THE 00Z/06Z MODELS AND ENSEMBLES, AS WELL AS THE  
PREFERRED FORECAST APPROACH AND RESULTING SYSTEM EVOLUTION, ARE  
FAIRLY SIMILAR TO THE PREVIOUS CYCLE. AN OPERATIONAL BLEND LOOKS  
GOOD EARLY IN THE PERIOD WITH A GRADUAL INCREASE OF THE 06Z  
GEFS/00Z ECMWF MEANS LATER IN THE FORECAST (REACHING 40 PERCENT  
TOTAL BY DAY 7 MONDAY) HELPING TO TONE DOWN LOWER-PREDICTABILITY  
DIFFERENCES.  
 
THROUGH THE PERIOD THERE ARE VARIOUS EMBEDDED DETAIL UNCERTAINTIES  
WITHIN A MORE AGREEABLE MEAN EVOLUTION. ON THURSDAY-FRIDAY  
DIFFERENCES ARE NOTICEABLE WITH THE EVOLUTION/TIMING OF THE ENERGY  
OFF SOUTHERN CALIFORNIA, WITH THE 12Z GFS/UKMET/CMC ALL SUGGESTING  
THE 00Z ECMWF MAY BE TOO SLOW AND CLOSED AS NORTHERN STREAM ENERGY  
DIGS DOWN FROM THE NORTH. THE 12Z ECMWF HAS INDEED TRENDED  
SOMEWHAT FASTER. EITHER WAY THE OVERALL PATTERN RETURNS TO FAIRLY  
GOOD AGREEMENT BY SATURDAY. THEN DIFFERENCES ARISE WITH HOW ENERGY  
WILL BE DISTRIBUTED WITHIN THE OVERALL UPPER TROUGH CROSSING THE  
EASTERN HALF OF THE COUNTRY, AND THUS WITH SURFACE EVOLUTION. THUS  
FAR ECMWF RUNS HAVE BEEN EITHER SUFFICIENTLY PHASED OR DIFFUSE  
ALOFT TO YIELD A FAIRLY CONSISTENT MODERATE SURFACE DEVELOPMENT  
THAT EVENTUALLY BECOMES DEEPER OVER OR NEAR THE CANADIAN  
MARITIMES. GFS RUNS HAVE BEEN QUITE VARIABLE, TENDING TO SHOW  
STRONGER DEVELOPMENT OVER THE NORTHEASTERN QUADRANT OF LOWER 48  
BUT ALSO SOMEWHAT MORE FLOW SEPARATION THAT WOULD SUPPORT A  
TRAILING WAVE (THE LATTER HINTED AT MORE IN THE NEW 12Z ECMWF).  
THE 12Z GFS HAS BACKED OFF WITH THE DEPTH OF PRIMARY LOW PRESSURE  
BUT STILL HAS A PRONOUNCED TRAILING WAVE. A MODEL/MEAN BLEND WITH  
THE MODEL COMPONENT TILTED SOMEWHAT MORE IN THE ECMWF DIRECTION  
PROVIDED GOOD CONTINUITY WHILE AWAITING MORE CONFIDENT ADJUSTMENTS  
IN SYSTEM EVOLUTION. ON THE OTHER HAND, FOR NOW THE MODELS AND  
MEANS SHOW BETTER THAN AVERAGE AGREEMENT RELATIVE TO TYPICAL DAYS  
6-7 FORECASTS FOR THE UPPER TROUGHING AND CORRESPONDING STORM  
SYSTEM OFF THE PACIFIC NORTHWEST COAST BY SUNDAY-MONDAY.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
A WAVY COLD FRONT OVER THE EAST ON THURSDAY WILL ALLOW FOR SOME  
LINGERING SHOWERS ACROSS MAINLY THE SOUTHEAST, ALONG WITH LIGHT  
PRECIPITATION OVER PARTS OF THE NORTHEAST (SNOW IN NORTHERN NEW  
ENGLAND). BY FRIDAY NIGHT THROUGH THE WEEKEND DEVELOPING LOW  
PRESSURE AND ASSOCIATED FRONTS WILL SPREAD A BROAD AREA OF  
PRECIPITATION ACROSS PARTS OF THE PLAINS AND ESPECIALLY THE EAST.  
BEST POTENTIAL FOR MODERATE TO LOCALLY HEAVY RAINFALL WILL BE  
AHEAD OF THE COLD FRONT, APPROXIMATELY FROM THE SOUTH-CENTRAL  
MISSISSIPPI VALLEY THROUGH THE LOWER OHIO VALLEY/TENNESSEE VALLEY  
AND INTO THE SOUTHERN APPALACHIANS. EXPECT SOME WINTRY WEATHER IN  
THE NORTHERN SIDE OF THE PRECIPITATION SHIELD--FROM PARTS OF THE  
NORTHERN/CENTRAL PLAINS THROUGH THE MIDWEST, GREAT LAKES, AND  
NORTHEAST. STRENGTH AND TRACK OF LOW PRESSURE AS WELL AS ANY  
FRONTAL WAVES WILL DETERMINE THE PRECISE DETAILS OF PLACEMENT AND  
TOTALS.  
 
ANOTHER ROUND OF HEAVY MOUNTAIN SNOW IS POSSIBLE FOR PORTIONS OF  
THE PACIFIC NORTHWEST/NORTHERN ROCKIES ON THURSDAY AHEAD OF A  
SHORTWAVE REACHING THE NORTHWEST BY EARLY FRIDAY. FARTHER SOUTH,  
THE UPPER TROUGH/POSSIBLE CLOSED LOW OFF SOUTHERN CALIFORNIA WILL  
DIRECT INCREASED MOISTURE INTO THE SOUTHWESTERN U.S. AND FOUR  
CORNERS REGION THROUGH FRIDAY OR SATURDAY. SNOW IS LIKELY IN THE  
HIGHER TERRAIN, ESPECIALLY OVER THE COLORADO ROCKIES WITH THE  
MERGING OF NORTHERN AND SOUTHERN STREAM UPPER SYSTEMS. FLOW AHEAD  
OF THE NEXT FRONTAL SYSTEM APPROACHING THE PACIFIC NORTHWEST  
SHOULD BRING YET ANOTHER ROUND OF HEAVY MOUNTAIN SNOW TO THE  
REGION BY THE WEEKEND. LOWER ELEVATION RAIN COULD BECOME MODERATE  
TO HEAVY FROM THE PACIFIC NORTHWEST INTO NORTHWESTERN CALIFORNIA  
BY EARLY NEXT WEEK.  
 
BITTERLY COLD TEMPERATURES SHOULD CONTINUE THROUGH AT LEAST  
SATURDAY PARTICULARLY FOR THE NORTHERN PLAINS WITH DAYTIME HIGHS  
(AND TO A LESSER EXTENT OVERNIGHT LOWS) 20 TO 35 DEGREES BELOW  
AVERAGE. SOME AREAS OF NORTHERN NORTH DAKOTA AND MONTANA MAY STAY  
BELOW 0F EVEN INTO NEXT WEEKEND, DESPITE TEMPERATURES MODERATING  
SLIGHTLY AS THE WEEK PROGRESSES. THE ADDITION OF 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 LIKELY TO BE BELOW NORMAL  
BY ABOUT 5-15 DEGREES ACROSS THE REST OF THE WESTERN U.S. THROUGH  
SUNDAY. MEANWHILE, EXPECT MILD TO WARM WEATHER ACROSS THE  
SOUTH-CENTRAL U.S. AND NORTHEASTWARD THROUGH SATURDAY AND AREAS  
NEAR THE EAST COAST INTO SUNDAY. MANY PLACES FROM THE SOUTHERN  
HALF OF THE PLAINS TO THE OHIO VALLEY/MID-ATLANTIC COULD BE 15-25  
DEGREES ABOVE NORMAL THROUGH SATURDAY. MORE DAILY RECORD HIGH  
MINIMUM AND MAXIMUM TEMPERATURES ARE LIKELY. A POTENTIAL PATTERN  
SHIFT MAY FINALLY BRING VALUES BACK CLOSER TO NORMAL, POSSIBLY  
BELOW AVERAGE IN THE SOUTH-CENTRAL U.S. BY SUNDAY AND THE EAST BY  
NEXT MONDAY. ALSO BY MONDAY PARTS OF THE HIGH PLAINS MAY SEE  
TEMPERATURES REBOUND TO MODERATELY ABOVE NORMAL.  
 
RAUSCH/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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