808  
FXUS02 KWBC 021859  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
159 PM EST THU DEC 02 2021  
 
VALID 12Z SUN DEC 05 2021 - 12Z THU DEC 09 2021  
   
..OVERVIEW
 
 
GUIDANCE SHOWS THE LARGE SCALE UPPER PATTERN SETTLING INTO A BROAD  
MEAN TROUGH ACROSS MUCH OF THE LOWER 48, BETWEEN A STRONG BUT  
SLOWLY FLATTENING EASTERN PACIFIC RIDGE AND ANOTHER AREA OF  
RIDGING OVER THE MID-LATITUDE ATLANTIC THROUGH THE CARIBBEAN AND  
SOUTHERN GULF OF MEXICO. THIS PATTERN WILL SUPPORT A SERIES OF  
SURFACE SYSTEMS AND FRONTS THAT WILL BRING AREAS OF MEANINGFUL  
RAIN/MOUNTAIN SNOW MAINLY TO NORTHERN AND CENTRAL PARTS OF THE  
WEST AND THEN PRECIPITATION TO THE EASTERN HALF OF THE COUNTRY  
WITH SNOW POSSIBLE OVER SOME NORTHERN AREAS. ALTHOUGH THE  
FORECAST MEAN PATTERN IS AGREEABLE, DETAILS OF EMBEDDED INDIVIDUAL  
SHORTWAVES BECOME INCREASINGLY UNCERTAIN FROM MONDAY ONWARD SO  
CONFIDENCE IN SOME OF THE IMPORTANT SENSIBLE WEATHER SPECIFICS IS  
A LOT LOWER.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
ASIDE FROM THE NEW 12Z UKMET THAT IS SLOW WITH ITS UPPER TROUGH,  
GUIDANCE IS SLOWLY BUT STEADILY IMPROVING ITS CLUSTERING FOR THE  
STRENGTHENING SYSTEM TRACKING FROM THE NORTHERN PLAINS INTO  
SOUTHEASTERN CANADA ALONG WITH THE TRAILING FRONT THAT CROSSES THE  
EASTERN HALF OF THE COUNTRY DURING SUNDAY-MONDAY. THERE IS STILL  
SOME DISAGREEMENT OVER EXACT STRUCTURE OF THE LOW AT TIMES WITH AN  
INTERMEDIATE SOLUTION REASONABLE, BUT AGAIN MINUS THE 12Z UKMET,  
CONSENSUS IS NOW GOOD FOR TIMING OF THE TRAILING FRONT.  
 
FARTHER WEST, SOME ASPECTS OF THE FORECAST APPEAR TO HAVE BECOME  
MORE DIVERGENT OVER THE PAST DAY. GUIDANCE CONTINUES TO HAVE  
DIFFICULTY IN RESOLVING THE CHARACTER OF SHORTWAVE ENERGY EXPECTED  
TO BE SOUTH OF ALASKA AT THE START OF THE PERIOD EARLY SUNDAY, AND  
IN ADDITION POSSIBLY SOME ENERGY FROM A SEPARATE FEATURE TO THE  
SOUTH, DROP INTO THE WESTERN U.S. OVER THE FOLLOWING COUPLE DAYS.  
AMONG INCOMING GUIDANCE THE 12Z GFS WAS ON THE STRONG/SOUTHERN  
SIDE OF THE SPREAD WITH ITS CONCENTRATED UPPER DYNAMICS AND  
ASSOCIATED SURFACE LOW NEARING THE NORTHWEST EARLY MONDAY.  
INTERESTINGLY THE NEW ECMWF HAS ADJUSTED A LOT CLOSER TO THE GFS  
AT THE SURFACE EVEN THOUGH THE ECMWF'S UPPER SHORTWAVE IS A LOT  
MORE OPEN. FROM THERE GUIDANCE DIFFERS OVER HOW MUCH THE ENERGY  
WILL AMPLIFY OVER THE WEST, WHETHER IT MAY HELP TO EJECT THE UPPER  
LOW OVER/NEAR BAJA CALIFORNIA, AND THE ULTIMATE AMPLITUDE AND  
TIMING DOWNSTREAM. THE GUIDANCE AVERAGE IS WAFFLING SOMEWHAT FOR  
HOW MUCH WESTERN AMPLIFICATION WILL OCCUR EARLY IN THE WEEK WITH  
THE 00Z/06Z CONSENSUS TRENDING A BIT DEEPER THAN PREVIOUS  
FORECAST. 12Z MODELS ARE NOW INCREASING THE POTENTIAL FOR QUICKER  
EJECTION OF THE BAJA UPPER LOW. ANOTHER BUNDLE OF NORTH PACIFIC  
ENERGY FLOWING AROUND THE EASTERN PACIFIC UPPER RIDGE SHOULD REACH  
WESTERN NORTH AMERICA AROUND WEDNESDAY WITH AMPLITUDE/TIMING  
DIFFERENCES ADDING TO THE UNCERTAINTY.  
 
GIVEN THE RAPIDLY DECLINING CONFIDENCE IN DETAILS WITH TIME, THE  
OPERATIONAL MODEL CONSENSUS EMPLOYED EARLY IN THE PERIOD  
TRANSITIONED TO 60 PERCENT TOTAL WEIGHT OF THE 18Z GEFS/00Z ECMWF  
MEANS BY DAY 7 THURSDAY TO EMPHASIZE THE MORE AGREEABLE MEAN  
PATTERN. OTHERWISE LINGERING INPUT FROM LATEST ECMWF/GFS RUNS  
PROVIDED A LITTLE ENHANCEMENT WHERE AGREEABLE TRAITS EXISTED BUT  
OTHERWISE OFFSET TO KEEP THE BLEND CLOSER TO THE MEANS.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
THE STRENGTHENING STORM SYSTEM TRACKING FROM THE NORTHERN PLAINS  
THROUGH THE GREAT LAKES AND INTO SOUTHEASTERN CANADA SUNDAY-MONDAY  
SHOULD PRODUCE A BAND OF MEANINGFUL SNOW TO THE NORTH OF THE LOW  
TRACK--MAINLY OVER THE EXTREME NORTHERN PLAINS INTO UPPER GREAT  
LAKES. ALSO THE STORM WILL LIKELY PRODUCE A PERIOD OF STRONG  
WINDS FROM THE NORTHERN PLAINS INTO GREAT LAKES, AND THEN AFTER  
PASSAGE, A PERIOD OF LAKE EFFECT SNOW. THERE SHOULD BE SUFFICIENT  
MOISTURE ALONG THE TRAILING COLD FRONT TO PRODUCE SOME LOCALLY  
MODERATE TO HEAVY RAINFALL FROM THE OHIO VALLEY SOUTHWESTWARD.  
RELATIVELY DRY CONDITIONS LEADING INTO THIS EVENT SHOULD HELP TO  
TEMPER WHAT ADVERSE EFFECTS COULD ARISE WHERE RAINFALL HAPPENS TO  
BE MORE INTENSE.  
 
WHILE THE DETAILS REMAIN A QUESTION MARK, THE SYSTEM PUSHING INTO  
THE NORTHWEST BY MONDAY AND THEN DROPPING SOUTHEASTWARD OVER THE  
WEST WILL PRODUCE AN AREA OF ENHANCED PRECIPITATION FROM THE  
PACIFIC NORTHWEST THROUGH THE NORTHERN AND CENTRAL ROCKIES.  
HIGHEST TOTALS WILL BE OVER FAVORED WINDWARD TERRAIN AND THE BEST  
POTENTIAL FOR HEAVY SNOW WILL BE OVER THE NORTHERN CASCADES AND  
NORTHERN ROCKIES. THE PRECISE SOUTHWARD EXTENT OF PRECIPITATION  
OVER THE WEST WILL DEPEND ON THE UNCERTAIN AMPLITUDE AND  
PROGRESSION OF DYNAMICS ALOFT. PROGRESSION OF THIS ENERGY INTO  
THE EAST, POSSIBLE WAVE DEVELOPMENT, AND INTERACTION OF GULF  
MOISTURE WITH A MEAN FRONTAL BOUNDARY SHOULD LEAD TO AN INCREASING  
AREA OF PRECIPITATION OVER THE EASTERN HALF OF THE COUNTRY TUESDAY  
ONWARD. SOME HEAVY RAINFALL WILL BE POSSIBLE, WITH DETAILS COMING  
INTO BETTER FOCUS WHEN GUIDANCE EVENTUALLY AGREES BETTER FOR  
IMPORTANT SHORTWAVE SPECIFICS. SOME NORTHERN LATITUDES MAY SEE  
SNOW WITH RELATIVELY HIGHER PROBABILITIES FROM THE GREAT LAKES  
INTO NEW ENGLAND. UPSTREAM PACIFIC ENERGY SHOULD PRODUCE ONE OR  
MORE ADDITIONAL EPISODES OF RAIN AND HIGHER ELEVATION SNOW OVER  
PORTIONS OF THE WEST FROM TUESDAY ONWARD.  
 
OVER A MAJORITY OF THE LOWER 48 THE PROGRESSIVE NATURE OF MOST  
SYSTEMS WILL PROMOTE VARIABLE TEMPERATURES FROM DAY TO DAY AND  
LIKELY AVERAGE OUT TO NEAR OR ABOVE NORMAL. THE ONE EXCEPTION  
SHOULD BE FROM THE EXTREME NORTHERN PLAINS INTO UPPER GREAT LAKES  
WHERE SNOW COVER IN THE WAKE OF THE SUNDAY-MONDAY SYSTEM MAY LEAD  
TO SOME READINGS 10-20F BELOW NORMAL EARLY NEXT WEEK. THE  
GREATEST ANOMALIES ON THE WARM SIDE SHOULD BE AHEAD OF THE  
AFOREMENTIONED SYSTEM, WITH A FAIRLY BROAD AREA OF PLUS 10-20F  
ANOMALIES FROM THE PLAINS INTO THE EAST EARLY IN THE PERIOD. ONE  
OR MORE ADDITIONAL EPISODES OF WARMTH MAY PASS THROUGH THE  
SOUTHWEST INTO THE SOUTHERN PLAINS. A FEW DAILY RECORDS FOR  
HIGHS/WARM LOWS MAY BE POSSIBLE OVER THE SOUTHWEST.  
 
RAUSCH  
 
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  
 

 
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page