773  
FXUS02 KWBC 302030  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
329 PM EST TUE NOV 30 2021  
 
VALID 12Z FRI DEC 03 2021 - 12Z TUE DEC 07 2021  
   
..OVERVIEW
 
 
DURING THE FRIDAY-TUESDAY PERIOD EXPECT AN EASTERN PACIFIC MEAN  
RIDGE ALOFT TO SUPPORT VARYING DEGREES OF CYCLONIC FLOW ACROSS  
SOUTHERN CANADA AND PORTIONS OF THE LOWER 48, WITH THE OVERALL  
MEAN TROUGH AXIS TENDING TO EXTEND FROM CENTRAL CANADA INTO THE  
GREAT LAKES/NORTHEAST. UPPER TROUGHING SHOULD BE FAIRLY SHALLOW  
LATE THIS WEEK AND THEN BECOME SOMEWHAT MORE AMPLIFIED AFTER  
SATURDAY. THIS PATTERN WILL CARRY ALONG A SERIES OF SURFACE WAVES  
AND FRONTAL SYSTEMS, MOST OF WHICH SHOULD PRODUCE LIGHT TO  
MODERATE PRECIPITATION. A FRONT REACHING THE WEST EARLY NEXT WEEK  
MAY SUPPORT A PERIOD OF HEAVIER ACTIVITY FROM THE PACIFIC  
NORTHWEST INTO THE ROCKIES. MEANWHILE ONE OR MORE PIECES OF  
ENERGY OVER THE EASTERN PACIFIC/FAR SOUTHWEST CORNER OF THE U.S.  
MAY CONSOLIDATE INTO AN UPPER LOW THAT SETTLES NEAR BAJA  
CALIFORNIA. THIS FEATURE SHOULD NOT PRODUCE MUCH SIGNIFICANT  
WEATHER DURING THE PERIOD.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE MOST PROMINENT UNCERTAINTIES OVER THE LOWER 48 SEEM TO ARISE  
FROM QUESTION MARKS WITH A COUPLE STREAMS OF FLOW WITHIN A GENERAL  
TROUGH TO THE SOUTH OF ALASKA. A LEADING SHORTWAVE REACHES THE  
PACIFIC NORTHWEST BY EARLY SATURDAY AND THERE ARE ALREADY  
AMPLITUDE DIFFERENCES AT THAT TIME WITH MORE SIGNIFICANT SPREAD  
FOR THE CORRESPONDING SURFACE PATTERN OFF THE PACIFIC NORTHWEST  
COAST. A PREFERRED INTERMEDIATE SOLUTION WOULD HAVE A MODERATE  
WAVE NOT FAR FROM THE 00Z ECMWF MEAN POSITION BUT SOMEWHAT WEAKER  
THAN IT OR THE 00Z ECMWF/12Z CMC. THE LATEST GFS/UKMET ARE QUITE  
WEAK AND SUPPRESSED WHILE THE 12Z ECMWF IS WEAKER/SOUTH VERSUS THE  
PRIOR RUN.  
 
EARLY DIFFERENCES WITH THIS SHORTWAVE AND THEN DIFFERENCES IN HOW  
STREAMS INTERACT WITHIN THE TROUGH SOUTH OF ALASKA LEAD TO RAPIDLY  
INCREASING SPREAD FOR LOW PRESSURE TRACKING FROM THE PLAINS INTO  
EASTERN CANADA/NORTHEAST U.S. BY SUNDAY-MONDAY. LATEST GFS/GEFS  
RUNS AND THE 12Z UKMET DIG THE INCOMING SHORTWAVE ENERGY RATHER  
STRONGLY INTO THE EAST-CENTRAL U.S., LEADING TO STRONG LOW  
PRESSURE OVER THE GREAT LAKES/SOUTHEASTERN CANADA BY EARLY MONDAY.  
IN CONTRAST THE 00Z ECMWF AND LATEST CMC RUNS ARE MORE  
PROGRESSIVE AND LESS AMPLIFIED, LEADING TO WEAKER AND FARTHER  
EAST/SOUTHEAST LOW PRESSURE. HIGHLIGHTING THE UNCERTAINTY,  
INDIVIDUAL ENSEMBLE MEMBERS ARE VERY DIVERSE AND GFS/ECMWF RUNS  
HAVE OCCASIONALLY SWITCHED PLACES IN RECENT DAYS. FOR EXAMPLE A  
NUMBER OF RECENT ECMWF RUNS HAD SOME SIMILARITY TO THE GFS BUT THE  
00Z ECMWF HAPPENED TO BE SIMILAR TO YESTERDAY'S 00Z GFS THAT WAS  
ON THE WEAKER/SUPPRESSED SIDE VERSUS MOST GFS RUNS. THE 12Z ECMWF  
TRENDED MUCH STRONGER AND A LITTLE NORTHWESTWARD WITH INTERMEDIATE  
TIMING. PREFER TO MAINTAIN A COMPROMISE APPROACH THAT IS FAIRLY  
CLOSE TO CONTINUITY.  
 
ADDITIONAL ENERGY SHOULD DROP INTO THE WESTERN HALF OF THE COUNTRY  
BY NEXT TUESDAY BUT WITH CONSIDERABLE DIFFERENCES FOR SPECIFICS.  
THE NEW 12Z GFS IS ON THE DEEP SIDE OF GUIDANCE BY EARLY TUESDAY.  
ALSO NOTE THAT 06Z/12Z GFS RUNS ARE ON THE EASTERN SIDE OF THE  
FULL GUIDANCE SPREAD FOR THE UPPER LOW NEAR BAJA CALIFORNIA BY  
SUNDAY.  
 
A BLEND OF THE 00Z/06Z GFS RUNS AND THE PAST TWO ECMWF RUNS  
THROUGH THE 00Z CYCLE, ALONG WITH MODEST 00Z CMC/UKMET INPUT,  
PROVIDED A GOOD REPRESENTATION OF FORECAST PREFERENCES DURING THE  
FIRST HALF OF THE PERIOD. THE FORECAST TRANSITIONED TO A MIX OF  
THE AFOREMENTIONED GFS/ECMWF/CMC RUNS AND 06Z GEFS/00Z ECMWF MEANS  
THEREAFTER.  
 
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
A WAVY FRONT PROGRESSING FROM THE WEST/PLAINS THROUGH THE EAST  
ALONG WITH ANOTHER WAVY FRONT FARTHER NORTH WILL BRING RELATIVELY  
LIGHT PRECIPITATION TO THE MIDWEST, GREAT LAKES, AND NORTHEAST  
LATE THIS WEEK INTO THE WEEKEND. MOISTURE INCREASING AHEAD OF THE  
COLD FRONT EXTENDING INTO THE SOUTHERN PLAINS SHOULD PRODUCE  
SCATTERED SHOWERS AND A FEW THUNDERSTORMS FROM THE DEEP SOUTH TO  
THE OHIO VALLEY OVER THE WEEKEND, BUT AMOUNTS ARE NOT LIKELY TO BE  
EXCESSIVE. A SHORTWAVE AND FRONTAL SYSTEM MAY BRUSH THE EXTREME  
NORTHERN PACIFIC NORTHWEST/ROCKIES WITH SOME PRECIPITATION DURING  
THE WEEKEND. THIS SYSTEM COULD THEN PRODUCE A BAND OF  
PRECIPITATION FROM THE NORTHERN PLAINS THROUGH THE NORTHEAST INTO  
EARLY NEXT WEEK. HOWEVER CONFIDENCE IS LOW FOR EXACT  
TIMING/TRACK/DEPTH OF THE SURFACE LOW, SO IT WILL TAKE ADDITIONAL  
TIME TO RESOLVE PRECIPITATION TYPE AND AMOUNTS ACROSS THIS BROAD  
AREA. A SHORTWAVE AND COLD FRONT REACHING THE PACIFIC NORTHWEST  
BY EARLY NEXT WEEK SHOULD BRING AN EPISODE OF MODERATE TO LOCALLY  
HEAVY RAIN AND MOUNTAIN SNOWS TO WESTERN WASHINGTON AND OREGON.  
EXPECT SOME OF THIS MOISTURE TO REACH FARTHER INLAND THROUGH THE  
WEST, WITH BEST POTENTIAL FOR HEAVY SNOW OVER THE NORTHERN  
ROCKIES--INCLUDING THE HIGHER ELEVATIONS OF NORTHERN IDAHO,  
WESTERN MONTANA, AND WESTERN WYOMING.  
 
TEMPERATURES WILL BE ABOVE NORMAL FOR EARLY DECEMBER ACROSS MOST  
OF THE NATION THROUGH EARLY NEXT WEEK. THE GREATEST POSITIVE  
ANOMALIES SHOULD BE OVER THE CENTRAL AND SOUTHERN PLAINS ON FRIDAY  
WITH HIGHS FORECAST TO BE 15-25F ABOVE AVERAGE. ANOTHER EPISODE  
OF WELL ABOVE NORMAL TEMPERATURES IS LIKELY FOR PARTS OF THE  
PLAINS NEXT MONDAY-TUESDAY AHEAD OF THE NEXT COLD FRONT WITH A  
BROAD AREA OF HIGHS AND MORNING LOWS AT LEAST 10-20F ABOVE NORMAL.  
A FEW LOCATIONS MAY APPROACH DAILY RECORD HIGHS WITH THESE MILD  
TEMPERATURES. THE NORTHEAST U.S. WILL BE AN EXCEPTION TO THE  
GENERALLY WARM THEME AS MEAN TROUGHING ALOFT WILL KEEP  
TEMPERATURES FAIRLY COLD WITH HIGHS RUNNING UP TO 10 DEGREES BELOW  
NORMAL THROUGH THE WEEKEND. HOWEVER, NO MAJOR ARCTIC AIRMASS  
INTRUSIONS ARE EXPECTED DURING THIS FORECAST PERIOD.  
 
RAUSCH/HAMRICK  
 
HAZARDS:  
- HEAVY PRECIPITATION ACROSS PORTIONS OF NORTHWESTERN CALIFORNIA,  
THE PACIFIC NORTHWEST INCLUDING THE CASCADES, AS WELL AS PARTS OF  
THE NORTHERN ROCKIES, SUN-MON, DEC 5-DEC 6.  
- FLOODING OCCURRING OR IMMINENT ACROSS PORTIONS OF NORTHWESTERN  
WASHINGTON STATE.  
- FLOODING LIKELY ACROSS PORTIONS OF NORTHWESTERN WASHINGTON STATE.  
 
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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