480  
FXUS02 KWBC 261909  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
208 PM EST FRI NOV 26 2021  
 
VALID 12Z MON NOV 29 2021 - 12Z FRI DEC 03 2021  
   
..OVERVIEW
 
 
THE INITIAL UPPER-LEVEL PATTERN AS THE MEDIUM RANGE PERIOD BEGINS  
MONDAY WILL CONSIST OF TROUGHING ACROSS THE EAST AND RIDGING OVER  
THE WEST, AND SHORTWAVES MOVING THROUGH THE BROADER FLOW MAY SERVE  
TO DEAMPLIFY THE PATTERN SOMEWHAT AS NEXT WEEK PROGRESSES. A  
SURFACE LOW OFF THE NORTHEAST COAST COULD SPREAD SOME  
PRECIPITATION TO THE REGION MONDAY, WHILE ROUNDS OF LIGHT  
PRECIPITATION ARE POSSIBLE FOR THE GREAT LAKES REGION AND INTERIOR  
NORTHEAST THROUGH THE WEEK. MEANWHILE, PERSISTENT MOIST INFLOW  
INTO THE PACIFIC NORTHWEST SHOULD CAUSE ADDITIONAL PRECIPITATION  
THERE, WITH THE HEAVIEST LIKELY TUESDAY INTO WEDNESDAY.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
MODEL AGREEMENT FOR THE 00Z/06Z CYCLE WAS FAIRLY GOOD REGARDING  
THE LARGE SCALE PATTERN EARLY IN THE PERIOD ON MONDAY, THOUGH  
UNCERTAINTIES IN ENERGY COMING INTO THE TROUGH IN THE EAST WILL  
AFFECT POSITIONING AND DEPTH OF A SURFACE LOW OFF THE NORTHEAST  
COAST. THE GENERAL TREND FOR THIS LOW HAS BEEN SOMEWHAT WEAKER  
COMPARED TO YESTERDAY, BUT ENSEMBLE GUIDANCE CONTINUES TO SHOW  
SPREAD WITH THE CENTRAL PRESSURE AS WELL AS ITS POSITION. THE 00Z  
UKMET CONTINUED TO BE A SLOW OUTLIER COMPARED TO CONSENSUS AND WAS  
NOT FAVORED. OTHER MODELS ARE MORE DIFFICULT TO RULE OUT, BUT THE  
00Z AND ESPECIALLY THE NEW 12Z CMC SHOW A TRACK SOMEWHAT SLOWER  
AND CLOSER TO LAND THAN WHAT WE WANTED TO FAVOR AT THIS TIME. THE  
WPC FORECAST ATTEMPTED TO FOLLOW A MIDDLE GROUND SOLUTION OF THE  
GFS AND ECMWF DETERMINISTIC AND ENSEMBLE MEAN POSITIONS FOR THE  
LOW. BUT UNCERTAINTY IS STILL FAIRLY HIGH WITH THIS FEATURE FOR A  
DAY 3 FORECAST ESPECIALLY AS RELATIVELY SMALL WOBBLES IN THE LOW  
TRACK CLOSER OR FARTHER FROM LAND COULD CAUSE IMPACTFUL  
PRECIPITATION DIFFERENCES, AND NEW 12Z GUIDANCE IS SHIFTING A BIT  
WESTWARD. AFTER THAT, THERE IS FAIRLY GOOD AGREEMENT FOR POTENT  
SHORTWAVE ENERGY TO MOVE ACROSS THE GREAT LAKES TUESDAY-WEDNESDAY,  
BUT WITH MORE VARIABILITY IN ITS EXIT NORTHEASTWARD AS WELL AS  
POTENTIALLY ADDITIONAL ENERGY APPROACHING THAT REGION BY THURSDAY.  
 
FARTHER UPSTREAM, INITIALLY MODEL GUIDANCE IS CLUSTERED FAIRLY  
WELL IN THE WEST, BUT DIFFERENCES INCREASE BY AROUND WEDNESDAY  
WITH MORE RIDGING SHOWN IN THE NORTHWEST IN GFS AND GEFS MEMBERS  
COMPARED TO THE DETERMINISTIC ECMWF AND EC ENSEMBLE MEMBERS AND  
THE MEAN. THIS AFFECTS AMOUNTS OF PRECIPITATION REACHING WESTERN  
WASHINGTON FROM BRITISH COLUMBIA AS WELL AS PRECIPITATION TYPE  
(THE RIDGIER GFS SUITE HAS LESS PRECIPITATION FARTHER SOUTH AND  
STAYING ALL RAIN, WITH THE EC/CMC WETTER AND LOWER SNOW LEVELS).  
EVEN GREATER DIFFERENCES ARISE IN THE LARGE SCALE PATTERN BY  
THURSDAY-FRIDAY IN THE WESTERN AND CENTRAL U.S. AS SHORTWAVES  
EITHER DIG OR PASS THROUGH THE BROADER FLOW TO VARYING DEGREES.  
THE EC MEAN AND TO A LESS EXTENT THE GEFS MEANS END UP SHOWING A  
MEAN TROUGH ACROSS THE CENTRAL U.S. ON FRIDAY, WHICH THE 00Z  
DETERMINISTIC ECMWF AND CMC WERE CLOSER TO COMPARED TO THE 00Z AND  
06Z GFS RUNS THAT KEPT LOW AMPLITUDE TROUGHING OVER THE NORTHWEST  
AND NORTHEAST INSTEAD. UNCERTAINTY IS RATHER HIGH BY THAT POINT  
GIVEN THE SMALL MAGNITUDE OF THE IMPACTFUL SHORTWAVES AND THE  
ENERGY ARISING FROM HIGH LATITUDES AND OVER THE PACIFIC.  
 
THE WPC FORECAST WAS BASED ON BASED ON A BLEND OF THE  
DETERMINISTIC 00Z ECMWF AND CMC AND 00Z AND 06Z GFS EARLY IN THE  
PERIOD. AS THE PERIOD PROGRESSED, WEIGHTED THE GEFS AND EC MEANS  
MORE TO MINIMIZE INDIVIDUAL MODEL DIFFERENCES, AS WELL AS  
LESSENING GFS INFLUENCE AS GFS RUNS WERE NOT ALIGNED WITH THE  
MEANS BY LATER IN THE WEEK.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
THE PACIFIC NORTHWEST, NAMELY THE OLYMPIC PENINSULA AND THE FAR  
NORTHERN CASCADES, SHOULD SEE RELATIVELY LIGHT PRECIPITATION  
CONTINUING THROUGH MONDAY BEFORE ANOTHER INCREASE IN MOISTURE  
BRINGS ANOTHER ROUND OF MODERATE TO LOCALLY HEAVY PRECIPITATION TO  
WESTERN WASHINGTON AROUND TUESDAY-WEDNESDAY, WITH SOME UNCERTAINTY  
IN SNOW LEVELS AT THIS POINT. PORTIONS OF THE NORTHERN ROCKIES ARE  
ALSO EXPECTED TO HAVE LIGHT PRECIPITATION WITH THIS FLOW.  
 
PERIODIC SHORTWAVE ENERGY ACROSS THE NORTH-CENTRAL TO NORTHEASTERN  
U.S. WILL CAUSE ROUNDS OF LIGHT PRECIPITATION CHANCES THROUGH  
THOSE AREAS. FAVORED LAKE ENHANCEMENT AREAS WILL SEE THE BEST  
CHANCE FOR SNOWFALL BECOMING MODERATE, ESPECIALLY DOWNWIND OF LAKE  
ONTARIO, AS NORTHWESTERLY FLOW PASSES ACROSS THE OPEN LAKES.  
ADDITIONALLY, THE LOW OFFSHORE OF THE NORTHEAST COULD CAUSE LIGHT  
WRAPAROUND RAIN OR SNOWFALL THROUGH MONDAY, WITH AMOUNTS DEPENDING  
ON THE LOW TRACK. OTHER THAN THESE TWO HIGHLIGHTED REGIONS, A  
MAJORITY OF THE COUNTRY WILL BE GENERALLY DRY OTHER THAN  
INCREASING MOISTURE FOR TEXAS MIDWEEK ONWARD AND SPREADING INTO  
THE MID-SOUTH BY THURSDAY/FRIDAY.  
 
TEMPERATURES WILL GENERALLY BE SLIGHTLY BELOW NORMAL ACROSS THE  
EASTERN U.S. MONDAY AS THE MEAN TROUGH BECOMES REINFORCED, WITH  
MODERATION BACK TOWARDS NORMAL WEDNESDAY AND THURSDAY. MUCH OF THE  
WESTERN AND CENTRAL STATES WILL MAINTAIN MILD TEMPERATURES THROUGH  
THE EXTENDED FORECAST THANKS TO THE RIDGING OVER THE REGION. A FEW  
LOCATIONS MAY REACH OR EXCEED DAILY MAXIMUM AND MINIMUM  
TEMPERATURE RECORDS, WHERE SOME PLACES COULD REGISTER 20 TO 30  
DEGREES ABOVE SEASONAL AVERAGE. AS THE FLOW FLATTENS DURING THE  
LATE PERIOD, ABOVE NORMAL TEMPERATURES SHOULD ALSO BEGIN EXPANDING  
INTO THE MIDWEST AND SOUTH.  
 
TATE/CAMPBELL  
 
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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