938  
FXUS02 KWBC 270659  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
159 AM EST SAT NOV 27 2021  
 
VALID 12Z TUE NOV 30 2021 - 12Z SAT DEC 04 2021  
   
..OVERVIEW
 
 
PERSISTENT MOIST INFLOW INTO THE PACIFIC NORTHWEST SHOULD CAUSE  
ADDITIONAL PRECIPITATION THERE, WITH THE HEAVIEST LIKELY TUESDAY  
INTO WEDNESDAY.  
 
MULTIPLE SHORTWAVES WILL PIVOT THROUGH THE MEAN TROUGH THAT WILL  
BE IN PLACE OVER THE CENTRAL AND EASTERN PORTIONS OF THE CONUS  
DURING THE EXTENDED PERIODS; WHICH WILL HELP DEAMPLIFY THE PATTERN  
SOMEWHAT AS NEXT WEEK PROGRESSES. ROUNDS OF LIGHT PRECIPITATION  
ARE POSSIBLE FOR THE GREAT LAKES REGION AND INTERIOR NORTHEAST  
THROUGH THE WEEK. PERSISTENT MOIST INFLOW INTO THE PACIFIC  
NORTHWEST SHOULD CAUSE ADDITIONAL PRECIPITATION THERE, WITH THE  
HEAVIEST LIKELY ON WEDNESDAY.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE 12Z/18Z GUIDANCE HAS A FAIRLY DECENT HANDLE ON THE LARGE SCALE  
PATTERN EVOLUTION WITH THE TROUGHINESS OVER THE EAST AND RIDGING  
OVER THE WEST. HOWEVER, UNCERTAINTIES REMAIN WITH ENERGY COMING  
INTO THE TROUGH IN THE EAST THAT WILL AFFECT POSITIONING AND DEPTH  
OF A SURFACE LOW OFF THE NORTHEAST COAST. ENSEMBLE GUIDANCE  
CONTINUES TO SHOW SPREAD WITH THE CENTRAL PRESSURE AS WELL AS ITS  
POSITION. THE 18Z RUN OF THE GFS HAS TRENDED SOMEWHAT SLOWER AND  
LESS AMPLIFIED THAN THE 12Z RUN WITH THE SHORTWAVES TRAVERSING  
CENTRAL AND EASTERN CANADA/UNITED STATES. BY THURSDAY EVENING, IT  
REVERSES AND THE PATTERN BECOMES MORE PROGRESSIVE ACROSS THE  
NORTHEAST U.S. THERE IS FAIRLY GOOD AGREEMENT FOR POTENT SHORTWAVE  
ENERGY TO MOVE ACROSS THE GREAT LAKES THROUGH WEDNESDAY; HOWEVER  
WITH POTENTIALLY MORE SHORTWAVE ENERGY PASSES THROUGH THE FLOW  
OVER THE GREAT LAKES REGION BY THURSDAY, THE SPREAD IN POSSIBLE  
TRACKS INCREASE AS IT TRACKS NORTHEASTWARD. THE WPC ONCE AGAIN  
ATTEMPTED TO TAKE A MIDDLE GROUND SOLUTION OF THE GFS AND ECMWF  
DETERMINISTIC AND ENSEMBLE MEAN POSITIONS FOR THE LOW.  
 
FOR THE WEST, AT LEAST INITIALLY, MODEL GUIDANCE IS CLUSTERED  
FAIRLY WELL WITH THE RIDGING BUT THIS CHANGES BEYOND WEDNESDAY.  
THESE DIFFERENCES MAY IMPACT THE FORECAST NOT ONLY IN REGARDS TO  
HOW MUCH MOISTURE IS DRAWN ONSHORE BUT ALSO THE 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). THE MODEL SPREAD INCREASES FURTHER  
BY THE MID-TO-LATE PERIODS WHICH DECREASES THE PREDICTABILITY AND  
FORECAST CONFIDENCE. THE EC MEAN AND 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 A COMBINATION OF THE DETERMINISTIC  
00Z ECMWF, OOZ CMC,00Z AND 06Z GFS INITIALLY. AS THE PERIOD  
PROGRESSED, WEIGHTED THE GEFS AND ECWMF 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
 
 
PORTIONS OF THE NORTHERN CASCADES AND THE OLYMPIC PENINSULA WILL  
BEGIN TO HAVE PRECIPITATION RAMPING BACK UP BECOMING MODERATE TO  
HEAVY THROUGH WEDNESDAY FOR THE WESTERN PORTIONS OF WASHINGTON.  
SOME OF THIS WILL LIKELY FALL AS SNOW NEAR THE HIGHEST TERRAIN BUT  
THERE IS UNCERTAINTY IN SNOW LEVELS. 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 ARE EXPECTED TO MOSTLY 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.  
 
CAMPBELL/TATE  
 
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  
 
HAZARDS:  
- HEAVY PRECIPITATION ACROSS PORTIONS OF THE PACIFIC NORTHWEST AND  
THE NORTHERN GREAT BASIN,  
TUE-WED, NOV 30-DEC 1.  
- HEAVY RAIN ACROSS PORTIONS OF THE PACIFIC NORTHWEST, TUE-WED,  
NOV 30-DEC 1.  
- FLOODING OCCURRING OR IMMINENT ACROSS PORTIONS OF THE PACIFIC  
NORTHWEST.  
- FLOODING LIKELY ACROSS PORTIONS OF THE PACIFIC NORTHWEST AND THE  
NORTHERN GREAT BASIN.  
 
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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