288  
FXUS02 KWBC 252012  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
312 PM EST THU NOV 25 2021  
 
VALID 12Z SUN NOV 28 2021 - 12Z THU DEC 02 2021  
   
..OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
MEAN TROUGHING IN THE EAST WILL BE REINFORCED SEVERAL TIMES BY  
SHORTWAVES AND MAINTAINED THROUGHOUT MUCH OF THE MEDIUM RANGE  
PERIOD (SUNDAY-THURSDAY). ELSEWHERE, AN AMPLIFIED RIDGE ACROSS THE  
WEST SHOULD WEAKEN AS A COUPLE OF SHORTWAVES MOVE THROUGH AND INTO  
THE CENTRAL U.S., WHICH MAY ALSO HELP TO FLATTEN THE OVERALL UPPER  
PATTERN SOME BY MID TO LATER NEXT WEEK.  
 
THE GUIDANCE CONTINUES TO SHOW REASONABLY GOOD AGREEMENT FOR THIS  
LARGE SCALE FLOW PATTERN BUT OFFERS PLENTY OF UNCERTAINTY STILL IN  
THE DETAILS. ENERGY RELOADING THROUGH THE WESTERN SIDE OF THE  
TROUGH IN THE EAST ON SUNDAY-MONDAY CONTINUES TO SHOW SOME SLIGHT  
TIMING AND MAGNITUDE DIFFERENCES, WHICH WOULD AFFECT A SURFACE LOW  
DEEPENING OFF THE NORTHEAST COAST. THE 00Z/06Z MODEL CYCLE TRENDED  
A BIT EASTWARD WITH THE SURFACE LOW AWAY FROM THE COAST COMPARED  
TO THE PREVIOUS MODEL CYCLE AND WPC FORECAST, WHICH WAS GENERALLY  
MAINTAINED WITH THE INCOMING 12Z MODELS. GUIDANCE WAS FAIRLY WELL  
CLUSTERED WITH THE LOW POSITION WITH THE MAIN EXCEPTION OF THE  
UKMET, WHICH HAS BEEN SLOWER AND FARTHER SOUTH. SHORTWAVES MOVING  
THROUGH THE TROUGH IN THE EAST AND THEIR POTENTIAL SPINUPS INTO  
CLOSED LOWS WITHIN THE MEAN TROUGH, AS WELL AS THE FATE OF  
SHORTWAVES TRACKING THROUGH THE EASTERN PACIFIC, BECOME LESS  
AGREEABLE IN DETERMINISTIC GUIDANCE AS THE PERIOD PROGRESSES FROM  
TUESDAY ONWARD, WITH NO PARTICULARLY CLEAR SOLUTION AT THIS POINT.  
THOUGH ONE SEEMINGLY CLEAR OUTLIER WAS THE 00Z CMC, WHICH SHOWED A  
STRONG SHORTWAVE MOVING INTO THE WEST DIVING SOUTH TO MEET UP WITH  
ENERGY OVER NORTHERN MEXICO TO CREATE A CLOSED LOW DIFFERENTLY  
THAN CONSENSUS.  
 
THUS, THE WPC FORECAST BEGAN WITH A BLEND OF THE 00Z/06Z GFS,  
ECMWF, AND CMC, GRADUALLY TRANSITIONING TO A BLEND THAT FAVORED  
THE GEFS AND EC ENSEMBLE MEANS (ALONG WITH SOME COMPONENT OF THE  
EC AND GFS) BY AROUND MIDWEEK TO MINIMIZE INDIVIDUAL MODEL  
DIFFERENCES. OVERALL THIS MAINTAINED GOOD CONTINUITY WITH THE  
PREVIOUS FORECAST, WITH THE MAIN CHANGE THE AFOREMENTIONED SHIFT  
OF THE ATLANTIC LOW AND ITS ASSOCIATED PRECIPITATION A BIT  
EASTWARD FOR THIS CYCLE, BUT WILL CONTINUE TO BE MONITORED.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
THE ATMOSPHERIC RIVER INTO THE PACIFIC NORTHWEST SHOULD BE  
WEAKENING BY SUNDAY, THOUGH SOME LINGERING MODERATE TO LOCALLY  
HEAVY RAINFALL WILL REMAIN POSSIBLE ACROSS THE OLYMPIC PENINSULA  
AND FAR NORTHERN CASCADES. RELATIVELY LIGHT PRECIPITATION IS  
LIKELY TO CONTINUE THERE THROUGH MONDAY BEFORE ANOTHER INCREASE IN  
MOISTURE BRINGS ANOTHER ROUND OF MODERATE TO LOCALLY HEAVY  
PRECIPITATION TO WESTERN WASHINGTON AROUND TUESDAY-WEDNESDAY,  
WHICH MAY BE MOSTLY RAIN BUT WITH SOME UNCERTAINTY IN SNOW LEVELS  
AT THIS POINT. LIGHT PRECIPITATION SHOULD SPILL INTO THE NORTHERN  
ROCKIES AS WELL. NORTHWESTERLY FLOW ACROSS THE GREAT LAKES SHOULD  
CONTINUE TO SUPPORT PERIODIC LAKE EFFECT/ENHANCED SNOWFALL WHILE  
UPPER LEVEL-ENERGY BRINGS WAVES OF LIGHT PRECIPITATION ACROSS THE  
MIDWEST TO INTERIOR NORTHEAST. DOWNWIND OF LAKE ERIE IS EXPECTED  
TO BE A FOCUS FOR LAKE EFFECT SNOW SUNDAY-MONDAY WHILE LAKE  
ONTARIO COULD BE A FOCUS LATER IN THE WEEK. GIVEN THE EASTERN  
TREND OF THE ATLANTIC LOW FOR THIS MODEL CYCLE, LESS PRECIPITATION  
IS SHOWN FOR NEW ENGLAND EARLY IN THE WEEK, BUT LIGHT RAIN AND  
SNOW COULD STILL WRAP AROUND THE LOW ONSHORE. ELSEWHERE, THE  
CONTIGUOUS U.S. SHOULD REMAIN MOSTLY DRY, OTHER THAN SOME EARLY  
WEEK SHOWERS FOR FLORIDA AND INCREASING MOISTURE FOR TEXAS MIDWEEK  
ONWARD.  
 
THE MEAN TROUGH OVER THE EAST SHOULD KEEP TEMPERATURES SLIGHTLY  
BELOW NORMAL INTO TUESDAY WITH SOME MODERATION BACK TOWARDS NORMAL  
WEDNESDAY AND THURSDAY. MEANWHILE, UPPER LEVEL RIDGING KEEPS  
CONDITIONS WARM THROUGHOUT THE WEST AND CENTRAL U.S. THE ENTIRE  
PERIOD, WITH A HANDFUL OF BOTH RECORD HIGHS AND WARM OVERNIGHT  
LOWS POSSIBLE. THE GREATEST ANOMALIES SHOULD BE ACROSS THE  
NORTHERN-CENTRAL PLAINS WHERE DAYTIME HIGHS COULD BE 20 TO 30  
DEGREES ABOVE NORMAL. AS THE FLOW FLATTENS DURING THE LATE PERIOD,  
ABOVE NORMAL TEMPERATURES SHOULD ALSO BEGIN EXPANDING INTO THE  
MIDWEST AND SOUTH.  
 
TATE/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  
 
HAZARDS:  
- HEAVY RAIN ACROSS PORTIONS OF THE PACIFIC NORTHWEST AND THE  
NORTHERN GREAT BASIN, SUN, NOV 28 AND  
TUE, NOV 30.  
- FLOODING POSSIBLE ACROSS PORTIONS OF THE PACIFIC NORTHWEST.  
 
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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