452  
FXUS02 KWBC 211854  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
154 PM EST SUN NOV 21 2021  
 
VALID 12Z WED NOV 24 2021 - 12Z SUN NOV 28 2021  
 
...POTENTIAL HEAVY RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS,  
LOWER MISSISSIPPI VALLEY, AND PACIFIC NORTHWEST MAY IMPACT TRAVEL  
FOR THE THANKSGIVING HOLIDAY...  
   
..OVERVIEW
 
 
AN AMPLIFIED PATTERN SHOULD PREVAIL THROUGH MUCH OF THE MEDIUM  
RANGE PERIOD (WED-SUN) BEGINNING WITH A TROUGH AXIS MOVING THROUGH  
THE INTERIOR WEST, A CLOSED LOW OFF THE NORTHEAST COAST, AND  
RIDGING IN BETWEEN. THE DEVELOPING CLOSED LOW OFF THE NORTHEAST  
COAST MAY LINGER FOR A FEW DAYS BUT LIKELY REMAINING FAR ENOUGH  
EAST NOT TO IMPACT THE NORTHEAST U.S. SIGNIFICANTLY. OUT WEST,  
GUIDANCE CONTINUES TO ADVERTISE THE TROUGH WILL LIKELY SPLIT INTO  
TWO SEPARATE STREAMS, WITH SOUTHERN ENERGY DIGGING INTO MEXICO AND  
NORTHERN STREAM ENERGY SLIDING ACROSS THE NORTHERN PLAINS AND  
GREAT LAKES. THE LATTER MAY EVENTUALLY COMBINE WITH A SHORTWAVE IN  
CENTRAL CANADA TO FORM A CLOSED LOW OVER EASTERN CANADA NEXT  
WEEKEND. THE SOUTHERN STREAM ENERGY AND MOISTURE FLOWING INTO A  
FRONTAL SYSTEM COULD PRODUCE LOCALLY HEAVY RAIN ACROSS THE  
SOUTH-CENTRAL U.S. DURING THE LATE WEEK.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE 00Z/06Z GUIDANCE CONTINUES TO SHOW THE LEADING TROUGH/CLOSED  
LOW LINGER NEAR THE NORTHEAST BEFORE PULLING AWAY FROM THE COAST  
FURTHER OUT INTO THE ATLANTIC OCEAN. THIS FEATURE WILL MOSTLY  
IMPACT THE CANADIAN MARITIME REGION. MEANWHILE ANOTHER LONGWAVE  
TROUGH MOVES INTO THE CENTRAL CONUS AND TRANSITIONS TO A SPLIT  
FLOW PATTERN. THE NORTHERN STREAM IS EXPECTED TO CONTINUE TO TRACK  
ACROSS THE MISSISSIPPI VALLEY TO THE MID-ATLANTIC AND NORTHEAST  
REGIONS WHILE THE SOUTHERN STREAM GETS HUNG UP NEAR THE BAJA  
PENINSULA FOR A FEW DAYS. THE SOUTHERN STREAM ENERGY CONTINUES TO  
TREND WEAKER IN THE MODELS HOWEVER THIS LEADS TO UNCERTAINTY ON  
WHEN ENERGY SLIDES SOUTHEAST INTO THE SOUTHERN PLAINS. THE CMC IS  
THE QUICKEST TO BRING THE ENERGY INTO THE SOUTHERN PLAINS NEXT  
WEEKEND AND IS QUITE GENEROUS WITH THE ASSOCIATED QPF. THE  
GFS/ECMWF HAVE SLOWER TIMING, BUT ALSO DIFFERING PLACEMENTS OF THE  
MAXIMUM QPF. AS NOTED ON THE LAST CYCLE, THE ENSEMBLE MEANS  
CONTINUE TO SUPPORT THE PARENT ECMWF/GFS SOLUTIONS. THE NORTHERN  
STREAM HAS HAD PERSISTENT, RUN-TO-RUN CONSISTENCY AND CLUSTERING  
WITH THE EVOLUTION AS THE LOW CLOSES OFF NEAR EASTERN CANADA,  
LIKELY A RESULT OF STRONGER SHORTWAVE ENERGY DROPPING IN BEHIND IT  
FROM WESTERN CANADA. THE NEXT SHORTWAVE SHOULD BEGIN IMPACTING THE  
WEST BY SATURDAY-SUNDAY. THE CMC IS MUCH SLOWER WITH THE SHORTWAVE  
AS IT DROPS SOUTHWARD FROM ALASKA LATE THIS WEEK AND QUICKLY  
BECOMES OUT OF PHASE WITH THE GFS, ECMWF, AND THE ENSEMBLE MEANS  
SHOWING STRONG RIDGING OVER THE WEST COAST ON DAY 7 RATHER THAN  
TROUGHING LIKE THE BETTER CONSENSUS WOULD SUGGEST.  
 
THE WPC MODEL BLEND CONSISTED OF 00Z CMC/ECWMF/00Z AND 06Z GFS AND  
THE THEIR ENSEMBLE MEANS THROUGH THE EXTENDED PERIOD - WEIGHTING  
OF THE MEANS INCREASED THROUGH THE MIDDLE AND LATER PERIODS. THIS  
COMBINATION HELPS MAINTAINS CONTINUITY WITH THE PREVIOUS WPC  
FORECAST.  
 
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
MODERATE RAINFALL, POSSIBLY HEAVY AT TIMES, IS EXPECTED OVER PARTS  
EASTERN TEXAS INTO THE LOWER MISSISSIPPI VALLEY ON WEDNESDAY AND  
INTO THANKSGIVING DAY AS A COLD FRONT PASSING THROUGH THE SOUTHERN  
PLAINS. THE BEST CHANCE FOR HEAVY RAINFALL MAY BE THURSDAY ALONG  
THE WESTERN GULF COAST WHERE INSTABILITY SHOULD BE GREATEST AND AT  
LEAST A LOCALIZED FLOOD THREAT MAY DEVELOP. LESS INTENSE AND MORE  
SCATTERED RAIN MAY EXTEND NORTHWARD ALONG THE COLD FRONT INTO  
PARTS OF THE OHIO VALLEY WEDNESDAY-THURSDAY AND EVENTUALLY THE  
MID-ATLANTIC/NORTHEAST ON FRIDAY. THE DEGREE OF RISK FOR EXCESSIVE  
RAINFALL WILL BE DEPENDENT ON THE HOW QUICKLY THE COLD FRONT  
PROGRESSES; IF IT SLOWS OR STALLS THE RISK FOR LOCAL FLOODING  
CONCERNS WILL ELEVATE.  
 
SHORTWAVE ENERGY MAY HELP DIRECT AN ELONGATED PLUME OF MOISTURE  
INTO THE PACIFIC NORTHWEST TO PRODUCE TERRAIN-ENHANCED, MODERATE  
TO HEAVY RAIN AND MOUNTAIN SNOWS ACROSS EASTERN PORTIONS OF  
WASHINGTON AND OREGON ON THANKSGIVING DAY. UNSETTLED CONDITIONS  
WILL PERSIST ACROSS THE WEST THROUGH THE WEEKEND, WITH THE  
PRECIPITATION WEAKENING IN INTENSITY FOR A FEW DAYS. THE SYSTEM TO  
FOLLOW HAS THE POSSIBILITY OF USHERING IN ANOTHER ROUND OF LOCALLY  
HEAVY RAINS TO THE PACIFIC NORTHWEST.  
 
FOR TEMPERATURES ACROSS THE CONUS, SPRAWLING HIGH PRESSURE IN THE  
EAST ON WEDNESDAY SHOULD CONTINUE WELL BELOW NORMAL DAYTIME HIGHS  
AND MORNING LOWS, PARTICULARLY FOR LOCATIONS IN THE SOUTHEAST AND  
MID-ATLANTIC STATES. DAILY LOW READING ARE EXPECTED TO BE NEAR OR  
BELOW FREEZING. TEMPERATURES MAY BRIEFLY WARM BACK TOWARDS NORMAL  
ON THURSDAY AND FRIDAY BUT COULD TREND CHILLIER AGAIN NEXT WEEKEND  
AS THE NEXT COLD FRONT MOVES THROUGH. THROUGH THE EXTENDED  
FORECAST, LOCATIONS ACROSS THE WEST ARE EXPECTED TO TREND WARMER,  
WHICH WILL SPILL OVER INTO THE PLAINS NEXT WEEKEND. DAYTIME HIGHS  
FOR THE NORTHERN AND CENTRAL HIGH PLAINS COULD BE 10 TO 20 DEGREES  
ABOVE NORMAL BY NEXT SUNDAY.  
 
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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