090  
FXUS02 KWBC 221711  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1210 PM EST MON NOV 22 2021  
 
VALID 12Z THU NOV 25 2021 - 12Z MON NOV 29 2021  
 
...POTENTIAL HEAVY RAINFALL FOR PORTIONS OF EASTERN TEXAS AND THE  
PACIFIC NORTHWEST MAY IMPACT TRAVEL FOR THE THANKSGIVING HOLIDAY...  
 
17Z UPDATE: THE DETERMINISTIC GUIDANCE IS IN ABOVE AVERAGE  
AGREEMENT THROUGH THE END OF THE WEEK ON THE SYNOPTIC SCALE  
PATTERN, WITH THE MAIN DIFFERENCE BEING A SLIGHTLY MORE AMPLIFIED  
CMC SOLUTION WITH THE BUILDING TROUGH OVER THE MIDWEST AND OHIO  
VALLEY, AND A STRONGER UPSTREAM RIDGE OVER THE NORTHERN PLAINS.  
EXCEPT FOR THE EAST COAST REGION WHERE THE UKMET IS STRONGER WITH  
THE OFFSHORE UPPER LOW, THIS MODEL IS ALSO IN GOOD AGREEMENT  
ACROSS MOST OF THE NATION. BY THE WEEKEND, THE CMC IS STRONGER  
WITH A NORTHERN STREAM SHORTWAVE TRACKING TOWARDS THE GREAT LAKES,  
AND THIS STRONGER SOLUTION DOES NOT HAVE AS MUCH ENSEMBLE SUPPORT  
AS THE GFS AND ECMWF. ONE OF THE MAIN CHANGES FOR THIS FORECAST  
CYCLE WAS INCREASED QPF DOWNWIND OF THE GREAT LAKES GIVEN GREATER  
PROSPECTS FOR LAKE EFFECT SNOW OWING TO STRONG COLD AIR ADVECTION  
IN PLACE. THE WPC FORECAST WAS PRIMARILY DERIVED FROM A  
GFS/ECMWF/UKMET/ECENS BLEND THROUGH FRIDAY, AND THEN MAINLY  
GFS/ECMWF AND THEIR RESPECTIVE MEANS FOR SATURDAY THROUGH MONDAY.  
THE PREVIOUS FORECAST DISCUSSION IS APPENDED BELOW FOR REFERENCE.  
/HAMRICK  
 
   
..OVERVIEW
 
 
AMPLIFIED AND ELONGATED TROUGHING ACROSS THE CENTRAL U.S. SHOULD  
BE SPLITTING INTO TWO SEPARATE STREAMS AS THE MEDIUM RANGE PERIOD  
BEGINS. THE SOUTHERN ENERGY SHOULD DROP SOUTHWARD AND LINGER  
OFF/OVER BAJA CALIFORNIA THROUGH NEXT WEEKEND WHILE THE NORTHERN  
ENERGY CLOSES OFF A LOW OVER THE GREAT LAKES AND SHIFTS INTO  
EASTERN CANADA. ADDITIONAL ENERGY FROM CANADA SHOULD ACT TO RELOAD  
AND AMPLIFY THE TROUGH OVER THE EASTERN U.S. BY EARLY NEXT WEEK AS  
UPPER LEVEL RIDGING BUILDS ACROSS THE NORTHWEST/ROCKIES THE SECOND  
HALF OF THE PERIOD.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
A CLOSED LOW OFF THE NORTHEAST COAST MAY HANG AROUND FOR A DAY OR  
TWO BEFORE IT WEAKENS AND THE ENERGY BECOMES ABSORBED BY NORTHERN  
STREAM ENERGY FROM THE WEST. WITH THIS, THE LATEST SUITE OF  
GUIDANCE SHOWS REASONABLE AGREEMENT IN THE LARGE SCALE AS A LOW  
CLOSES OFF OVER THE GREAT LAKES, WITH ONLY SOME MINOR  
TIMING/PLACEMENT DIFFERENCES. THERE'S MORE UNCERTAINTY WITH  
SECONDARY ENERGY DIVING INTO THE WESTERN FRINGE OF THE TROUGH (CMC  
MUCH STRONGER THAN THE GFS/ECMWF) AND THEN EVOLUTION AND DETAIL  
DIFFERENCES AS THE TROUGH RE-AMPLIFIES OVER THE EAST DAY 6-7. THE  
MODELS ACTUALLY SHOW REALLY GOOD AGREEMENT WITH THE SOUTHERN  
STREAM CLOSED LOW OVER BAJA CALIFORNIA WHICH MAY TRY TO INCH INTO  
THE SOUTHWEST BY NEXT MONDAY. ELSEWHERE, THERE ARE SOME  
DIFFERENCES IN THE EAST PACIFIC WITH THE NEXT TROUGH TOWARDS THE  
WEST COAST WITH THE CMC BEGINNING TO GET OUT OF PHASE WITH THE  
GFS/ECMWF AND ENSEMBLE MEANS.  
 
WPC USED A BLEND OF THE DETERMINISTIC GUIDANCE (MORE WEIGHTING  
TOWARDS THE GFS AND ECMWF) FOR DAYS 3-5, WITH INCREASING  
CONTRIBUTIONS FROM THE AGREEABLE ENSEMBLE MEANS BY LATE PERIOD  
WHICH SHOULD HELP MITIGATE THE DIFFERENCES DISCUSSED ABOVE. THIS  
COMBINATION HELPS MAINTAIN GOOD CONTINUITY WITH THE PREVIOUS WPC  
FORECAST FOR DAYS 3-6.  
 
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
MODERATE RAINFALL, POSSIBLY HEAVY AT TIMES, IS EXPECTED OVER PARTS  
EASTERN TEXAS INTO THANKSGIVING DAY. THE GUIDANCE STILL SHOWS THE  
BEST CHANCE FOR HEAVY RAINFALL MAY BE ALONG THE WESTERN GULF  
COAST/FAR SOUTH TEXAS WHERE INSTABILITY SHOULD BE GREATEST AND AT  
LEAST A LOCALIZED FLOOD THREAT MAY DEVELOP. 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. LESS INTENSE AND MORE SCATTERED  
RAIN MAY EXTEND NORTHWARD ALONG THE COLD FRONT INTO PARTS OF THE  
LOWER MISSISSIPPI, TENNESSEE AND OHIO VALLEYS THURSDAY AND  
EVENTUALLY THE MID-ATLANTIC/NORTHEAST ON FRIDAY.  
 
SHORTWAVE ENERGY SHOULD 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 NEXT WEEKEND.  
 
ONE DAY OF NEAR NORMAL TEMPERATURES ARE EXPECTED OVER THE EAST,  
BUT AFTER THANKSGIVING TEMPERATURES SHOULD ONCE AGAIN TREND MUCH  
COOLER FROM THE MIDWEST INTO THE EAST UNDERNEATH OF AMPLIFIED  
UPPER TROUGHING. PERIODS OF 10-15 BELOW NORMAL ARE POSSIBLE FROM  
THE GULF COAST TO THE MIDWEST/OHIO VALLEY ON FRIDAY, WITH  
GENERALLY MINUS 5-10 DEGREES INTO THE EAST NEXT WEEKEND.  
MEANWHILE, AMPLIFIED TROUGHING INTO THE WEST WILL PROMOTE A  
WARMING TREND ACROSS THE WEST AND INTO THE PLAINS THROUGH THE  
PERIOD. BY NEXT SUNDAY AND MONDAY, DAYTIME HIGHS AND MORNING LOWS  
ACROSS THE NORTHERN/CENTRAL HIGH PLAINS COULD BE 10 TO 20 DEGREES  
ABOVE NORMAL.  
 
SANTORELLI/HAMRICK  
 
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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