833  
FXUS02 KWBC 210655  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
155 AM 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 12Z/18Z MODEL RUNS FROM YESTERDAY REMAIN IN FAIRLY GOOD  
AGREEMENT WITH THE LINGERING CLOSED LOW OFF THE NORTHEAST COAST  
WED-FRI. SOME LINGERING UNCERTAINTY REGARDING PLACEMENT OF THE  
UPPER LOW AND SURFACE LOW, BUT THE BULK OF THE MODEL GUIDANCE DOES  
AGREE THAT REGARDLESS, THESE FEATURES WILL MAINLY BE A MARINE AND  
SOUTHEAST CANADA THREAT.  
 
IN THE WEST, THE GUIDANCE AGREES THE MAIN TROUGH OVER THE  
ROCKIES/SOUTHWEST SHOULD SPLIT INTO TWO STREAMS. THE SOUTHERN  
STREAM ENERGY CONTINUES TO TREND WEAKER IN THE MODELS, BUT SHOULD  
LINGER FOR AT LEAST A FEW DAYS NEAR BAJA CALIFORNIA. QUESTIONS  
BEGIN TO ARISE BY LATER IN THE PERIOD REGARDING TIMING OF THE  
ENERGY AS IT EVENTUALLY SLIDES EASTWARD. THE CMC IS THE QUICKEST  
TO BRING THE ENERGY INTO THE SOUTHERN PLAINS NEXT WEEKEND WHILE  
THE LATEST RUNS OF THE GFS/ECMWF HOLD IT BACK THROUGH THE WEEKEND.  
THE ECMWF AND GFS HAVE MORE SUPPORT FROM THE ENSEMBLE MEANS. THE  
NORTHERN STREAM ENERGY SHOWS BETTER AGREEMENT AND RUN TO RUN  
CONSISTENCY. BY NEXT WEEKEND THOUGH, THE CMC IS FASTER TO LIFT A  
NEWLY FORMED CLOSED LOW INTO EASTERN CANADA, LIKELY A RESULT OF  
STRONGER SHORTWAVE ENERGY DROPPING IN BEHIND IT FROM WESTERN  
CANADA. AGAIN, THE ENSEMBLE MEANS ARE CLOSER TO THE GFS AND ECMWF  
SOLUTIONS, BOTH OF WHICH ARE IS GOOD AGREEMENT WITH EACH OTHER.  
 
FINALLY, 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.  
 
OVERALL, THE DETERMINISTIC SOLUTIONS SHOWED GOOD ENOUGH AGREEMENT  
FOR A GENERAL MODEL BLEND DAYS 3-4. AFTER THIS, THE ENSEMBLE MEANS  
WERE USED IN PLACE OF THE CMC FOR THE VARIOUS ISSUES LISTED ABOVE.  
WAS ABLE TO MAINTAIN A 60/40 BLEND OF THE GFS/ECMWF WITH THE  
ENSEMBLE MEANS DAYS 6 AND 7 WHICH SHOULD HELP GIVE SOME ADDED  
DEFINITION TO SYSTEMS NORMALLY WASHED OUT BY THE MEANS. THIS  
APPROACH MAINTAINS GOOD CONTINUITY WITH THE PREVIOUS WPC FORECAST  
AS WELL FOR DAYS 3-6.  
 
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
THE WESTERN U.S. COLD FRONT SHOULD REACH THE PLAINS BY WEDNESDAY  
AND BEGIN INTERACTING WITH GULF MOISTURE TO PRODUCE AN EXPANDING  
AREA OF MODERATE TO POSSIBLY HEAVY RAINFALL FROM EASTERN TEXAS  
INTO PARTS OF THE LOWER MISSISSIPPI VALLEY ON WEDNESDAY AND  
ESPECIALLY INTO THANKSGIVING DAY. 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.  
 
SHORTWAVE ENERGY MAY HELP DIRECT AN ELONGATED PLUME OF MOISTURE  
INTO THE PACIFIC NORTHWEST TO PRODUCED TERRAIN ENHANCED MODERATE  
TO HEAVY RAIN AND MOUNTAIN SNOWS ACROSS EASTERN PORTS OF  
WASHINGTON AND OREGON ON THURSDAY. CONDITIONS REMAIN UNSETTLED  
INTO THE WEEKEND OUT WEST, BUT PRECIPITATION SHOULD WEAKEN IN  
INTENSITY FOR A FEW DAYS. THE NEXT SYSTEM MOVING TOWARDS THE  
REGION SATURDAY INTO SUNDAY COULD BRING ANOTHER PERIOD 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 ACROSS ESPECIALLY THE SOUTHEAST/MID-ATLANTIC  
WHERE MORNING LOWS NEAR OR BELOW FREEZING ARE LIKELY. 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. OUT WEST, TEMPERATURES SHOULD TREND WARMER THROUGH  
THE PERIOD AND EVENTUALLY 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.  
 
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  
 
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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