824  
FXUS02 KWBC 201856  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
156 PM EST SAT NOV 20 2021  
 
VALID 12Z TUE NOV 23 2021 - 12Z SAT NOV 27 2021  
 
...POTENTIAL HEAVY RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS  
AND LOWER MISSISSIPPI VALLEY MAY IMPACT TRAVEL FOR THE  
THANKSGIVING HOLIDAY...  
   
..OVERVIEW
 
 
THE MEDIUM RANGE PERIOD WILL BEGIN TUESDAY RATHER AMPLIFIED IN THE  
UPPER LEVELS, WITH A TROUGH AXIS NEAR THE WEST COAST, A DEEP  
TROUGH IN THE EAST, AND RIDGING IN BETWEEN. THE EASTERN TROUGH IS  
FORECAST TO DEVELOP A CLOSED UPPER LOW AROUND WEDNESDAY AND SPIN  
OVER THE ATLANTIC ALONG WITH ITS ASSOCIATED SURFACE LOW, LIKELY  
FAR ENOUGH EAST NOT TO IMPACT THE NORTHEAST U.S. SIGNIFICANTLY  
FROM MIDWEEK ONWARD. THE TROUGH IN THE WEST WILL MOVE EASTWARD AS  
THE WEEK PROGRESSES, AND IT IS BECOMING MORE LIKELY IT WILL 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. ON WEDNESDAY INTO THANKSGIVING DAY.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
00Z/06Z MODEL AGREEMENT WAS FAIRLY GOOD WITH THE DEPARTING TROUGH  
ACROSS THE EAST, WITH SOME LINGERING QUESTIONS REGARDING TIMING  
WHEN THE CLOSED MID-UPPER LOW DEVELOPS AND PLACEMENT OF SURFACE  
LOWS. THE BULK OF MODEL GUIDANCE AGREES THAT THESE FEATURES WILL  
MAINLY CAUSE MARINE AND SOUTHEAST CANADA THREATS, WITH THE SYSTEM  
FAR ENOUGH FROM THE NORTHEAST U.S. DURING THE MEDIUM RANGE PERIOD  
NOT TO CAUSE SIGNIFICANT IMPACTS.  
 
IN THE WEST, GUIDANCE CONTINUES TO SHOW PLENTY OF RUN TO RUN  
VARIABILITY REGARDING PLACEMENT AND STRENGTH OF SHORTWAVES MOVING  
THROUGH THE MEAN TROUGH, WHICH LEADS TO DIFFERENCES IN TIMING AND  
PROGRESSION OF THE SEPARATION INTO NORTHERN AND SOUTHERN STREAMS  
DURING THE LATTER HALF OF THE WEEK. THE 00Z/06Z MODEL CYCLE  
GENERALLY FLIPPED BACK TO A SOLUTION WITH MORE SEPARATION OF  
STREAMS, WITH MANY DETERMINISTIC MODELS (THOUGH NOT THE 00Z GFS)  
AND EVEN THE 06Z GEFS MEAN AND 00Z EC MEAN AT TIMES INDICATING A  
SOUTHERN STREAM CLOSED LOW OVER NORTHERN MEXICO THAT LINGERS THERE  
FRIDAY AND SATURDAY. THE 00Z DETERMINISTIC ECMWF SHOWED THIS 500  
MB CLOSED LOW FARTHEST WEST, ALONG WITH A GOOD PORTION OF THE EC  
ENSEMBLE MEMBERS, PUSHING THE 00Z EC ENSEMBLE MEAN TROUGH/LOW  
FARTHER WEST COMPARED TO CMC AND GEFS ENSEMBLE SOLUTIONS THAT ARE  
NOT AS SLOW TO MOVE THE SOUTHERN STREAM TROUGH/LOW. THE STREAM  
SEPARATION DOES CAUSE SENSIBLE WEATHER DIFFERENCES AS WELL--THE  
COLD FRONT AHEAD OF THE NORTHERN STREAM TROUGH COULD MOVE FASTER  
WITH MORE SEPARATION, AND QPF IN THESE SOLUTIONS IS LOWER OVER THE  
SOUTH-CENTRAL U.S., POSSIBLY BECAUSE THE PATTERN HOLDS BACK MORE  
PACIFIC MOISTURE. THE UPDATED WPC FORECAST DID FOLLOW THE TREND  
TOWARD A CLOSED LOW AND ATTEMPTED TO TAKE A MIDDLE GROUND SOLUTION  
IN PLACEMENT BETWEEN THE SLOWER EC SUITE AND THE FASTER GUIDANCE.  
GIVEN THE VARIABILITY BETWEEN MODELS AND FROM RUN TO RUN, THERE IS  
STILL CONSIDERABLE UNCERTAINTY WITH THIS EVOLUTION. CURRENTLY  
INCOMING 12Z GUIDANCE REMAINS SPLIT, WITH THE ECMWF AND GFS STILL  
SHOWING CONSIDERABLE STREAM SEPARATION (WITH THE NEW 12Z EC A BIT  
FASTER THAN THE 00Z RUN AND THE 12Z GFS ALIGNED WITH ITS 06Z RUN),  
WHILE THE CMC SHOWS A MORE PHASED SOLUTION.  
 
OVERALL THE WPC FRONTS/PRESSURES BEGAN WITH A BLEND OF THE  
DETERMINISTIC GUIDANCE EARLY IN THE PERIOD, LESSENING THE  
PROPORTION OF THE SLOW 00Z ECMWF AS THE PERIOD PROGRESSED,  
SPLITTING THE DIFFERENCE WITH THE FASTER 00Z AND SLOWER 06Z GFS,  
WHILE INCREASING THE PROPORTION OF THE 06Z GEFS AND 00Z EC  
ENSEMBLE MEANS.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
THE COLD FRONT ASSOCIATED WITH THE NORTHEAST LOW PRESSURE SHOULD  
BE WELL OFF THE ATLANTIC COAST BY TUESDAY WITH SPRAWLING HIGH  
PRESSURE IN ITS WAKE ACROSS MUCH OF THE EASTERN THIRD OF THE  
NATION. AS A RESULT, DAYTIME HIGHS AND MORNING LOWS TUESDAY AND  
WEDNESDAY COULD BE 10-20 DEGREES BELOW NORMAL FROM PORTIONS OF THE  
TENNESSEE/OHIO VALLEY INTO THE MID-ATLANTIC AND THE SOUTHEAST.  
NEAR TO BELOW FREEZING MORNING LOWS COULD EXTEND RATHER FAR INTO  
PARTS OF THE SOUTH, RESULTING IN THE POTENTIAL FOR THE FIRST FROST  
OR FREEZE FOR SOME LOCATIONS, MOST CERTAINLY IMPACTING SENSITIVE  
VEGETATION. LOW PRESSURE LINGERING OFF THE NORTHEAST COAST MAY  
BRING PERIODS OF BREEZY/GUSTY WINDS TO PARTS OF NEW ENGLAND AND  
COMBINED WITH ALREADY CHILLY TEMPERATURES COULD MAKE FOR SOME  
UNPLEASANT WIND CHILLS DURING AT LEAST THE BEGINNING OF NEXT WEEK.  
 
SOME LIGHT TO PERHAPS LOCALLY MODERATE PRECIPITATION IS EXPECTED  
TO SPREAD ACROSS THE ROCKIES AROUND TUESDAY-WEDNESDAY AS A COLD  
FRONT MOVES ACROSS. THEN BY MIDWEEK, THE COLD FRONT SHOULD REACH  
FAR ENOUGH INTO THE PLAINS TO 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 FOR WEDNESDAY AND ESPECIALLY INTO THANKSGIVING DAY, WITH  
AMOUNTS CURRENTLY EXPECTED TO LESSEN BY FRIDAY AS THE FRONT MOVES  
OFFSHORE. THOUGH THIS UPDATE LOWERED FORECAST RAINFALL AMOUNTS PER  
THE LATEST MODEL CONSENSUS, STAY TUNED FOR FORECAST UPDATES AS  
TOTALS COULD INCREASE AGAIN, AND THE RAIN COULD HAVE IMPACTS FOR A  
COUPLE OF THE BUSIEST TRAVEL DAYS OF THE YEAR. ADDITIONAL  
SHORTWAVE ENERGY INTO THE PACIFIC NORTHWEST MAY ALSO BRING ANOTHER  
ROUND OF MODERATE TO LOCALLY HEAVY COASTAL RAIN AND MOUNTAIN SNOWS  
ON THURSDAY AND FRIDAY, WHICH COULD IMPACT TRAVEL AS WELL.  
 
ABOVE NORMAL TEMPERATURES AHEAD OF THE WESTERN FRONT WILL ACROSS  
THE PLAINS TUESDAY-WEDNESDAY WITH ANOMALIES +10 TO +20F POSSIBLE.  
MUCH OF THE COUNTRY LOOKS TO BE GENERALLY NEAR NORMAL ON  
THANKSGIVING, BEFORE MODESTLY ABOVE NORMAL TEMPERATURES MAY BEGIN  
TO SPREAD ACROSS THE WEST AND HIGH PLAINS BY LATER IN THE WEEK.  
 
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  
 
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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