714  
FXUS02 KWBC 200701  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
200 AM 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 AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
AN AMPLIFYING TROUGH OVER THE EAST COAST TO START THE PERIOD ON  
TUESDAY WILL EVENTUALLY CLOSE OFF AN UPPER LOW OFF THE NORTHEAST  
COAST WHICH MAY LINGER FOR A COUPLE OF DAYS BEFORE FINALLY  
WEAKENING AND DRIFTING NORTHWARD. MEANWHILE, A WESTERN U.S. TROUGH  
MAY SPLIT INTO TWO SEPERATE STREAMS WITH SOUTHERN ENERGY DIGGING  
INTO MEXICO/THE SOUTHERN PLAINS AND NORTHERN STREAM ENERGY SLIDING  
ACROSS THE NORTHERN PLAINS AND GREAT LAKES. THIS ENERGY MAY  
EVENTUALLY COMBINE WITH A SHORTWAVE IN CENTRAL CANADA TO FORM A  
POSSIBLE CLOSED LOW OVER EASTERN CANADA NEXT WEEKEND.  
 
MODELS CONTINUE TO SHOW GOOD AGREEMENT ON THE DEPARTING TROUGH  
ACROSS THE EAST WITH LINGERING QUESTIONS MAINLY SURROUNDING HOW  
CLOSE TO THE COAST THE LOW LINGERS (CMC CLOSEST, GFS FARTHEST  
AWAY) OR HOW QUICKLY THE ENERGY PHASES WITH A SHORTWAVE/CLOSED LOW  
INTO EASTERN CANADA LATE PERIOD. THE ENSEMBLE MEANS PROVIDED A  
REASONABLE MIDDLE GROUND FOR THE SECOND HALF OF THE WEEK.  
 
THE GUIDANCE CONTINUES TO SHOW PLENTY OF RUN TO RUN VARIABILITY  
REGARDING SHORTWAVE ENERGY INTO THE WEST COAST ON TUESDAY WHICH  
SHOULD PHASE WITH CLOSED LOW ENERGY SOUTHWEST OF CALIFORNIA. THERE  
IS GOOD AGREEMENT THAT THIS NEW TROUGH SHOULD EVENTUALLY SPLIT  
INTO NORTHERN/SOUTHERN STREAM ENERGY. PREVIOUS RUNS OF THE  
DETERMINISTIC MODELS INDICATED POTENTIAL FOR A SOUTHERN STREAM  
CLOSED UPPER LOW TO FORM AND LINGER OVER NORTHERN MEXICO, BUT THE  
MOST RECENT RUNS HAVE SOMEWHAT BACKED OFF ON THIS SCENARIO NOW  
INDICATING A BIT MORE PROGRESSIVE OF A SYSTEM, BUT STILL IMPACTFUL  
NONETHELESS. THERE ARE STILL A NUMBER OF ENSEMBLE MEMBERS  
INDICATING A CLOSED LOW, SO THE POTENTIAL IS STILL THERE. BY DAYS  
6-7, THERE IS A GREAT DEAL OF UNCERTAINTY REGARDING STRENGTH AND  
TIMING OF THIS SYSTEM BUT THE ENSEMBLE MEANS SEEM TO HANDLE THE  
UNCERTAINTY WELL AND PROVIDED A GOOD STARTING POINT FOR THE  
FORECAST. THE NORTHERN STREAM ENERGY ACROSS THE NORTHERN PLAINS  
INTO THE GREAT LAKES IS HANDLED RELATIVELY WELL BY THE  
DETERMINISTIC SOLUTIONS THE FIRST HALF OF THE PERIOD. THEREAFTER,  
SOME MINOR TIMING DIFFERENCES BEGIN TO EMERGE, BUT NOTHING OUT OF  
THE ORDINARY FOR THIS TIME RANGE.  
 
THE WPC PROGS USED A MAJORITY BLEND OF THE DETERMINISTIC SOLUTIONS  
FOR DAYS 3-5, WITH MORE EMPHASIS TOWARDS THE ENSMELBE MEANS DAYS  
6-7 TO HELP MITIGATE THE FORECAST UNCERTAINTIES. THIS APPROACH  
MAINTAINS GOOD CONTINUITY WITH THE PREVIOUS WPC FORECAST FOR DAYS  
3-6.  
   
..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 NORTHEAT COAST MAY  
BRING PERIODS OF BREEZY/GUSTY WINDS TO PARTS OF NEW ENGLAND AND  
COMBINED WITH ALREADY CHILLY TEMPERATURES COULE MAKE FOR SOME  
UNPLEASANT WIND CHILLS DURING AT LEAST THE BEGINNING OF NEXT WEEK.  
 
THE PACIFIC NORTHWEST INTO NORTHERN ROCKIES SHOULD SEE MOSTLY  
LIGHT TO PERHAPS LOCALLY MODERATE PRECIPITATION DURING THE FIRST  
HALF OF NEXT WEEK WITH A SHORTWAVE/COLD FRONT ARRIVING FROM THE  
PACIFIC. THE COMBINATION OF THIS ENERGY ALOFT AND THE  
OPENING/PROGRESSION OF THE UPPER LOW SOUTHWEST OF CALIFORNIA MAY  
SPREAD SOME MOISTURE INTO OTHER PARTS OF THE WEST BY  
TUESDAY/WEDNESDAY, BUT STILL WITH MODEST CONFIDENCE FOR  
PRECIPITATION COVERAGE/AMOUNTS. BY MID WEEK, THE ASSOCIATED 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, BEFORE SHIFTING FOCUS TO THE CENTRAL GULF COAST ON FRIDAY.  
SOME MODELS SHOW THE POTENTIAL FOR SEVERAL INCHES OF RAIN.  
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. FOR BOTH REGIONS, THE SOUTH  
AND PACIFIC NORTHWEST, THIS RAINFALL COULD IMPACT TRAVEL FOR THE  
BUSY THANKSGIVING HOLIDAY.  
 
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 THURSDAY,  
THOUGH MODESTLY ABOVE NORMAL TEMPERATURES MAY BEGIN TO SPREAD  
ACROSS THE WEST BY LATER IN THE WEEK.  
 
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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