746  
FXUS63 KUNR 250422  
AFDUNR  
 
AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY  
NATIONAL WEATHER SERVICE RAPID CITY SD  
1022 PM MDT THU SEP 24 2020  
   
UPDATE
 
 
ISSUED AT 658 PM MDT THU SEP 24 2020  
 
BASED ON UPSTREAM OBS AND THE EXPECTATION OF ENHANCED PRESSURE  
RISES BEHIND AN APPROACHING COLD FRONT, CONFIDENCE IS INCREASING  
IN A STRONG WIND EVENT TONIGHT FOR MUCH OF THE WRN SD PLAINS. MAIN  
POINT OF CONSIDERATION THIS EVENING HAS BEEN THE POTENTIAL FOR  
NOCTURNAL STABILIZATION TO LIMIT ABILITY FOR WINDS TO MAKE IT TO  
THE SFC, PARTICULARLY GIVEN A LACK OF APPRECIABLE CAA BEHIND THE  
FRONT. NEVERTHELESS, BASED ON FCST SOUNDINGS AND OBSERVED POST-  
FRONTAL PRESSURE RISE OF 5 MB/2 HR AT KMLS, THINK CAA WILL BE  
JUST ENOUGH TO PROMOTE STEEP LAPSE RATES IN THE LOWEST 1-2 KFT  
ABOVE GROUND LAYER, FACILITATING THE TRANSFER OF SOME GUSTS 40+  
KT TO THE SFC. THUS, HAVE INTRODUCED A WIND ADVISORY FOR MUCH OF  
THE AREA THROUGH 10Z, WITH THE EXPECTATION THAT THE WINDS WILL  
DEVELOP FROM NW TO SE ACROSS THE ADVISORY AREA BEGINNING SHORTLY.  
SUSPECT HIGHEST GUSTS WILL BE IN THE "SQUEEZE" ZONE EAST OF THE  
BLACK HILLS, LIKELY BEGINNING CLOSER TO 04-06Z.  
 
EXPIRATION TIME OF 02Z FOR RED FLAG WARNING OVER NERN WY STILL  
LOOKS TO BE ON TRACK. PREFRONTAL SW WINDS ARE GRADUALLY  
DIMINISHING AND SHOULD MOSTLY GIVE WAY TO NOCTURNAL STABILIZATION  
IN THE NEXT HOUR OR SO. AS THE FRONT CROSSES THE AREA LATER THIS  
EVENING, WINDS WILL INCREASE OUT OF THE NW, BUT RH WILL ALSO RISE.  
 

 
   
DISCUSSION
 
(THIS EVENING THROUGH THURSDAY)  
ISSUED AT 146 PM MDT THU SEP 24 2020  
 
WATER VAPOR SHOWS UPPER RIDGE EXTENDING FROM THE DESERT SW INTO  
THE NORTHERN PLAINS, WITH UPPER TROF OVER THE PAC NW INTO THE  
NORTHERN ROCKIES. AT THE SURFACE, LOW IS CENTERED OVER  
NORTHEASTERN MT WITH DRYLINE EXTENDING SOUTHWARD INTO NORTHEASTERN  
WY, AND COLD FRONT THROUGH SOUTH CENTRAL MT. DRY/GUSTY SOUTHWEST  
WINDS ACROSS NORTHEASTERN WY EARLY THIS AFTERNOON WITH EXPANDING RED  
FLAG CONDITIONS.  
 
DRY COLD FRONT ASSOCIATED WITH TROF MOVING THROUGH THE NORTHERN  
ROCKIES WILL CREATE A PERIOD OF GUSTY NORTHWEST WINDS THIS  
EVENING AND OVERNIGHT, WITH WINDS DECREASING ON FRIDAY. HIGHS  
FRIDAY WILL BE A BIT COOLER THAN TODAY.  
 
MEAN UPPER TROF SHIFTS THROUGH THE NORTHERN ROCKIES INTO THE  
NORTHERN PLAINS LATE FRIDAY INTO SATURDAY. STRONG COLD FRONT MOVES  
THROUGH EARLY SATURDAY PRODUCING GUSTY NORTHWEST WINDS AND COOLER  
TEMPS. NOT MUCH MOISTURE TO WORK WITH, BUT THERE COULD BE A FEW  
SHOWERS. GIVEN THE STRENGTH OF WINDS AND COLD ADVECTION, SOME  
WIND HIGHLIGHTS MAY BE NECESSARY ESPECIALLY FOR NORTHWEST SD.  
 
A STRONG JET STREAK WILL DEEPEN THE TROF OVER THE NORTHERN PLAINS  
ON SUNDAY, DRIVING ANOTHER COLD FRONT THROUGH THE AREA. ANOTHER  
PERIOD OF GUSTY NORTHWEST WINDS AND POTENTIAL WIND HIGHLIGHTS CAN  
BE EXPECTED WITH HIGHS IN THE 50S/60S. AGAIN, NOT MUCH MOISTURE  
TO WORK WITH BUT A FEW -SHOWERS EXPECTED, POSSIBLY MIXING WITH  
SNOW AT TIMES IN THE HIGHER ELEVATIONS.  
 
UPPER TROF SETS UP OVER THE UPPER MIDWEST/GREAT LAKES THROUGH THE  
END OF NEXT WEEK, WITH PERIODIC DRY COLD FRONTS AND BOUTS OF GUSTY  
NORTHWEST WINDS AND ELEVATED FIRE WEATHER CONDITIONS. TEMPS LIKELY  
TO BE NEAR AND BELOW CLIMO FOR MOST OF THE PERIOD.  
 

 
   
AVIATION
 
(FOR THE 06Z TAFS THROUGH 06Z FRIDAY NIGHT)  
ISSUED AT 1022 PM MDT THU SEP 24 2020  
 
VFR CONDS SHOULD CONTINUE THROUGH THE TAF PERIOD. LOCALIZED  
REDUCTIONS TO VSBYS DUE TO SMOKE AND HAZE CANNOT BE RULED OUT.  
BREEZY N/NW WINDS BEHIND A COLD FRONT WILL CONTINUE TO OVERSPREAD  
THE AREA FROM NW TO SE THROUGH THE NIGHT. OCCASIONAL GUSTS OVER 40  
KT WILL BE POSSIBLE, ESPECIALLY OVER THE WRN SD PLAINS IMMEDIATELY  
EAST OF THE BLACK HILLS. WINDS AND GUSTS WILL DIMINISH LATE  
TONIGHT INTO THE MORNING.  
 

 
   
FIRE WEATHER
 
 
ISSUED AT 146 PM MDT THU SEP 24 2020  
 
DRY/GUSTY SOUTHWEST WINDS BEHIND A DRY LINE HAVE CREATED RED FLAG  
CONDITIONS OVER NORTHEAST WY THIS AFTERNOON. RED FLAG CONDITIONS  
ARE EXPECTED TO CONTINUE INTO THE EVENING, BEFORE A COLD FRONT  
BRINGS SLIGHTLY HIGHER RELATIVE HUMIDITY BACK INTO THE REGION  
TONIGHT. THERE WILL BE A PERIOD OF GUSTY NORTHWEST WINDS BEHIND  
THE COLD FRONT.  
 
GUSTY NORTHWEST WINDS WILL DEVELOP BOTH SATURDAY AND SUNDAY ACROSS  
THE REGION AS A SERIES OF COLD FRONT MOVES THROUGH. ALTHOUGH A FEW  
SHOWERS WILL BE POSSIBLE, MOISTURE WILL BE LIMITED AND MOST AREAS  
WILL REMAIN DRY. MINIMUM RELATIVE HUMIDITY OF 20-35 PERCENT BOTH  
DAYS COMBINED WITH THE GUSTY WINDS WILL CREATE ELEVATED FIRE  
WEATHER CONDITIONS THROUGH THE WEEKEND.  
 
MUCH OF NEXT WILL BE NEAR OR A BIT COOLER THAN AVERAGE. A SERIES  
OF DRY COLD FRONTS WILL BRING GUSTY NORTHWEST WINDS AND LOW  
RELATIVE HUMIDITY AT TIMES ALONG WITH ELEVATED FIRE WEATHER  
CONDITIONS.  
 

 
   
UNR WATCHES/WARNINGS/ADVISORIES
 
 
SD...WIND ADVISORY UNTIL 4 AM MDT FRIDAY FOR SDZ001-002-012>014-025-  
026-031-032-043-072-073.  
 
WY...NONE.  

 
 

 
 
UPDATE...SHERBURN  
DISCUSSION...JOHNSON  
AVIATION...SHERBURN  
FIRE WEATHER...JOHNSON  
 
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