919  
FXUS63 KFSD 212256  
AFDFSD  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SIOUX FALLS SD  
556 PM CDT THU SEP 21 2017  
   
SHORT TERM
 
(THIS EVENING THROUGH FRIDAY)  
ISSUED AT 326 PM CDT THU SEP 21 2017  
 
LATE THIS AFTERNOON AND EVENING AN ISOLATED SHOWER OR THUNDERSTORM  
WILL BE POSSIBLE OVER PARTS OF SOUTHWEST MN AND NORTHWEST IA ON THE  
NOSE OF THE STRONG LOW LEVEL MOISTURE ADVECTION, BUT A CAPPING  
INVERSION SHOULD EITHER KEEP THE ACTIVITY AT BAY OR VERY ISOLATED.  
 
OVERALL A VERY WARM AND HUMID FORECAST FOR MANY OVER THE NEXT 24  
HOURS. SOUTHERLY FLOW WILL REMAIN STRONG TONIGHT INTO FRIDAY AND NOT  
OUT OF THE QUESTION TO POSSIBLY NEED A WIND ADVISORY ON FRIDAY  
AFTERNOON. SUSPECT THAT WINDS WILL AVERAGE ABOUT 20 TO 25 MPH WITH  
GUSTS OF 35 TO 40 MPH ALONG AND EAST OF THE JAMES RIVER. FOR NOW  
WILL NOT ISSUE BUT IT WILL BE VERY WINDY REGARDLESS. WITH STRONG  
SOUTHERLY FLOW AND VERY HIGH DEW POINT TEMPERATURES FOR THIS TIME OF  
YEAR LOWS TEMPERATURES TONIGHT WILL BE IN THE 70 TO 75 DEGREE RANGE  
IN MOST LOCATIONS, WHICH MIGHT SET SOME WARM RECORD LOWS FOR  
SEPTEMBER 22ND. THERE WILL BE A VERY SMALL CHANCE FOR SHOWERS AND  
THUNDERSTORMS ALONG AND NORTH OF HIGHWAY 14 OVERNIGHT ON THE EDGE OF  
THE CAPPING INVERSION BUT FOR NOW SEE THE BETTER CHANCE NORTH OF  
THERE.  
 
MOST OF THE DAY ON FRIDAY SHOULD REMAIN DRY AS A FAIRLY STRONG  
CAPPING INVERSION WILL BE IN PLACE. HOWEVER NEAR AND JUST WEST OF  
THE JAMES RIVER TEMPERATURES WILL LIKELY HEAT OUT ENOUGH TO ALLOW A  
STORM OR TWO TO DEVELOP AFTER ABOUT 22Z. PLENTY OF SHEAR AND  
INSTABILITY TO SUPPORT SEVERE STORMS AND WITH FAIRLY DRY AIR ALOFT  
WHICH SHOULD SUPPORT ISOLATED SUPERCELLS. FOR NOW WILL LEAN TOWARDS  
THE MAIN THREATS AS GOLF BALL SIZE HAIL AND 70 MPH WIND GUSTS, BUT  
WITH THE POTENTIAL FOR ABOUT 15 TO 20 KNOTS OF SURFACE TO 1 KM SHEAR  
AN ISOLATED TORNADO WILL ALSO BE POSSIBLE.  
   
LONG TERM
 
(FRIDAY NIGHT THROUGH THURSDAY)  
ISSUED AT 326 PM CDT THU SEP 21 2017  
 
BY FRIDAY EVENING THE SURFACE BASED THUNDERSTORM THREAT MAY LIFT  
NORTH OUT OF THE AREA BUT WITH AN INCREASING LOW LEVEL JET  
ADDITIONAL THUNDERSTORMS WILL LIKELY DEVELOP NORTHWARD FROM  
CENTRAL AND NORTH CENTRAL NEBRASKA INTO CENTRAL SD. WHILE A FEW OF  
THESE STORMS COULD BE SEVERE THE MAIN THREATS WILL BE LIGHTNING  
AND HEAVY RAINFALL. ANOTHER NIGHT OF VERY WARM LOWS RANGING FROM  
THE MID 50S IN CENTRAL SD TO THE LOWER 70S IN NORTHWEST IA.  
 
SATURDAY INTO SATURDAY NIGHT, ESPECIALLY SATURDAY NIGHT, WILL LIKELY  
BE THE BEST THREAT FOR HEAVY RAINFALL. THE WARM CLOUD DEPTH SHOULD  
BE AT ITS DEEPEST, CLOSE TO 13000 FEET. THIS IS ALSO THE TIME WHEN  
THE RIGHT ENTRANCE REGION OF THE UPPER LEVEL JET WILL SHIFT INTO THE  
AREA PROVIDING BROAD ASCENT TO THE AREA. THE SEVERE WEATHER THREAT  
WILL BE MINIMIZED BUT THE HEAVY RAIN THREAT WILL BE HIGHER. AT THIS  
TIME LOCATIONS BETWEEN THE JAMES RIVER AND INTERSTATE 29 WILL HAVE  
THE HIGHEST CHANCE FOR HEAVY RAIN ON SATURDAY NIGHT.  
 
THE RIGHT ENTRANCE REGION OF THE UPPER LEVEL JET DOES NOT MOVE MUCH  
AT ALL THROUGH MONDAY MORNING WHICH MEANS THAT THE THREAT FOR  
OCCASIONAL RAIN AND THUNDERSTORMS WILL REMAIN IN PLACE AND THE LOW  
LEVEL BOUNDARY WILL BE SLOW TO SHIFT EAST. WILL LIKELY SEE THE MOST  
WIDESPREAD THREAT FOR RAIN AND THUNDERSTORMS AS WE MOVE INTO SUNDAY  
AFTERNOON AND NIGHT AS THE BROAD FORCING IS IN PLACE, THE BOUNDARY  
IS NEAR AND INSTABILITY IS WEAK, ALLOWING FOR MORE OF A GENERAL RAIN  
WITH ISOLATED THUNDERSTORM THREAT.  
 
MONDAY AFTERNOON INTO MONDAY NIGHT THE UPPER LEVEL JET WILL FINALLY  
START SHIFTING EAST TAKING THE HIGHER CHANCES FOR RAIN WITH IT.  
LOOKS LIKE A WET DAY STILL AS RAIN CHANCES CONTINUE UNTIL LATE  
MONDAY NIGHT.  
 
TUESDAY THROUGH THURSDAY WILL SEE COOL AIR SETTLE IN WITH HIGHS IN  
THE 60S AND LOWS IN THE 40S AND NOT MUCH OF A THREAT FOR RAINFALL.  
TUESDAY MAY EVEN SEE A FEW SITES STRUGGLE TO REACH 60 DEGREES.  
 

 
   
AVIATION
 
(FOR THE 00Z TAFS THROUGH 00Z FRIDAY EVENING)  
ISSUED AT 546 PM CDT THU SEP 21 2017  
 
BREEZY SOUTHERLY WINDS WILL SLACK OFF A BIT OVERNIGHT. WITH THE  
DEVELOPMENT OF A LOW LEVEL JET, LOW LEVEL WIND SHEAR WILL OCCUR AT  
KFSD AND KSUX AFTER 02Z. ON FRIDAY, SOUTHERLY WINDS WILL ONCE  
AGAIN PICK UP, GUSTING AROUND 25 TO 30 KTS THROUGH THE AFTERNOON.  
 

 
   
FSD WATCHES/WARNINGS/ADVISORIES
 
 
SD...NONE.  
MN...NONE.  
IA...NONE.  
NE...NONE.  

 
 

 
 
SHORT TERM...08  
LONG TERM...08  
AVIATION...JM  
 
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