369  
FXUS63 KFSD 222241  
AFDFSD  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SIOUX FALLS SD  
541 PM CDT FRI SEP 22 2017  
   
SHORT TERM
 
(THIS EVENING THROUGH TONIGHT) ISSUED AT 322 PM CDT  
FRI SEP 22 2017  
 
THE NORTHERN PLAINS CONTINUES TO SIT UNDER STRONG SOUTHWESTERLY FLOW  
ALOFT, BETWEEN THE UPPER LEVEL TROUGH OVER THE WESTERN U.S., AND A  
LARGE SCALE RIDGE OVER THE EASTERN STATES. AT THE SURFACE, THIS HAS  
ALLOWED WARM MOIST AIR TO CONTINUE STREAMING NORTHWARD, BRINGING  
GUSTY WINDS AND NEAR RECORD HEAT TO THE AREA. THOSE WITH OUTDOOR  
PLANS THIS AFTERNOON WILL WANT TO STAY HYDRATED AND TAKE STEPS TO  
AVOID HEAT EXHAUSTION IN THE SUMMER-LIKE HEAT.  
 
DESPITE BOUNTIFUL CAPE THIS AFTERNOON, A STRONG CAPPING INVERSION  
HAS SUPPRESSED CONVECTION OVER THE AREA. IN FACT, EVEN CUMULUS  
CLOUDS HAVE BEEN SCARCE DUE TO THE STRENGTH OF THE 700 TO 800 HPA  
WARM LAYER. MODELS SHOW A SHORTWAVE TROUGH MOVING NORTHWARD THIS  
EVENING WHICH WILL LIKELY BE ABLE TO ERODE THE CAP SUFFICIENTLY AND  
PROVIDE LIFT TO ALLOW THUNDERSTORM FORMATION OVER SOUTHEAST SOUTH  
DAKOTA. THESE STORMS ARE EXPECTED TO THEN MOVE NORTHEASTWARD,  
STAYING ROUGHLY NORTH AND WEST OF A YANKTON, SD TO SIOUX FALLS TO  
MARSHALL, MN LINE. THE TIME PERIOD WITH GREATEST LIKELIHOOD FOR  
THUNDERSTORMS SHOULD BE 6-11PM CDT. THE THREAT FOR SEVERE STORMS IS  
ONLY MARGINAL, WITH LIGHTNING AND STRONG OUTFLOW WINDS BEING THE  
MORE WIDESPREAD THREATS. FURTHER TO THE SOUTHEAST IN NW IOWA, THE  
AIR MASS WILL LIKELY REMAIN CAPPED, AND STORMS ARE NOT EXPECTED  
HERE.  
   
LONG TERM
 
(SATURDAY THROUGH FRIDAY) ISSUED AT 322 PM CDT FRI  
SEP 22 2017  
 
THE WEEKEND CONTINUES TO LOOK VERY WET, WITH MANY LOCATIONS LIKELY  
SEEING ONE TO TWO INCHES OF RAIN OVER A 72-HOUR PERIOD AS A QUASI-  
STATIONARY BOUNDARY SETS UP OVER THE REGION. AN AIR MASS WITH 1.5  
INCH PLUS PRECIPITABLE WATER VALUES WILL CONTINUE TO FUEL SHOWERS  
AND SCATTERED THUNDERSTORMS ALONG THIS BOUNDARY AS THE LARGE SCALE  
UPPER LEVEL TROUGH SLOWLY PUSHES EAST OVER THE WEEKEND. WEST OF I-  
29, THE HEAVIEST RAIN LOOKS TO MOVE THROUGH SATURDAY AFTERNOON AND  
EVENING. THE SEVERE THREAT IS ONLY MARGINAL SATURDAY, AS THE  
STRONGEST SHEAR IS EXPECTED TO REMAIN WEST OF THE SURFACE BOUNDARY  
DURING THE AFTERNOON, AND LAPSE RATES ARE ONLY MARGINAL, AT ABOUT  
7 C/KM. FOR LOCATIONS EAST OF I-29, SUNDAY AFTERNOON AND EVENING  
LOOK MOST FAVORABLE FOR THE HEAVIEST RAIN. TEMPERATURES SATURDAY  
AND SUNDAY WILL NOT BE AS WARM AS ON FRIDAY, AS ABUNDANT CLOUD  
COVER LIMITS HEATING AND COOLER AIR SLOWLY PUSHES IN FROM THE  
NORTHWEST. THE BOUNDARY AND FOCUS FOR RAIN MOVE EAST INTO  
SOUTHWEST MN AND NORTHWEST IA ON MONDAY, WITH RAIN CHANCES  
DECREASING FOR EASTERN SD.  
 
BY THE PM HOURS TUESDAY, NORTHWEST SURFACE FLOW IN THE WAKE OF THE  
FRONT IS FORECAST TO DRY THE BOUNDARY LAYER AND BRING AN END TO  
PRECIPITATION. A LARGE SCALE PATTERN CHANGE WITH THE SHIFTING EAST  
OF THE UPPER LEVEL LONGWAVE TROUGH WILL BRING QUIETER WEATHER AND  
CLOSER TO SEASONAL TEMPERATURES FOR THE MIDDLE TO END OF NEXT  
WEEK.  
 

 
   
AVIATION
 
(FOR THE 00Z TAFS THROUGH 00Z SATURDAY EVENING)  
ISSUED AT 535 PM CDT FRI SEP 22 2017  
 
SCATTERED THUNDERSTORMS WILL BE POSSIBLE THROUGH THE JAMES RIVER  
VALLEY INTO EAST CENTRAL SD DURING THE OVERNIGHT HOURS. THERE WILL  
BE A PERIOD OF LOW LEVEL WIND SHEAR AT KSUX LATER TONIGHT AS A LOW  
LEVEL JET DEVELOPS ACROSS THE REGION. SHOWER AND THUNDERSTORM  
CHANCES WILL CONTINUE ON SATURDAY ROUGHLY OVER THE SAME AREAS,  
WITH BETTER COVERAGE ON SATURDAY AFTERNOON.  
 

 
   
FSD WATCHES/WARNINGS/ADVISORIES
 
 
SD...HEAT ADVISORY UNTIL 6 PM CDT THIS EVENING FOR SDZ056-062-066-067-  
069>071.  
 
MN...HEAT ADVISORY UNTIL 6 PM CDT THIS EVENING FOR MNZ071-072-080-081-  
089-090-097-098.  
 
IA...HEAT ADVISORY UNTIL 6 PM CDT THIS EVENING FOR IAZ001>003-012>014-  
020>022-031-032.  
 
NE...HEAT ADVISORY UNTIL 6 PM CDT THIS EVENING FOR NEZ013-014.  
 

 
 

 
 
SHORT TERM...VANDENBOOGART  
LONG TERM...VANDENBOOGART  
AVIATION...JM  
 
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