808  
FXUS63 KFSD 181155  
AFDFSD  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SIOUX FALLS SD  
655 AM CDT TUE SEP 18 2018  
   
SHORT TERM
 
(TODAY AND TONIGHT)  
ISSUED AT 321 AM CDT TUE SEP 18 2018  
 
THIS MORNING WILL BRING INCREASING THUNDERSTORM COVERAGE AS A  
DISTINCT SHORTWAVE MOVES ALONG THE ELEVATED BOUNDARY DRAPED ACROSS  
THE REGION. MOISTURE FOR THESE STORMS IS PRIMARILY LOCATED IN THE  
800 TO 900 HPA LAYER, AND STORMS ARE EXPECTED TO BUILD TO THE SOUTH  
OVER TIME AS THE WEAK LOW-LEVEL JET PROVIDES ADDITIONAL MOISTURE  
ADVECTION. PRECIPITABLE WATER VALUES ARE UNSEASONABLY HIGH IN THE  
INCOMING AIR MASS, SO HEAVY RAIN WILL BE POSSIBLE UNDER STORMS;  
THANKFULLY, THIS CLUSTER OF CELLS LOOKS TO REMAIN FAIRLY  
PROGRESSIVE. THE PRIMARILY THREAT FROM THESE STORMS WILL BE ABUNDANT  
LIGHTNING, BRIEF HEAVY RAINFALL, AND LOCALIZED HAIL.  
 
DURING THE AFTERNOON AND EARLY EVENING, ANOTHER SUBTLE UPPER-LEVEL  
WAVE HAS THE POTENTIAL TO FIRE OFF ADDITIONAL STORMS AS THE  
STATIONARY FRONTAL BOUNDARY REMAINS OVER THE REGION. OVERALL, SHORT-  
RANGE MODELS FAVOR LOCATIONS SOUTH OF I-90 FOR THIS ACTIVITY, SO  
HAVE INCREASED POPS AROUND THE SIOUX CITY VICINITY.  
 
LATE THIS EVENING INTO THE OVERNIGHT HOURS, COOLER AIR FROM THE  
NORTH IS FORECAST TO PUSH THE BOUNDARY SLIGHTLY FARTHER TO THE  
SOUTH, ESPECIALLY OVER SOUTHWEST MN AND NORTHWEST IA. THIS WILL  
BRING THE NORTHERN TERMINUS OF THE LOW-LEVEL JET INTO THE TRI-STATE  
AREA, AND INCREASE THE THREAT FOR TRAINING THUNDERSTORMS CAUSING  
HEAVY RAIN AND LOCALIZED FLASH FLOODING ISSUES. THE ULTIMATE  
LOCATION OF WHERE THIS BOUNDARY SETS UP IS STILL UNCERTAIN, GIVEN  
LITTLE CONSENSUS AMONG CAMS; HOWEVER, TODAY WOULD BE A GOOD TIME TO  
MAKE SURE THE SUMP PUMP IS WORKING WELL, ESPECIALLY IF YOU LIVE  
ALONG AND EAST OF I-29. FLASH FLOOD WATCHES MAY STILL BE NEEDED  
DURING THIS TUESDAY NIGHT INTO WEDNESDAY MORNING TIMEFRAME, BUT HAVE  
AGAIN HELD OFF DUE TO A LACK OF CONFIDENCE ON WHERE THIS BAND OF  
HEAVIEST RAINFALL WILL END UP. FOR NOW, WILL CONTINUE TO MONITOR  
TRENDS TO DETERMINE WHERE THE INGREDIENTS BEST LINE UP.  
   
LONG TERM
 
(WEDNESDAY THROUGH MONDAY)  
ISSUED AT 321 AM CDT TUE SEP 18 2018  
 
WEDNESDAY INTO WEDNESDAY NIGHT, THUNDERSTORMS CONTINUE TO BE A  
THREAT AS DYNAMICS INCREASE AHEAD OF AN INCOMING LONGWAVE TROUGH.  
WEDNESDAY EVENING IN PARTICULAR LOOKS TO BE A FAVORABLE TIME FOR  
ADDITIONAL HEAVY RAIN, GIVEN THE INTENSIFYING SURFACE LOW  
PRESSURE SYSTEM. ON THURSDAY, THE STRONG UPPER TROUGH STILL LOOKS  
TO PUSH THROUGH THE NORTHERN PLAINS. THIS WILL BRING A DRIER  
CANADIAN AIRMASS INTO THE REGION BY THE AFTERNOON. FRIDAY THROUGH  
THE WEEKEND LOOKS DRY AND COOLER WITH SEASONAL AUTUMN CONDITIONS.  
 

 
   
AVIATION
 
(FOR THE 12Z TAFS THROUGH 12Z WEDNESDAY MORNING)  
ISSUED AT 650 AM CDT TUE SEP 18 2018  
 
THUNDERSTORMS WILL BE POSSIBLE THROUGHOUT THE DAY. A STATIONARY  
FRONT OVER THE REGION WILL BRING VARYING WIND DIRECTION, WITH ABUNDANT  
CLOUDS, AND LOW CLOUDS POSSIBLE IN THE VICINITY OF STORMS. HIGH  
RESOLUTION MODELS CURRENTLY HIGHLIGHT THE MID- AFTERNOON AND  
EVENING AS BEING RELATIVELY THUNDERSTORMS FREE, BUT MODELS HAVE  
ALSO BEEN VERY INCONSISTENT RUN-TO-RUN, SO CONFIDENCE ON  
THUNDERSTORM TIMING IS LOW. ONE TREND THAT HAS BEEN STEADY  
OVERNIGHT IS A BAND OF HEAVY RAIN DURING THE EARLY AM HOURS  
WEDNESDAY. CURRENTLY, THESE STORMS LOOK TO FOCUS NORTH OF I-90,  
AND COULD AFFECT KFSD AND KHON OVERNIGHT INTO WEDNESDAY MORNING.  
 

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

 
 

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