668  
FXUS63 KFSD 182353  
AFDFSD  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SIOUX FALLS SD  
653 PM CDT WED JUL 18 2018  
   
SHORT TERM
 
(THIS EVENING THROUGH THURSDAY)  
ISSUED AT 445 PM CDT WED JUL 18 2018  
 
FOCUS FOR THE FORECAST WAS GENERALLY ON THE FIRST 24-36 HOURS TODAY  
WITH THE MULTI-FACETED THREAT FOR SEVERE STORMS AND HEAVY RAINFALL.  
 
CONVECTION FIRED EARLY THIS AFTERNOON NEAR K9V9 NEAR A BROAD  
WARM FRONTAL BOUNDARY AS LEADING PV IMPULSE PUSHING THROUGH CENTRAL  
SD. DESPITE NOT BEING TOO STRONGLY CAPPED, WAS GENERALLY A SOLO CELL  
UNTIL BETWEEN 20-21Z WHEN ADDITIONAL DESTABILIZATION WITH TEMPS  
CLOSER TO 80 DEGREES EXPANDED IMPACT OF LIFT NEAR AND NORTH OF THE  
WARM FRONT. SURFACE BOUNDARY EXTENDS NORTHWEST BACK TO LOW PRESSURE  
WHICH IS A REFLECTION OF A SECONDARY UPPER WAVE NOTED BACK ACROSS  
NORTHWEST SD. ATMOSPHERE IS NOT HIGHLY UNSTABLE WITH DEEPER MOISTURE  
REMAINING LOCKED UP THROUGH THE CENTRAL PLAINS, BUT ENOUGH FOR UP TO  
1500 J/KG MLCAPE. DEEP-LAYER SHEAR IS PRIMARILY FOCUSED THROUGH THE  
MISSOURI RIVER CORRIDOR WITH 35-45 KNOTS, LESSER TO AREAS NORTHEAST,  
BUT ALSO FAIRLY HEALTHY 30-40 KNOTS FROM 0-3KM. AREA OF CURRENT TOR  
WATCH COVERS THE STRONGER PARAMETERS LATE THIS AFTERNOON AND EARLY  
THIS EVENING WITH GOOD UPDRAFT ACCELERATION FROM HIGHER AXIS OF 0-  
3KM CAPE CONCENTRATING ALONG PRESENCE OF WEST-NORTHWEST TO EAST-  
SOUTHEAST CONVERGENCE AXIS, AS WELL AS LOW LCLS TO SUGGEST THE  
BETTER TORNADIC THREAT. THREAT SHOULD CONTINUE INTO EARLY EVENING  
WITH SOME TENDENCY FOR EVOLUTION TOWARD SHORTER BOWING SEGMENTS  
FEATURING WIND AND MARGINALLY LARGE HAIL. HOWEVER, AS ADVERTISED  
MORE ON THAN OFF IN THE HIGH RESOLUTION SOLUTIONS OVER THE LAST 3-6  
HOURS, A DEVELOPING LINE OF STORMS BACK TOWARD THE SECONDARY PV  
/JET EXIT MAXIMUM ACROSS NORTHWEST SD IS EXPECTED TO EVOLVE INTO A  
FAIRLY DISTINCT AND LARGER SCALE BOWING SEGMENT, WHICH SHOULD  
PROPAGATE DOWN THE INSTABILITY/SHEAR AXIS THROUGH THE MISSOURI RIVER  
CORRIDOR LATER IN THE EVENING AND EARLY IN THE OVERNIGHT WITH A  
LIKELY STRONGER THREAT FOR DAMAGING WIND. 0-3KM SHEAR VECTORS WOULD  
INTRODUCE POTENTIAL FOR SOME SMALLER ENHANCEMENTS AND PERHAPS  
MESOVORTEX GENERATION ALONG THE LINE DURING THE EVENING. BETTER WIND  
THREAT LOOKS TO DIMINISH AFTER 06Z-07Z AROUND KSUX AS THE STRONGER  
AREA EXITS SOUTHEAST.  
 
THERE WILL ALSO BE A THREAT FOR HEAVY RAINFALL OVER THE NEXT 18-24  
HOURS AS WELL. PRECIPITABLE WATER IS AROUND 1.5-1.7 INCHES IN THE  
PRE-CONVECTIVE ENVIRONMENT. THE EARLIER STORM ACROSS BRULE COUNTY,  
DESPITE A SMALL STATURE, WAS ABLE TO PRODUCE A MEASURED RAINFALL  
OVER 2" IN AN HOUR AT K9V9, AND MUCH MORE ESTIMATED TO THE SOUTH  
WHERE FLASH FLOODING OCCURRED EARLIER IN THE AFTERNOON. SLOW MOTION  
TO STORMS ALONG WITH SOME BACKBUILDING TENDENCY WILL LIKELY PRODUCE  
SOME LOCALLY HEAVY RAINFALL HEADING INTO THE EARLY EVENING HOURS.  
AFTER THIS, THE HEAVIER PRECIPITATION THREAT MAY REFOCUS TOWARD  
AREAS EAST OF THE ADVANCING MID-LEVEL WAVE SLIDING TOWARD SOUTHWEST  
MN LATER TONIGHT AND EARLY THURSDAY MORNING, WHERE A MORE BROAD AND  
HEAVY RAINFALL COULD OCCUR.  
 
TREND IN SOLUTIONS IS TO WRAP UP THE MID-LEVEL CIRCULATION FURTHER  
SOUTH AND SLOWER AS ANTICIPATED YESTERDAY AS IT SHIFTS FROM  
NORTHEAST SOUTH DAKOTA INTO SOUTHERN MN THROUGH THE DAY. FOR A BRIEF  
TIME COULD SEE MOST PRECIPITATION LIFT NORTH AND EAST OF THE CWA,  
BUT SOUTHWARD WRAP TO FEATURE SHOULD RETURN PRECIPITATION AS WELL AS  
SOME STRONGER WINDS AROUND LOW PRESSURE IN SOUTHWESTERN MN. COULD BE  
A VERY STRONG GRADIENT IN TEMPS ACROSS THE AREA ON THURSDAY,  
ESPECIALLY IF CONVECTION CONTINUES THROUGH MUCH OF THE DAY ACROSS  
SOUTHWEST MN. SHOULD AT LEAST VARY FROM MID 70S IN SOUTHWEST MN TO  
UPPER 80S IN SOUTH CENTRAL SD. ON THE EDGE OF CLOUDS/PRECIPITATION  
AND ALONG THE CONVERGENCE AXIS NEAR THE EASTERN PORTIONS OF  
SOUTHWEST MN/NORTHWEST IA, COULD SEE A COUPLE OF RANDOM STRONGER  
STORMS TOMORROW AFTERNOON. THE SHEAR IS FAIRLY CONCERNING FROM IA  
GREAT LAKES SOUTHWARD IF CAN COUPLE WITH SOME GREATER INSTABILITY,  
PERHAPS ENOUGH FOR A LOW-TOPPED TORNADIC THREAT TOWARD HIGHWAY 71.  
   
LONG TERM
 
(THURSDAY NIGHT THROUGH WEDNESDAY)  
ISSUED AT 445 PM CDT WED JUL 18 2018  
 
AS THE LOW CONTINUES TO WRAP UP AND PULL AWAY ON FRIDAY, SHOULD  
REMAIN COOL ENOUGH ALOFT, CYCLONIC ENOUGH AT LOW-LEVELS, AND MOIST  
ENOUGH TO BUBBLE UP ISOLATED INSTABILITY SHOWERS/STORMS ON FRIDAY  
AFTERNOON NEAR/EAST OF I-29. AFTER THIS, HIGH PRESSURE WILL  
PRESENT A TERRIFIC WEEKEND FOR MOST WITH TEMPS A BIT BELOW NORMAL,  
A DROP IN HUMIDITY, AND WINDS DECREASING FROM A NORTH TO  
NORTHEAST DIRECTION THROUGH SATURDAY. THE UPPER RIDGE WILL BE  
KNOCKED DOWN SUNDAY BY A WEAKER LEADING WAVE WHICH COULD PUSH A  
COUPLE OF STORMS INTO THE JAMES RIVER VALLEY WITHIN THE ELEVATED  
WARM ADVECTION ZONE, WITH A BETTER CHANCE FOR THUNDERSTORMS ACROSS  
THE AREA WITH A COLD FRONTAL ZONE LATER SUNDAY NIGHT INTO MONDAY.  
ANOTHER NORTHWEST FLOW SYSTEM AROUND TUESDAY LIKELY TO PRODUCE A  
SCATTERING OF SHOWERS/STORMS, WITH TEMPS INCREASINGLY BELOW NORMAL  
EARLY NEXT WEEK.  
 

 
   
AVIATION
 
(FOR THE 00Z TAFS THROUGH 00Z THURSDAY EVENING)  
ISSUED AT 639 PM CDT WED JUL 18 2018  
 
VERY COMPLICATED SET OF TAFS WITH SCATTERED THUNDERSTORMS ACROSS  
THE AREA. WAVES OF SHOWERS AND THUNDERSTORMS EXPECTED ACROSS THE  
AREA AS SLOW MOVING LOW PRESSURE SYSTEM MOVES THROUGH. TREID TO  
TIME THUNDERSTORMS BASED ON LATEST GUIDANCE ACROSS THE AREA, BUT  
MODELS ARE SOMEWHAT INCONSISTENT.  
 
FOR NOW, FOCUSED ON TIMING FOR THE FIRST 9 HOURS OF THE TAF. COULD  
SEE SOME MVFR TO IFR CEILINGS DEVELOP BEHIND A WAVE OF  
THUNDERSTORMS APPROACHING THE I-29 TOWARDS 06Z WEDNESDAY.  
 

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

 
 

 
 
SHORT TERM...CHAPMAN  
LONG TERM...CHAPMAN  
AVIATION...BT  
 
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