875  
FXUS63 KFSD 201717  
AFDFSD  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SIOUX FALLS SD  
1217 PM CDT SUN AUG 20 2017  
   
SHORT TERM
 
(TODAY AND TONIGHT)  
ISSUED AT 402 AM CDT SUN AUG 20 2017  
 
STRONGER ZONAL FLOW ALOFT OVER THE NORTHERN SECTIONS OF THE REGION  
AS A WAVE ZIPS ALONG THE INTERNATIONAL BORDER EARLY THIS MORNING.  
ANOTHER WAVE CLOSER TO THE SYNOPTIC WARM FRONTAL BOUNDARY WAS  
LOCATED UPSTREAM OF A LARGE COMPLEX OF THUNDERSTORMS WHICH CONTINUES  
TO PRESS SOUTHEAST THROUGH SOUTHEAST NEBRASKA AND SOUTHERN IOWA.  
THIS HAS LIKELY DISRUPTED THE LOW-LEVEL JET ENOUGH TO ONLY KEEP A  
MINIMAL CHANCE FOR CONTINUED SHOWERS/THUNDERSTORMS EARLY MORNING  
MAINLY ALONG/EAST OF HIGHWAY 60. OTHERWISE, THE NORTHERN WAVE IS  
PROMOTING AN INCREASE IN MID-LEVEL FRONTOGENETIC FORCING ACROSS  
CENTRAL/NORTHERN SOUTH DAKOTA, WHICH HAS RESULTED IN A QUICKER AND  
MORE EXPANSIVE COVERAGE TO CONVECTION THAN EVEN HIGH-RES MODEL  
SOLUTIONS WOULD SUGGEST. ELEVATED INSTABILITY ON ORDER OF 300-600  
J/KG, SO WOULD EXPECT AT LEAST SOME SPOTTY THUNDER IN BAND WHICH  
WILL STREAK EASTWARD THROUGH EARLY AFTERNOON. WHILE THE BEST FORCING  
LOOKS TO REMAIN JUST NORTH OF THE AREA, OFTEN TIMES THERE IS A  
TENDENCY FOR THESE BANDS TO SAG SOUTHWARD WITH CONTINUED CONVECTIVE  
DEVELOPMENT, AND HAVE CARRIED SOME LOWER CHANCE-LEVEL POPS THROUGH  
THE HIGHWAY 14 CORRIDOR.  
 
THROUGH THE DAY, A FRONTAL ZONE WILL MOVE GRADUALLY SOUTHWARD,  
SETTLING TOWARD I-80 BY LATE DAY. ONCE THE MID-LEVEL BAND ACROSS THE  
NORTH WEAKENS BY MIDDAY, MUCH OF THE DAY WILL LIKELY REMAIN QUIET  
FROM A CONVECTIVE PERSPECTIVE, WITH A MODERATE CAP BECOMING  
ESTABLISHED WITH THE DIFFERENTIAL THERMAL ADVECTION. WILL MAINTAIN A  
LATER AFTERNOON LOW CHANCE FOR CONVECTION NEAR AND JUST NORTH OF THE  
FRONTAL BOUNDARY GENERALLY SOUTH OF A KTKC TO KYKN LINE, AS HEATING  
PRODUCES A GREATER POTENTIAL INSTABILITY, BUT CONCERNS FOR CAPPING  
REMAIN AND BOUNDARY LIKELY TO SINK FAR ENOUGH SOUTH.  
 
AS WE ADVANCE TOWARD THE LATE EVENING AND INTO THE EARLY OVERNIGHT  
HOURS, THE LOW-LEVEL JET INDUCING WARM ADVECTION INTO AREAS NORTH OF  
THE STALLED FRONTAL BOUNDARY SHOULD FEED AN INCREASING THREAT OF  
THUNDERSTORMS FROM FAR SOUTHEAST SOUTH DAKOTA AND NORTHEAST NEBRASKA  
INTO NORTHWEST IOWA, SPREADING SLOWLY NORTHWARD TOWARD SOUTHWEST MN.  
THE AIRMASS TO THE SOUTH WILL FEATURE AT LEAST 2000 J/KG ELEVATED  
INSTABILITY, AND THERE IS EFFECTIVE SHEAR WHICH INCREASING  
CONSIDERABLY HEADING NORTHWARD. STORMS COULD ORGANIZE WITHIN THE  
ELEVATED REGIME DURING THE EVENING AND EARLY OVERNIGHT, WITH THE  
PRIMARY THREAT LIKELY LARGE HAIL, ALTHOUGH DEVELOPMENT OF  
MESOCYCLONES AND MORE SHALLOW STABLE LAYER TOWARD THE SOUTH COULD  
BRING SOME DAMAGING WIND THREAT INTO PARTS OF NORTHWEST IOWA.  
THE STORMS SHOULD BEGIN TO DECREASE LATER IN THE NIGHT AS WAVE PULLS  
EASTWARD ALONG WITH VEERING LOW LEVEL FLOW, AND THIS COULD OPEN THE  
DOOR FOR SOME PATCHY FOG DEVELOPMENT SOUTH OF A KSPW TO KYKN LINE.  
   
LONG TERM
 
(MONDAY THROUGH SATURDAY)  
ISSUED AT 402 AM CDT SUN AUG 20 2017  
 
MONDAY SHOULD BE THE MOST ACTIVE DAY OF THE MID-LONG RANGE,  
PRIMARILY LATE AFTERNOON INTO THE EVENING. DESPITE WHAT SHOULD BE A  
RELATIVELY DRY PERIOD DURING THE MIDDLE OF THE DAY FOR THE ECLIPSE,  
MODEL TRENDS ARE LOOKING LESS FAVORABLE FOR CLEAR VIEWING, WITH MOST  
INDICATING THE POTENTIAL FOR EXTENSIVE STRATUS, AT LEAST FOR AREAS  
NEAR AND SOUTH OF I-90, IN THE WAKE OF ANY OVERNIGHT/EARLY MORNING  
CONVECTION. BY MIDDAY, STRATUS SHIFTS SLIGHTLY NORTHWARD WITH SLOW  
MOVING WARM FRONT, WHILE HIGH CLOUDS INCREASE FROM THE NORTHWEST IN  
ADVANCE OF APPROACHING WAVE.  
 
THIS WAVE WILL BRING INCREASING CHANCES OF STORM DEVELOPMENT BY LATE  
AFTERNOON, AS IT SWINGS SOUTHEAST ACROSS THE DAKOTAS/MINNESOTA AND  
INTERACTS WITH FRONTAL BOUNDARY LOCATED SOUTH OF I-90. STABILITY  
FORECASTS VARY, DEPENDING ON WHETHER INDIVIDUAL MODEL BREAKS OUT OF  
THE STRATUS AND ALLOWS SUFFICIENT SURFACE WARMING. WILL ALSO HAVE  
ADDED EFFECT OF ECLIPSE KEEPING TEMPERATURES A FEW DEGREES COOLER  
INTO THE EARLY AFTERNOON THAN WOULD OTHERWISE TYPICALLY OCCUR.  
HOWEVER, EVEN IN THE ABSENCE OF STRONG SURFACE WARMING, SHOULD SEE  
ABUNDANT ELEVATED INSTABILITY ABOVE ANY STABLE/STRATUS LAYER,  
WITH STEEP MID-LEVEL LAPSE RATES CONTRIBUTING TO ELEVATED CAPE  
VALUES IN EXCESS OF 1000-1500 J/KG NORTH OF THE BOUNDARY, AND  
MIXED LAYER CAPE IN EXCESS OF 2000 J/KG NEAR AND SOUTH OF THE  
BOUNDARY. THIS ALONG WITH INCREASED LIFT FROM MID-UPPER LEVEL  
TROUGH SWINGING THROUGH THE REGION SHOULD SUPPORT DEVELOPMENT OF  
STRONG TO SEVERE STORMS DURING THE LATE AFTERNOON AND EVENING.  
INITIAL THREATS WOULD MORE LIKELY FOCUS ON LARGE HAIL DUE TO THE  
MID-LEVEL LAPSE RATES, WITH ISOLATED STRONG WINDS GUSTS NEAR/SOUTH  
OF THE BOUNDARY WHERE STORMS ARE MORE LIKELY TO BE SURFACE-BASED.  
AS THE EVENING PROGRESSES, COULD SEE EVOLUTION TO A LOCALLY HEAVY  
RAIN THREAT FOR PORTIONS OF NORTHWEST IOWA, BEFORE STORMS SHIFT  
SOUTHEAST OF THE AREA WITH ADVANCING SURFACE FRONT BY 06Z OR  
SHORTLY AFTER.  
 
BEYOND MONDAY EVENING, MIDWEEK PATTERN WILL FEATURE COOL SURFACE  
HIGH PRESSURE OVER THE UPPER MISSISSIPPI VALLEY, WITH CONTINUED  
NORTHWEST FLOW ALOFT AHEAD OF BUILDING RIDGE OVER THE ROCKIES AND  
HIGH PLAINS. THIS SHOULD ALLOW FOR BELOW NORMAL TEMPERATURES AND  
GENERALLY DRY CONDITIONS ACROSS THE REGION THROUGH WEDNESDAY.  
 
LOW LEVEL RIDGE AND UPPER TROUGH SHIFT FAR ENOUGH EAST BY WEDNESDAY  
NIGHT THAT AN INCREASING SOUTHERLY FLOW/ASSOCIATED WARM ADVECTION  
BENEATH SLOWLY EASTWARD ADVANCING UPPER RIDGE WILL BEGIN TO HAVE  
GREATER INFLUENCE ACROSS WESTERN PORTIONS OF THE FORECAST AREA.  
PRECIPITATION CHANCES LATE IN THE WORK WEEK WILL REMAIN SPOTTY, BUT  
TEMPERATURES SHOULD GRADUALLY WARM FROM MID-WEEK LEVELS. COULD SEE  
MORE UNSETTLED PATTERN RETURN BY NEXT WEEKEND, AS THE UPPER RIDGE  
SHIFTS INTO THE GREAT LAKES AND ALLOWS ANOTHER TROUGH TO DROP INTO  
THE NORTHERN PLAINS.  
 

 
   
AVIATION
 
(FOR THE 18Z TAFS THROUGH 18Z MONDAY AFTERNOON)  
ISSUED AT 1215 PM CDT SUN AUG 20 2017  
 
SHOWERS AND THUNDERSTORMS WILL BECOME POSSIBLE LATE THIS EVENING  
AND CONTINUE INTO THE OVERNIGHT HOURS. THE BETTER CHACE WILL BE  
FAR SOUTHERN SD INTO NORTHWEST IA. ADDITIONAL CHANCE FOR SHOWERS  
AND THUNDERSTORMS WILL OCCUR VERY LATE TONIGHT INTO MONDAY MORNING  
FROM CENTRAL SD INTO EAST CENTRAL SD. OUTSIDE OF THE THUNDERSTORMS  
VFR CONDITIONS ARE EXPECTED, ALTHOUGH THERE WILL BE A CHANCE FOR  
SOME MVFR CEILINGS IN NORTHWEST IA ON MONDAY MORNING.  
 

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

 
 

 
 
SHORT TERM...CHAPMAN  
LONG TERM...JH  
AVIATION...08  
 
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