778  
FXUS63 KFSD 240907  
AFDFSD  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SIOUX FALLS SD  
407 AM CDT THU AUG 24 2017  
   
SHORT TERM
 
(TODAY AND TONIGHT)  
ISSUED AT 407 AM CDT THU AUG 24 2017  
 
SCATTERED SHOWERS AND A FEW ISOLATED THUNDERSTORMS CONTINUE ON RADAR  
EARLY THIS MORNING BEING FUELED BY PERSISTENT MID LEVEL WARM AIR  
ADVECTION EVIDENT IN THE 850-700 MB THETA-E SURFACES. THIS TREND IS  
EXPECTED TO CONTINUE THROUGH THE MORNING AS A JET MAX CONTINUES TO  
PIVOT TO OUR EAST PLACING US IN THE FAVORABLE ENTRANCE REGION. A  
REINFORCING WEAK WAVE DROPPING THROUGH FROM THE NORTHWEST WILL ALSO  
LIKELY CONTINUE TO AID IN SHOWER AND ISOLATED THUNDERSTORM CHANCES  
WITH THE BETTER CHANCES EVER SO SLOWING DRIFTING TO OUR SOUTHEAST  
COUNTIES BY MID TO LATE MORNING. BY LATE AFTERNOON, ANOTHER  
SHORTWAVE APPROACHES THE AREA THROUGH NORTHERN NEBRASKA UNDER THE  
BROADER RIDGE AND WITH A STRENGTHENING LOW LEVEL JET, WILL LIKELY  
SEE A RE-EXPANSION OF PRECIPITATION ACTIVITY FROM THE LATE AFTERNOON  
INTO THE OVERNIGHT HOURS. ELEVATED INSTABILITY IS PRETTY MARGINAL  
AND COMBINED WITH LESS THAN IMPRESSIVE DEEP LAYER SHEAR, DON'T  
FORESEE MUCH IN THE WAY OF SEVERE WEATHER. ALL IN ALL, SHOULDN'T BE  
A WASHOUT OF A DAY WITH JUST HIT OR MISS SHOWERS/STORMS, MOST  
PREVALENT OVER NORTHWEST IOWA AND ADJACENT AREAS.  
 
SURFACE WINDS WILL BE HAVE A LITTLE MORE OF AN EASTERLY COMPONENT  
TODAY AND SHOULD LEAD TO TEMPERATURES A DEGREE OR TWO COOLER THAN  
YESTERDAY AT MOST LOCATIONS.  
   
LONG TERM
 
(FRIDAY THROUGH WEDNESDAY)  
ISSUED AT 407 AM CDT THU AUG 24 2017  
 
MESSY START TO THE INTERMEDIATE TERM. LOW-LEVEL JET WILL CONTINUE  
INTO THE MORNING, NOT EXCEPTIONALLY STRONG, BUT LEADS TO A  
PERSISTENT PERIOD OF THETA-E ADVECTION OF WHICH THE STRONGER FOCUS  
APPEARS TO SLOWLY LIFTS NORTH AND EASTWARD INTO SOUTHERN MN/NORTHERN  
IA THROUGH THE MORNING. HARD TO SAY WITH CERTAINTY THAT  
PRECIPITATION WILL END, WHICH SEEMS MORE LIKELY ON THE SURFACE AS  
HEIGHTS TRY AND NUDGE UP A BIT THROUGH THE DAY, BUT MORE LIKELY HAVE  
SOME POCKETS OF REGENERATING SHOWERS/THUNDER DUE TO THE WARM  
ADVECTION. SHOULD BE QUITE A FEW CLOUDS REGARDLESS, AND TEMPS WILL  
REMAIN SOMEWHAT LIMITED IN WARMING POTENTIAL, MID 70S TO LOWER 80S.  
 
MORE COHERENT SHORTWAVE TROUGHING WILL MOVE INTO THE WESTERN DAKOTAS  
BY LATE DAY FRIDAY, AND COULD ENHANCE ELEVATED LIFT A BIT MORE NORTH  
OF I-90 LATE IN THE DAY. STILL LOOKS QUITE FAVORABLE FOR CONVECTION  
TO BECOME FAIRLY WIDESPREAD ACROSS THE AREA FROM FRIDAY EVENING  
THROUGH AT LEAST EARLY SATURDAY, WITH INITIATION EXPECTED CLOSER TO  
UPSLOPE/CONVERGENCE IN WESTERN PLAINS LATE FRIDAY AFTERNOON.  
PARAMETERS FOR SEVERE STORMS ARE NOT PARTICULARLY ROBUST BY ANY  
MEANS, WITH INSTABILITY PROFILES GENERALLY FAIRLY THIN OUTSIDE THOSE  
AREAS WEST OF THE JAMES RIVER IN THE LATE AFTERNOON TO EVENING.  
WITH WEAK MID-LEVEL WINDS, MOST OF THE SHEAR IS DUE TO STRONG  
VEERING IN LOWEST 3 KM AND WITH SPEED CHANGES WITHIN THE LOW-LEVEL  
JET, SUGGESTING AN OPPORTUNITY FOR RAPID TRANSITION TO SHORT BOWING  
SEGMENTS OR MORE OUTFLOW/COLD POOL DOMINANCE AFTER INITIAL STORMS  
PROGRESSING TO PERHAPS THE I-29 CORRIDOR BY LATE EVENING.  
 
IF ANYTHING, PRECIPITATION MAY PULL OUT QUICKER TO THE EAST ON  
SATURDAY WITH IMPACT OF ORGANIZED CONVECTION, ALTHOUGH THE SYNOPTIC  
BOUNDARY AND UPPER FORCING ARE LIKELY TO LOW AS TROUGH BEGINS TO DIG  
TOWARD THE UPPER MIDWEST THROUGH SATURDAY NIGHT AND EARLY SUNDAY.  
HAVE MAINTAINED FAIRLY HIGH EARLY DAY POPS TOWARD SW MN, BUT  
GENERALLY WILL SEE VEERING OF LOW LEVEL JET AND PROGRESSION OF MID-  
LEVEL TROUGH DECREASE CHANCES FROM WEST TO EAST THROUGH THE MIDDAY  
AND EARLY AFTERNOON. LINGERING LOW-LEVEL FRONTAL BOUNDARY ACROSS  
SOUTHEAST SD/NORTHWEST IA TOWARD EVENING COULD REIGNITE ADDITIONAL  
CONVECTION, BUT THAT WILL ALSO PROGRESS FAIRLY QUICKLY SOUTHEAST  
DURING THE EVENING.  
 
AT THIS POINT, SUNDAY COULD VERY WELL FEATURE ON AREA OF  
PRECIPITATION EARLIER IN THE DAY RIPPLING ALONG LINGERING MID-LEVEL  
THERMAL AXIS THROUGH THE MISSOURI RIVER VALLEY, AND THEN A DIURNALLY  
DRIVEN SHOWER FIELD MAINLY NEAR/EAST OF I-29 BY MIDDAY AND THROUGH  
THE AFTERNOON. DIFFERENTIAL THERMAL CHANGES LIKELY TO YIELD SEVERAL  
HUNDRED J/KG MLCAPE IN PLACE BY AFTERNOON IN SUPPORT OF THUNDER.  
ALSO, WINDS ALOFT AND THE PRESENCE OF DEEPER COLD ADVECTION SUGGESTS  
WINDS WILL BE QUITE A BIT STRONGER THAN THE BLEND, AND HAVE HEDGED  
UPWARD WITHIN COLLABORATION ALLOWANCE, WITH EXPECTATION THAT BLENDS  
WILL EVENTUALLY CATCH ON TO POTENTIAL FOR GUSTS 20-25 KTS AROUND  
PEAK HEATING.  
 
NOT MUCH SOLID EVIDENCE TO TAMPER WITH THE LONGER RANGE GRIDS BEYOND  
SUNDAY, AS UPPER WAVE DRIFTS SLOWLY AWAY TO THE EAST AND GRADUALLY  
BUILD RIDGING SURFACE AND ALOFT, BUT ENOUGH VARIANTS IN SOLUTIONS  
WITH THE WAVE. DOES NOT APPEAR TO BE ANY APPRECIABLE PRECIPITATION  
THREATS THE FIRST HALF OF NEXT WEEK WITH DIURNAL INSTABILITY MUCH  
DAMPED ON MONDAY NEAR UPPER WAVE COMPARED TO SUNDAY, THEN LIKELY TO  
SEE RIDGING TAKE OVER. COULD END UP A FEW MORE CLOUDS AT LEAST  
MONDAY AND TUESDAY IF MOISTURE HANGS AROUND AT LOWER LEVELS  
NEAR/EAST OF THE LOW-LEVEL RIDGE AXIS, SUGGESTED VERY STRONGLY IN  
OVERAMPLIFIED ECMWF SOLUTION. ABOUT THE ONLY IMPACT THIS WOULD HAVE  
IS ON TEMPERATURES, WHICH APPEAR TO REMAIN AT TO BELOW NORMAL  
THROUGH THE LONGER RANGE PERIOD.  
 

 
   
AVIATION
 
(FOR THE 06Z TAFS THROUGH 06Z THURSDAY NIGHT)  
ISSUED AT 1020 PM CDT WED AUG 23 2017  
 
MOST HIGH RESOLUTION MODELS CONTINUE TO HINT THAT SHOWERS AND  
ISOLATED THUNDERSTORMS MAY BEGIN TO REDEVELOP AFTER MIDNIGHT AS A  
WAVE APPROACHES FROM THE NORTH. SOUNDINGS THOUGH SHOW A FAIRLY  
NARROW CORRIDOR OF MOISTURE WITH SLIGHT INSTABILITY ALOFT ABOVE  
IT. WILL PLAY A BIT OF SCATTERED STORM COVERAGE FOR THE TERMINALS,  
BUT DUE WILL ONLY INCLUDE A VCSH AS THERE IS TOO MUCH UNCERTAINTY  
ON WHERE SHOWERS WILL ACTUALLY TRAVEL. ANY IMPACT WILL BE MINIMAL  
TO THE TERMINALS HOWEVER, WITH NO RESTRICTIONS EXPECTED.  
 
MID-LEVEL CLOUDS WILL LINGER INTO THE MID-MORNING HOURS, WITH A  
CHANCE OF DIURNAL CU REFORMING IN THE AFTERNOON. WINDS WILL REMAIN  
LIGHT AND SOUTHERLY TO SOUTHEASTERLY THROUGH THURSDAY.  
 

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

 
 

 
 
SHORT TERM...KALIN/08  
LONG TERM...CHAPMAN  
AVIATION...DUX  
 
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