textproduct: Sioux Falls

KEY MESSAGES

- Warm and humid conditions return today and increase further through the weekend. Afternoon heat indices may approach 100F in portions of the Missouri River Valley Saturday/Sunday as temperatures climb into the 90s.

- A Marginal Risk (level 1 of 5) for severe thunderstorms remains in place for this afternoon and overnight. Overall confidence in afternoon storm development is low, with better chances tonight. Stronger storms may produce up to two inch hail and 60 mph wind gusts.

- Periodic shower and storm chances continue this weekend into early next week. Confidence on any details is low.

UPDATE

Issued at 439 AM CDT Fri Aug 28 2026

Forecast remains on track early this morning, with just minor adjustments to coverage of early morning low pops for very weak ACCAS showers drifting southeast. East of this more expansive cloud cover, temperatures have dipped into the lower 50s, but anticipate readings will level off or perhaps rise slightly as the clouds slide east before daybreak.

Issued at 844 PM CDT Thu Aug 27 2026

Main change to the forecast this evening was to add some isolated pops to the area west of I-29 late tonight into early Friday morning, with a very isolated cell already developing west of the James this evening. Enough dynamic forcing should exist with a subtle wave, WAA, and elevated instability near the nose of the LLJ for some showers and isolated thunderstorms to develop. Severe weather is not expected. Storms should move to and east of I-29 between daybreak and the morning rush, tapering off by mid morning. Still a warm, humid, and breezy day Friday.

Biggest questions for Friday remain convective chances during the afternoon and early evening, which would be highly influenced by coverage of morning clouds and convective debris, which may keep us more stable. With guidance split on any waves moving through (and the strength of these) during the day, confidence in any showers/storms during the evening remains low. Guidance is also split on the timing of the waves tomorrow night. GFS is the quicker of the models with the main wave entering our western forecast area around mid/late evening, while Canadian and NAM are slower, closer to midnight. No changes to the severe risk/potential, with severe storms possible into the overnight with the slower solutions - max hail size golf ball to 2 inches and wind gusts to 60 mph in the strongest storms.

DISCUSSION

Issued at 304 PM CDT Thu Aug 27 2026

One more very pleasant day today with highs in the upper 70s to mid 80s, with low relative humidity, and light winds. Outside of some thin smoke from wildfires, skies are expected to be mostly clear today and tonight. Good news for those of you who wish to catch a glimpse of the partial Lunar eclipse tonight. With one caveat.... Those in south central South Dakota have a low chance (<30%) of showers and thunderstorms this afternoon and into the evening. A subtle short wave and weak to mid-level convergence may work together to trigger convection. MUCAPE values range widely between short term guidance, from 500 to nearly 2000 J/kg and a moderately strong cap will be in place. However, steep low-level lapse rates and 35 kts of 0-6 bulk shear could support a stronger updraft or two. Risks with stronger storms is damaging wind gusts to 60 mph and marginally severe hail to 1 inch in diameter. Lows for tonight will fall to the 50s and low 60s.

Friday a few scattered storms may linger along an area of weak mid- level convergence. These should cease by late morning. Then we will see the return of higher relative humidity as southerly winds usher in a rich Theta-E ridge. Highs are expected to climb to the 80s with a few 90s sprinkled in. To the west we see a stronger mid-level wave and associated surface low pressure pushing east out of the Rockies. This will work to tighten the SPG and induce a strong LLJ through the afternoon. With good mixing we'll see gusts along and west of I- 29 in the 25-35 mph range. South central South Dakota will see the highest gusts. To the east of I-29 gusts will be 25 mph or less. As the low pushes closer a warm front will drift north through the CWA, possibly triggering showers and thunderstorms in the mid to late afternoon. Through the remainder of the afternoon and evening the system will become more compact. The LLJ ramps up rapidly through the evening and WAA intensifies which could potentially expand storm coverage and severity around midnight. A marginally unstable environment will be in place with MUCAPE values of 1000-2000 J/kg, mid-level lapse rates of 7 C/km or higher, and 35-45 kts of 0-6 bulk shear. There remains a couple of factors that keep confidence in storms low. While marginally unstable, soundings indicate a strong cap will be in place across most of the region. Without stronger forcing it is uncertain if convection will be able to initiate. However, should storms form they have the potential to rapidly become strong to severe. Risks include large hail to 2 inches in diameter and wind gusts to 60 mph.

Storms will continue into Saturday morning before pushing east of the region. The rest of Saturday looks to be dry, warm, and humid with highs climbing into the mid 80s to mid 90s, warmest over south central South Dakota. Heat Index values will be very near to critical (100 F) through the James River Valley. Sunday will be very similar, with the 90s expanded east into northwest Iowa. Heat Index values will once again be very near to critical. We will continue to monitor these trends. Sunday afternoon a cold front will move through the region, possibly bringing another round of showers and thunderstorms.

For the first half of next week the upper ridge we've been under will be squashed somewhat by a strong upper low over central Canada. Flow remains southwest with multiple short waves traversing through. This will keep shower and thunderstorm chances in the forecast through at least Wednesday.

AVIATION /18Z TAFS THROUGH 18Z SATURDAY/

Issued at 1255 PM CDT Fri Aug 28 2026

VFR conditions for the first half of the period. Thunderstorms move in from central South Dakota this evening and proceed east through the overnight. An isolated storm or two could become strong to severe shortly after convection initialization, producing heavy rain, 1.5 to 2 inch hail, and damaging wind gusts to 65 mph. As storms progress east they will likely become elevated and the primary threat will transition to damaging wind gusts. Under storms IFR to possibly LIFR ceilings and visibilities are possible. Areas along and north of I-90 are most likely to see severe weather. Storms move to the east through mid-Saturday morning.

LLWS is possible beginning late this evening and continuing through mid-morning. Areas along and south of Highway 18, including KSUX, are the most likely to see longer duration directional shear.

FSD WATCHES/WARNINGS/ADVISORIES

SD...None. MN...None. IA...None. NE...None.