textproduct: Sioux Falls

KEY MESSAGES

- Hot and humid conditions will persist through at least Saturday. Afternoon temperatures in the 90s will push heat index values near 100 at times. Outdoor events should plan accordingly to the heat risks.

- Isolated showers and thunderstorms will diminish this afternoon, with quiet conditions expected into Friday morning. Weakening storms could still produce gusty winds.

- Occasional thunderstorms will be possible again Saturday and Sunday, though remain highly unpredictable on location. A few stronger storms could be possible, the the widespread risk of severe weather remains low.

- A shifting pattern may bring higher rain chances, slightly cooler temperature, and a severe weather risk early next week.

DISCUSSION

Issued at 1158 AM CDT Thu Sep 3 2026

THIS AFTERNOON: Another hot and humid early September day as temperatures are rising into the 90s. With dew points in the upper 60s to lower 70s, heat index values will climb between 95 and 100 degrees. Mid-lvl vorticity that has been strung across the CWA continues to produce high based showers and isolated storms. While updrafts themselves are not significant, high LCLs and significant DCAPE may continue to lead to pockets of stronger winds as they track northeast and pulse. As low-lvl temperatures increase and vorticity stretches out and moves east, this activity should dissipate by mid-afternoon. Some localized areas may have an impact on their high temperature due to the accompanying cloud cover.

TONIGHT: Shortwave ridging arrives for the overnight hours, with skies clearing this evening. However we'll remain very warm in the low-lvl and any surface wind will result in low temperatures in the 70s. Convection risks continue to stay on the lower end, with any focus for convection tied to convergence on the LLJ to the east of the CWA. We may have to watch for some fog focused in areas along and north of Highway 14.

FRIDAY: A variable wind early will give way to southerly winds quickly on Friday. With highly anomalous 925:850 mb temperatures remaining in place, we'll again see high temperatures rise into the middle to even upper 90s. The focus for the warmest temperatures may be in the James River valley where more significant impacts from the drought have continued. In these areas both HREF and the NBM indicate a 40-60% probability of reaching 100 degrees. Heat index values will top out near 100 degrees for a couple hours mid- afternoon, though the duration may not be long enough to warrant an advisory attm. Outside of daybreak convection in the far eastern CWA, significant capping should limit the potential for afternoon and evening convection.

Convection risks may increase slightly by daybreak Saturday as some guidance hints at the redevelopment of LLJ centered through central South Dakota. As has been the normal, very isolated strong convection could be possible, and with steep mid-lvl lapse rates, thin stable boundary inversion, and considerable dry air in the sub- cloud area, additional heat bursts may be possible.

SATURDAY-SUNDAY: A bit more uncertainty lingers for the daytime hours of Saturday. Any morning convection could generate mid- lvl vorticity that could cause persistent lingering iso-sct storms into the early afternoon hours. Should convection form and linger, then it may mess with the afternoon temperature forecast. Currently anticipating another hot and humid day with highs in the 90s, and a few hours in the mid-late afternoon with heat index values either side of 100. Anyone with afternoon/evening outdoor activities may want to consider adjusting the timing of their plans due to the heat.

The broad ridge remains planted overhead on Sunday, with some potential for mid-lvl vorticity to ride over the top of the ridge Saturday night and Sunday. These waves are highly unpredictable, and lead to spurious MCS development that may or may not develop. Typically these MCS would track along the periphery of the EML (sometimes tracked by the +10 to +12C temperature line). This could cause a weakening MCS to track southward into the Tri-state area early in the day.

MONDAY-WEDNESDAY: Upper troughing digging into the western CONUS will suppress the Central Plains ridge and push it slightly further to the east. The resulting southerly mid-lvl flow will pull stronger return flow northward and lead to an increase in PoPs for the early portions of the week. Depending on the positioning of the surface fronts and upper jet, an increased severe weather signal could develop. This signal seems to also be hinted at by the AI learning models for Monday and Tuesday.

AVIATION /18Z TAFS THROUGH 18Z FRIDAY/

Issued at 1158 AM CDT Thu Sep 3 2026

A narrow corridor of high-based showers and isolated thunderstorms continues to track northeast and may impact the Sioux Falls airport over the next 1-2 hours. Localized MVFR visibility could be possible in heavier rain.

Elsewhere VFR conditions will persist into the evening and overnight hours as winds turn variable. By daybreak, patchy fog may develop with high boundary layer moisture in place. VFR conditions with a developing southerly wind arrive late morning Friday.

FSD WATCHES/WARNINGS/ADVISORIES

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