textproduct: Sioux Falls

KEY MESSAGES

- Hot and humid conditions will persist through at least Saturday with gradual improvement into the new work week. Afternoon temperatures in the 90s will push heat index values near 100 at times. Outdoor events should plan accordingly to the heat risks but know clouds could also impact Saturday and Sunday.

- Scattered thunderstorms may develop after midnight west of I-29. Pockets of quarter size hail and 50 mph wind gusts may be possible into mid-morning Saturday. Locally heavy rain may also develop due to slow moving activity.

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

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

DISCUSSION

Issued at 122 PM CDT Fri Sep 4 2026

THIS AFTERNOON: It remains hot and very humid across the Tri-State area, as temperatures have already risen into the upper 80s to low 90s. The generally weak surface flow has allowed dew points to remain in the upper 60s to lower 70s, with even mid-70 dew points in NW Iowa. Heat index values will continue to rise to 95 to 105 degrees and current heat advisory will remain in place.

Through the region a few clusters of elevated showers and thunderstorms continue. This activity is tied to both weak vorticity moving through the High Plains, but also west of a slightly richer 850:700 mb theta-e airmass. Latest guidance continues to suggest that cluster of activity just west of the CWA will continue to move north and northeast but could graze Beadle county later this afternoon. Activity over south central Nebraska is also expected to slowly crawl eastward, but without a strong theta-e push, may diminish by time it would reach Sioux City this evening. The development of any SFC based convection to the west and north seems limited given the strong EML in the area. If a storm could go, then large hail could be possible.

TONIGHT: There seems to be at least modest agreement in guidance tonight, suggesting continued mid-lvl theta-e advection will place a corridor of increased moisture west of I-29 and towards the James River valley after midnight. Guidance also seems in agreement that another weak perturbation will leave Colorado and move northeast, inducing a 40 knot LLJ that will converge near the western half of the CWA. Scattered showers and thunderstorm development would be favorable by 2-3am in this scenario. Environmental soundings would suggest anywhere from 1500-2000 J/KG MUCAPE lifted off the 700 mb layer, with modest shear AOA 25-30 knots. This would favor mostly multicells with some potential supercell characteristic, with brief bouts of quarter to ping pong ball hail. Feel perhaps the greatest risk given dry subcloud layer and steep low-lvl lapse rates would be some brief gusty winds. Storm motion may be very slow with mean winds only around 10 knots, suggesting that with deep warm cloud depths and increasing PWATs, some localized heavy rain might be an issue. HREF/REFS 3hr PMM and max guidance indicates pockets of rainfall of 2-4" could be possible.

SATURDAY: Lingering morning activity will continue to pose a few severe risks in the morning if any cold pool could develop. Though as the LLJ wanes and veers off, the convection risks may drop by early afternoon. Lingering cloud debris, if storms develop, will pose a challenge to the afternoon forecast with some guidance keeping this festering cloud cover right through the heart of the CWA. Currently forecast highs in the 90s are contingent on less cloud cover by mid-day. If cloud cover doesn't impact us as much, then another advisory could be needed south of I-90 as dew points remain in the 70s. Subsidence appears to remain in place for most of Saturday night, though the LLJ may once again spur isolated to scattered convection by daybreak Sunday. The various mesoscale waves topping the ridge into Sunday also pose an issue, with one or more convective clusters moving east and southeast out of North Dakota by daybreak.

SUNDAY: Uncertainty grows on Sunday with the Tri-State are remaining under the synoptic ridge. Convection on the periphery of this ridge will pose both a risk of dipping into the eastern CWA early Sunday, but moreso throwing a convective outflow boundary westward. If a boundary develops, it would certainly pose a risk for redevelopment later in the day despite a continued EML overhead. There remains far too much uncertainty in the forecast to be much more specific on severe weather risks, but those with outdoor plans should continue to monitor. Again, convection will have some impact on temperatures, but another day with highs in the upper 80s to lower 90s will be possible.

MONDAY-TUESDAY: The apex of the mid-lvl ridge slides east of the Tri- State by Monday as a deep upper trough moves into the Northern. Rockies. What will this pattern shift signal for us? Lee-side troughing will induce stronger return flow into the Plains, with gusty surface winds by Monday afternoon. An approaching front late on Monday into Monday night could spur isolated to scattered thunderstorms over the Dakotas and Nebraska. Should boundary layer moisture not mix-out too significantly, a severe weather risk may develop into Tuesday night. Those limitations to any widespread risks may be the fact the deepest moisture transport is pushed north of the CWA along with the strongest forcing, suggesting lift along the boundary may be weaker further south. Temperatures will remain in the upper 80s, with the front moving southeast Tuesday. Again, we'll watch the positioning of this boundary for potential convection Tuesday afternoon.

WEDNESDAY-THURSDAY: Continued signals for a slight cooldown for the middle of the week behind the passing front. Stronger CAA may allow for highs to fall back to the 70s and lower 80s.

AVIATION /18Z TAFS THROUGH 18Z SATURDAY/

Issued at 1155 AM CDT Fri Sep 4 2026

VFR conditions will continue into the evening hours. Mid-lvl clouds associated with persistent but weak lift in central South Dakota may move into the Huron area later this afternoon. However clouds should avoid most areas of the Tri-State area into the evening.

Heading into the overnight hours, surface winds may turn variable, as mid-lvl clouds begin to expand eastward into the James River and I-29 corridor after midnight. Isolated to scattered showers and thunderstorms may develop west of I-29, with weak flow potentially sagging this activity towards Sioux Falls after daybreak.

This activity may linger into the morning, leaving VFR ceilings through mid-day.

FSD WATCHES/WARNINGS/ADVISORIES

SD...Heat Advisory until 6 PM CDT this evening for SDZ063>071. MN...None. IA...Heat Advisory until 6 PM CDT this evening for IAZ012-013- 020>022-031-032. NE...Heat Advisory until 6 PM CDT this evening for NEZ013-014.