textproduct: Sioux Falls

KEY MESSAGES

- Thunderstorms will continue to persist through at least mid- morning. While severe weather risks continue to drop, occasional small hail and 50 mph winds will be possible. Heavy rain remains a risk.

- Hot and humid conditions persist this weekend with gradual improvement into next work week. Outdoor events should plan accordingly to the heat risks but know clouds could also impact temperatures today and Sunday.

- Occasional thunderstorms are possible again this 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 Labor Day into early next week.

UPDATE

Issued at 816 AM CDT Sat Sep 5 2026

MCS gradually shifting southeast this morning. VAD winds at 850mb from KLNX continue out of the west AOA 40 mph. Convergence on this LLJ will continue over northern Nebraska into northwest Iowa. While there may be brief bouts of small hail, the primary risk will continue to 40 to 50 mph winds but moreso locally heavy rainfall. Behind this MCS, we're seeing stronger SE surface winds up to 30 - 40 mph gusts develop as subsidence increases. These winds will continue for a few hours until the MCS completely weakens.

Convection is likely to continue through late morning or even mid-day, and there could be lingering ACCAS across NW Iowa/Nebraska deeper in the afternoon.

Temperatures will be highly impacted today and have backed off significantly on the hourly rises the NBM has suggested. We'll still likely see highs in the middle to upper 80s (perhaps into the 90s in central and south central SODAK). Am more concerned that these temperature drops will be replaced by surging low-lvl dew point with some guidance indicating mid 70 dewpoints in most areas. Despite the lower temperatures this would still result in heat index values between 95 and 103 this afternoon and early evening.

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 /12Z TAFS THROUGH 12Z SUNDAY/

Issued at 625 AM CDT Sat Sep 5 2026

Area of showers and thunderstorms continues to move east over the area this morning, with MVFR and lower conditions due to stratus and heavy rain. For KSUX, TAF issued at 1124z reflects the timing of the leading development, not the storms over northern NE as these may begin to move more to the northeast and miss the terminal. Showers and storms should taper off late this morning.

Patchy fog and stratus east of I-29 this morning has been leading to some MVFR and lower conditions, with a few brief visibility obs below 2 miles and ceilings at/below 500ft.

Once storms and stratus/fog diminish, VFR conditions return. Additional showers/storms are possible late tonight into early Sunday morning, but omitted for now due to low confidence in timing/coverage.

FSD WATCHES/WARNINGS/ADVISORIES

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