textproduct: Sioux Falls

KEY MESSAGES

- Isolated severe weather threat will wind down through midnight.

- Stratus showers are expected for Sunday. These are unlikely to be severe. Better chances for strong to severe storms exist late Sunday evening into Monday morning.

- An active pattern keeps periodic storm chances in our forecast for much of the upcoming week.

UPDATE

Issued at 856 PM CDT Fri Sep 11 2026

Diurnally driven thunderstorms along a dry line ignited this afternoon and are currently shifting southeast. While the environment continues to point towards strong to very isolated severe potential, updrafts thus far have struggled and remained fairly pulsy. With the pulsy nature and a freezing level around 13000 feet, larger hail has been hard to come by. A few locations received a little hail, but less than quarter size. Once the ongoing convection shifts southeast between about 11 pm to midnight, the threat for thunderstorms will come to an end as more stable air moves in behind.

DISCUSSION

Issued at 207 PM CDT Fri Sep 11 2026

Today winds will be southerly and strong, gusting to 30-40 mph with areas along the Buffalo Ridge possibly seeing 45 mph gusts. This is in response to an approaching upper wave and surface low pressure system tightening the SPG. Winds will decrease in the evening, but remain gusty as the front comes through. Highs today will be in the low to mid 80s east of I-29, mid to upper 80s west of I-29, and in the 90s west of the James River Valley. Through the day a narrow plume of Theta-e will advect in warm and moist air. Instability will increase in concert along and east of the plume. By mid-afternoon 1500-2500 J/kg of MUCAPE will be available. Steepening mid- level lapse rates, 30-40 kts deep layer shear, and clock-wise deeply curved hodographs all set the stage for strong to severe thunderstorms this afternoon into the evening. This is outlined in the SPC Day 1 Marginal (level 1 of 5) Risk. However, uncertainty remains leading to lower confidence in this forecast. Firstly, we are strongly capped across much of eastern South Dakota, and western Minnesota and Iowa. Model soundings and the 14Z CAMs guidance have begun to increase the amount of moisture in the 850-750 mb layer. Beneath that layer air remains very dry displaying a classic inverted "V". From here there are two possible scenarios. The first scenario is that the cap cannot be over come, and elevated storms form along the 850 mb front south of I-90, and east of I-29 after 7 pm. These storms may produce large hail to the size of a ping-pong ball (1.5"), damaging wind gusts of 70 mph, and brief heavy rain. This is the most likely scenario. The second scenario is the cap is overcome and isolated surface based supercells form along and east of Highway 81 after 5 pm. Surface based storms could produce hail the size of a golf ball (1.75"), damaging wind gusts to 70 mph, and brief heavy rain. In addition, though the risk is low (<2%) a tornado cannot be ruled out. These storms will grow upscale as they move east forming a broken line with possible bowing segments. The threats then transition to large hail and damaging winds. In either scenario, storms should be east of the region by 1 am Saturday.

Highs Saturday will be cooler, in the low to mid 70s thanks to the CAA behind the front. Winds will be northerly and light at 5-10 mph. Scattered light, elevated showers are possible Saturday. These should have very little impact and accumulations will be a trace to a couple hundredths at best. Modest moisture return in the mid- levels for Sunday will support stratus showers as a low amplitude wave passes to the north of the CWA and WAA pushes in from the south. Rain totals will be modest. LREF guidance indicates a less than 50% chance for 0.25 inches of rain for areas north of Highway 20 through midnight. In addition to showers, highs for Sunday will be cool, in the 60s.

Late Sunday night into early Monday morning an upper trough over the Pacific Northwest will begin to move east. A surface lee trough and low pressure will form along the east side of the northern Rockies. This will eject out onto the central Plains and accelerate northeast. A warm front will push out ahead of it with warm, moist air pooling behind it. Shortly before sunrise the LLJ ramps up south of the front, while at the same time a prefrontal trough takes shape and passes through the region. The combination of this forcing and warm moist air riding up over the established cold air over our region will result in more showers and isolated thunderstorms. Long- term guidance is in very low agreement on the convective environment. While it is possible an isolated stronger to severe storm may form, confidence at this time is low. Highs for Monday will be slightly warmer than Sunday, in the 70s.

By late Monday evening into early Tuesday morning, the main upper wave swings north over the region and the attendant cold front will move through from the northwest to the southeast. This fast moving system brings better chances for strong to severe thunderstorms that will pass quickly into central Iowa. An unsettled pattern will keep storm chances in our forecast through the end of the week. Highs during this time will be 5-10 degrees cooler than average for this time of year, in the 60s to low 70s.

AVIATION /00Z TAFS THROUGH 00Z SUNDAY/

Issued at 646 PM CDT Fri Sep 11 2026

Thunderstorms will be the main concern, with the main chances before 6z and mainly east of the James River, Otherwise strong southerly winds will turn west and north, then gradually taper off a bit through the night. Outside of any thunderstorms, VFR conditions are expected.

FSD WATCHES/WARNINGS/ADVISORIES

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