textproduct: Sioux Falls

KEY MESSAGES

- Isolated severe storms remain possible from roughly 4 pm to 9 pm in portions of northwest Iowa. Damaging winds and perhaps an isolated tornado will be the main hazards if storms can develop.

- Fog is likely tonight into Saturday morning. For now widespread dense fog is not expected, but the better chances should be in and around southwest MN.

- Below normal temperatures will persist through next week. Additional rain chances will continue into early next week, but thunder chances are low and severe weather is not expected.

DISCUSSION

Issued at 1251 PM CDT Fri Sep 18 2026

Shower and thunderstorm chances will likely come in tow waves tonight, but focused on mostly northwest IA and northeast NE. The first will be late afternoon and early evening, centered around the potential for something close to surface based if enough heating can occur. The warm nose is currently working towards Sioux City with Wayne NE at 85 degrees and a 73 dew point. Latest soundings suggest surface heating into the upper 80s will be needed to break the cap. The best chance for this should be south of an Elk Point to Okoboji line. If the cap can break CAPE values around 2500 J/kg are expected with marginal shear and a freezing level around 14000 feet AGL. This would support wind gusts to 65 mph and potentially an isolated tornado if a storm can become rooted in the boundary layer. The latest HRRR runs suggest that surface based development may tough to come by with slightly elevated storms developing closer to Jackson and Okoboji about 23-0z.

Later this evening and early overnight a wave moves northeast from NE and may bring another threat for showers and storms. Currently these should not be severe with marginal shear and instability expected.

Weak north to northeast flow develops at the surface tonight and continues through Saturday. Model soundings showing a saturated low level air mass in place tonight into Saturday morning, shallowest near the Missouri River and deepest near highway 14. Given the weaker, cool flow suspect that fog will become fairly widespread with some locally dense. Given the cooler northerly flow, this fog will be slow to lift on Saturday morning and will likely become a stratus layer through the day Saturday, leading to a cool and cloudy day.

A slow moving low pressure will move onto the Plains Sunday into Monday bringing the next threat for rain and isolated thunderstorms. For now severe weather looks very unlikely. Heavy rain also looks a bit unlikely with CAPE values pretty low. Could see some extended periods of moderate rainfall, but unlikely to make a push towards any flash flooding with river flooding also pretty low.

Broad upper level riding will build into the area behind the slow moving low pressure, with temperatures expected to warm but remain a bit below normal. The models are very disagreeable on some weak upper level energy mid week next week so confidence on rainfall is low.

AVIATION /18Z TAFS THROUGH 18Z SATURDAY/

Issued at 1205 PM CDT Fri Sep 18 2026

Widespread IFR conditions will persist through the period, possibly some LIFR in fog and stratus later tonight into Saturday morning. Parts of northwest IA will see VFR conditions for a few hours this afternoon. Otherwise scattered thunderstorms will be possible in parts of southwest MN and northwest IA from about 23z through 4z this evening, with spotty showers and drizzle possible after that into Saturday morning.

FSD WATCHES/WARNINGS/ADVISORIES

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