textproduct: Sioux Falls

KEY MESSAGES

- Fog has begun to form and will continue into Saturday morning. Patchy dense fog is possible along rivers, streams, valleys, and low areas. Visibility will fall to 2 miles or less at times, especially for areas of southwest Minnesota. Fog will be slow to burn off, and may linger into late Saturday morning to early afternoon.

- Another round of showers are possible early Saturday morning. These will most likely remain along and south of Highway 18 focused over northwest Iowa. These will clear out by mid- morning. Scattered sprinkles may continue until early afternoon.

- 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.

UPDATE

Issued at 804 PM CDT Fri Sep 18 2026

Currently isolated showers and thunderstorms are ongoing over northeast Nebraska, northwest Iowa, and southwest Minnesota. There remains a low risk (around 5%) that one or two of these storms could become strong to severe. Main risks are marginally severe hail, brief heavy rainfall, and wind gusts to 55-60 mph. Confidence in severe storm development is low as surface heating has ceased since the sun set and the lower levels have begun to stabilize. These storms will progress east out of the CWA around or shortly after midnight.

Low stratus to fog has begun to develop and is expected to become widespread across the CWA as winds become light and variable. Low areas, river and stream beds, and valleys could see brief periods of dense fog. Be prepared for rapidly changing visibility conditions tonight and into Saturday morning. Overnight clouds are expected to deepen, which will slow down fog dissipation into the late morning to early afternoon.

A second wave will work through the region early Saturday morning. Showers and thunderstorms are expected to develop over northeast Nebraska, southeast South Dakota, northwest Iowa, and into southwest Minnesota after 4 am. Storms are not expected to be severe as the environment is pretty worked over at this point. However, should something lift off the 850 mb layer it will have 500-1000 J/kg to work with. Most likely these will produce brief heavy rain, but an isolated stronger storm could produce wind gusts up to 55 mph and small hail. These will move northeast and dissipate through late Saturday morning. The rest of Saturday should be mostly dry, though cloudy and on the dreary side. Highs are only expected to climb into the 60s with a few low 70s possible over northwest Iowa.

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 /06Z TAFS THROUGH 06Z SUNDAY/

Issued at 1240 AM CDT Sat Sep 19 2026

Low stratus and fog will prevail for the first half of the period. Current ceilings and visibilities are down to IFR to LIFR. Periods of drizzle are also possible. Winds are light and variable and will continue into Saturday morning. Overnight the cloud depth will increase, resulting in slower fog dissipation. Most likely fog will hang around until the late morning when winds increase to around 10 mph out of the north. Due to the elevation of the Buffalo Ridge area, fog is likely to hang around until the afternoon.

Another round of showers and weak thunderstorms are possible early Saturday morning. These look to remain along and south of Highway 18 and focused over northwest Iowa. These will move northeast through mid-Saturday morning. After that, scattered light showers will remain in the forecast through the afternoon. Skies will remain overcast to broken, however visibility and ceilings will rise to mostly MVFR.

FSD WATCHES/WARNINGS/ADVISORIES

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