textproduct: Sioux Falls

KEY MESSAGES

- Dense fog this morning in portions of southwest Minnesota, especially higher elevations along the Buffalo Ridge. These conditions should improve by 10 AM, though areas of fog may persist in the higher elevations through the afternoon into tonight.

- Early morning elevated showers/isolated non-severe storms will be focused near/south of Vermillion SD to Spencer IA through 9-10 AM. Patchy fog/drizzle may linger into the 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 410 AM CDT Sat Sep 19 2026

Band of elevated showers/isolated storms has developed from northeast Nebraska into northwest Iowa, generally south of a line from Vermillion SD to Spencer IA, as of 4 AM. This activity in response to a weak wave interacting with an elevated warm front will slowly exit from west to east by mid-late morning.

Elsewhere, low stratus continues to produce areas of drizzle and/or fog, with the fog most widespread and dense in the higher elevations along the Buffalo Ridge in southwest Minnesota, and also in a small area west of the James River Valley along/north of I-90. A Dense Fog Advisory was issued earlier this morning for southwest Minnesota and was just issued for Jerauld, Brule and Aurora counties in South Dakota. Will monitor conditions to see if further expansion will be needed.

Latest guidance shows the stratus persisting over much of the area today into tonight, remaining low enough for areas of fog to linger along the Buffalo Ridge at least into this evening. With no substantial change in pattern, could see a repeat of at least patchy dense fog along the Ridge tonight into early Sunday and this will be evaluated more closely through today.

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 /12Z TAFS THROUGH 12Z SUNDAY/

Issued at 640 AM CDT Sat Sep 19 2026

IFR-LIFR ceilings will persist through much of this period, briefly climbing into the lower end of MVFR range during the mid afternoon to early evening. Low stratus will be accompanied by areas of MVFR-IFR visibility in fog/drizzle this morning. Visibility is expected to improve to P6SM most areas by 16Z-18Z, with the exception of the higher elevations of southwest MN where low stratus will result in lingering fog much of the period.

FSD WATCHES/WARNINGS/ADVISORIES

SD...Dense Fog Advisory until 10 AM CDT this morning for SDZ052-057- 058. MN...Dense Fog Advisory until 10 AM CDT this morning for MNZ071-072- 080-081-097. IA...None. NE...None.