textproduct: Sioux Falls

KEY MESSAGES

- Pockets of drizzle will continue through mid-morning mainly east of I-29.

- Patchy Fog will continue through late morning. Locally dense patches could lead to visibilities of 1/2 mile or less. Be prepared for rapidly changing visibility conditions.

- Showers and thunderstorms will lift through the area starting tonight. While severe weather is not expected, locally heavy rainfall will be the main threat. Widespread accumulations of 1-2+ inches are expected along and north of I-90. Minor impacts to rivers and streams are possible.

- Near normal temperatures persist this weekend and into next week. Any precipitation chances are most likely (30-70%) to remain light and non-severe through the weekend and into next week.

UPDATE

Issued at 420 AM CDT Thu Sep 17 2026

Taking a look across the area, the dreary conditions continue with mainly overcast skies, pockets of drizzle east of I-29, and pockets of locally dense fog developing. Starting with the fog, multiple surface web camera's continue to show patchy fog across the area this morning with a few locally dense patches across northwestern IA. While we decided to pull the trigger on a small Dense Fog Advisory for portions of northwestern IA through 11am, looks like the lower stratus is keeping things just warm enough at the surface to prevent more widespread dense fog from developing. Nonetheless, where there isn't as much fog east of I-29, the drizzlefest continues. With the lower LCLs and linger low-levels saturation expect the pockets of drizzle to hang around through at least mid- morning before dissipating as the column dries. Looking forward, dreary conditions will continue for most of the day as highs peak in the low to mid 70s for the day.

From here, the focus continues to be on tonight's rain chances. For the most part, most of the highlights remain the same as listed below in the discussion section. However, we've starting to see a slight southern shift in the axis of heaviest accumulations with the 00z/06z runs of guidance. With this in mind, could see pockets of 1+ inches as far south as the Iowa Great Lakes. However, still highly confident in widespread 1-2+ inches along and north of I-90. Timing wise, still thinking the heaviest accumulations fall between 12am to 8am Friday then exit most areas by late morning. As a far as Friday's severe risk, still conditionally dependent on how the morning activity pans out and if we can sufficiently recover by the afternoon to evening hours. Given the lingering cloud cover and potential for light showers, my gut is saying the window for any development will be fairly limited and isolated to the Highway-20 corridor and surrounding areas. Nonetheless, can't say with any confidence that a stronger storm will be make it through that environment at this time.

DISCUSSION

Issued at 243 PM CDT Wed Sep 16 2026

We are starting to see sprinkles to very light showers break out across parts of the area this afternoon as warm air advection (WAA) strengthens aloft. With saturation in the low levels but a dry dendritic growth zone (DGZ), sprinkles/drizzle is expected. At the same time, low level stratus not only continues to blanket most of the forecast area but is lowering in height. These conditions will persist through the rest of the afternoon and evening hours before rain chances ending after midnight. Though with lowering ceilings, sprinkles/drizzle looks to persist through the night. At the same time, with sufficient low level moisture in place, ceilings look to lower to potentially down to fog across the area. As of now, think that it is mainly across the higher elevation areas along the Buffalo Ridge in southwest Minnesota, downs through northwest Iowa, and along the Missouri River Valley. With the stratus/fog in place, temperatures will not move much, only lowering to the 50s to low 60s.

Fog along with sprinkles/drizzle will continue into the morning hours before lifting through the morning hours. This should allow for temperatures to warm to the 70s for the afternoon hours. While the afternoon hours will be quiet, things will change Thursday evening and especially for Thursday night as a shortwave trough rounds the western flank of an upper level ridge, parked over the southern CONUS. This wave will strengthen low level WAA and begin to develop showers across central South Dakota Thursday evening. The WAA will be driven by the low level jet (LLJ), tightening the temperature gradient, resulting in frontogenesis (FGEN). This band of FGEN looks to be the main driver for rain as the rain becomes more widespread as the forcing for ascent increases. There also looks to be instability above the front which looks to result in a tighter band of rain. This tighter band of rain continues to look most likely near and north of highway-14. The best forcing for ascent will push off to the northeast of the area Friday morning, resulting in slowly waning rain across the area. Could still see lighter rain persist through the rest of the morning hours. There remains some uncertainty in where this heaviest band sets up. Latest 12z deterministic guidance shows a slight wobble back southwards from the previous 00z guidance. However, the ensembles show similar placement with this band near and north of highway-14. The ensembles mean QPF remains a broad 0.5 inches to up to about 2.0 inches but their 90th percentile QPF also remains up to 2.0 to 3.0+ inches. Think this 90th percentile is representative of the highest amounts that fall within the band itself. The ensembles probabilities do support these totals as they all have a near 100% chance for 0.50 inches or more of rain. The probability for an inch or more is between 40-90% and lowers to a 20-40% chance for 2 inches or more. Have slightly trended total QPF upwards and think the overall rainfall amounts will be a broad 0.5 inch to 2.0 inches of rain across the area. Locally higher amounts up to and exceeding 3.0 inches is possible, mainly under the band itself. This may lead to minor rises in rivers and streams as well all ponding on roadways and poor drainage areas.

A small break in the action is possible for part of the afternoon hours on Friday as some breaks in the clouds occur. This will allow for high temperatures to warm to the low 70s to mid 80s, warmest south of I-90. The pockets of sunshine and diurnal heating may allow for sufficient CAPE values to return northwards, potentially resulting in strong to severe storms. This potential is highly conditional and dependent on how quickly morning showers and cloud cover can exit the area. If a severe weather threat were to materialize, it would be Friday afternoon and evening for locations south of I-90.

Saturday looks to be a dry day across the area with high temperatures near seasonable in the upper 60s to low 70s. The break from the rain will be short lived as another upper level wave looks to eject into the Plains on Sunday. This chance for rain is supported by the ensembles as they show a broad 30-70% chance for rainfall amounts to exceed a tenth of an inch. Good news is that instability is not expected, so this looks to be more stratiform rain rather than convective rain.

Medium range guidance diverges in how the upper level pattern will evolve for next week. There could be additional chances for rain but details are uncertain this far out. Temperatures on the other hand look to remain seasonable in the 60s and 70s.

AVIATION /06Z TAFS THROUGH 06Z FRIDAY/

Issued at 1233 AM CDT Thu Sep 17 2026

Patchy dense fog is expected to expand throughout most of the region over the next few hours. Current ceilings and visibility run from MVFR down to LIFR. This is expected to remain the case through mid to late Thursday morning before fog finally lifts. Light and variable winds become easterly and slightly breezy by afternoon. Gusts of 15-20 kts are expected for areas along and west of the James River Valley.

After 18.00Z gusty winds will expand east as a surface low and warm front approach from the southwest. Precipitation will begin over western South Dakota and Nebraska and flow northeast into the CWA. Severe weather is not anticipated at this time, however very heavy rainfall is possible at times for areas along and north of I-90. Moderate to heavy rain continues through the end of the period.

FSD WATCHES/WARNINGS/ADVISORIES

SD...None. MN...None. IA...Dense Fog Advisory until 11 AM CDT this morning for IAZ021-022- 032. NE...None.