textproduct: Sioux Falls

KEY MESSAGES

- Areas of fog, with visibility locally a mile or less, is expected to dissipate by mid-morning.

- Widespread showers with a few isolated thunderstorms will return late tonight into Tuesday. While severe weather is not expected, locally heavy rainfall will be the primary concern.

- Additional rain may impacts areas especially south of I-90 through the day Wednesday and possibly into early Thursday. Prolonged rainfall on top of recent rainfall could lead to some flooding concerns. Keep up to date with the latest forecast.

UPDATE

Issued at 500 AM CDT Mon Sep 28 2026

No significant changes to the forecast early this morning. Clearing of stratus in the far northeast part of the forecast area allow areas of dense fog to form earlier tonight, so issued a Dense Fog Advisory for Lyon County Minnesota. See clearing is expanding southward into Cottonwood as well over the past hour, so will monitor need for a small expansion of the advisory.

Farther west, watching an area of showers/thunderstorms moving through central South Dakota. This activity is a bit farther southeast than earlier high-res solutions indicated, but with movement toward the northeast, think our far western areas will see only a low (<30%) potential for showers or an isolated storm brushing the area early this morning. Made minor adjustments to account for this low potential, but main rain concerns will come late tonight, then off and on through Wednesday night. This has been addressed in previous discussions below, with no major changes to this portion of the forecast at this time.

Issued at 940 PM CDT Sun Sep 27 2026

Another dreary night is expected as the holes in the stratus continue to fill in this evening. Similar to last night, additional areas of fog and drizzle are expected as ceiling lower with the cooling temperatures. Fog-wise, can't completely rule out more widespread of coverage given the smaller dew point depressions and lighter surface winds with the surface high. However, locally dense patches will be a bit more scattered with most short-range guidance focusing in on portions of southwestern MN and northwestern IA. Either way, we'll have to continue monitoring surface observations and web camera as a dense fog advisory will likely be needed tonight for areas east of I-29. Otherwise, should see most of this fog gradually erode by mid to late morning (9am-11am).

Overall, Monday should be mostly a carbon copy of today with highs mainly in the low to mid 70s. The main difference comes with the breezier conditions and increasing precipitation chances. While were still on track for a few sprinkles by Monday afternoon, the main push of precipitation won't likely occur until after midnight into Tuesday morning as the better forcing and moisture arrives in our western-most counties. While severe weather is not expected, locally heavy rainfall will be the main threat as PWATs (1.25"-1.50") approach the top 0.5 percentile of ensemble guidance according the ESAT tables. Expect these increased rain chances to continue working their way across the area throughout the day on Tuesday with additional rainfall expected on Wednesday. Lastly, while overall amounts are still a bit uncertain; the highest accumulations will likely set up somewhere east of a Tyndall to Sioux Falls to Marshall, MN line with pockets of 1-2 inches over a 24-48 hour period possible.

DISCUSSION

Issued at 248 PM CDT Sun Sep 27 2026

Finally starting to see some stratus eroding and leading to some sunshine in parts of the area this afternoon. We may continue to see some erosion of the stratus so long as the sun is up as the relatively high sun angle (at least compared to winter) seems to be able to thin some of these lower clouds. Soundings still show some shallow instability within the lower cloud layer though, so we may continue to see some sprinkles to very light showers develop mainly through the afternoon and early evening. A few returns have popped up on radar west of the James River, but the lack of droplets on area cameras supports the idea that any rain will be very light in nature.

Stratus should build back in through the overnight hours along with some more fog. Not expecting fog to be as widespread or as dense as it was earlier this morning, but still locally dense fog will be possible mainly near and east of I-29 as this is where winds will be lightest. Models are a bit split on how shallow this stratus that develops tonight will be, so there remains some uncertainty as to if there will be some drizzle or not tomorrow morning. Enough confidence to mention at least some patchy drizzle during the morning commute so you may have to deal with some drizzle on your way to work or school. Temperatures will once again be slow to fall off through the overnight hours, especially after the low clouds redevelop and thicken late tonight. Think most of us will end up in the lower 60s for morning lows, though some pockets of upper 50s will be possible mainly near and east of I-29. Raised the NBM a bit by blending in some CONSAll to minimum temperatures given the returning southerly flow expected overnight.

Monday is looking to be a similar day to today. We'll start off with some low stratus that will gradually become more shallow into the early afternoon. At the same, there will once again be a shallow layer of instability to allow for some more sprinkles or light rain showers especially into the afternoon hours. At the same time, a trough will be digging towards the Desert Southwest and send an initial area of vorticity into the Northern Plains through the day. This should keep the more organized rain showers and thunderstorms tomorrow north of the area, though can't rule out some of this activity clipping our far western areas depending on the ultimate track of the wave. If this activity were to clip the area, thunderstorms appear unlikely as any parcels that rise will only have access to decent instability if it could overcome a strong cap around the 850/700 mb layer, which appears unlikely with the better forcing expected to our north and west. Also similar to today, most high temperatures will be in the upper-60s to lower-70s, perhaps slightly warmer west of the James.

The aforementioned Desert Southwest trough will send another upper wave into the region Monday night into Tuesday, this time much closer to home. With that, rain and storms are expected (70-90+%) especially late Monday night into the day on Tuesday. Modest shear will be in place, but instability looks fairly weak and so severe weather looks unlikely. Heavy rain appears to be the main threat with this system, especially when looking a bit ahead as the main trough closes off and absorbs the remnants of Hurricane Polo and very slowly moves eastward. This will mean that the trough continues to send upper waves into our region and so higher rain chances may continue especially south of I-90 through the day on Wednesday and potentially as late as into Thursday morning. Based on the position of the trough/closed upper low, the better instability will likely stay south of the area. So instead of any convectively driven flooding concerns, will have to watch how rivers respond to prolonged rain on top of the rain we've already seen this past week. The latest excessive rainfall outlooks from the WPC paint the southeastern half of the area with level 2 of 4 Slight Risks for Tuesday and Wednesday, so continue to monitor the forecasts for the latest information.

AVIATION /12Z TAFS THROUGH 12Z TUESDAY/

Issued at 645 AM CDT Mon Sep 28 2026

IFR-LIFR ceilings are again fairly widespread this morning, though a bit more broken with patches of MVFR ceilings mixed in. Fog is likewise less prevalent this morning, but pockets of dense fog will reduce visibility below a mile through 14Z. A few elevated thunderstorms in central South Dakota will brush through areas west of Highway 281 this morning. A non-zero risk for thunder at KHON within the next 2-4 hours, but confidence is too low to include in the TAF at this time. Overall conditions should improve to VFR throughout the region by early to mid afternoon.

Attention then turns increasing rain chances which will move into areas west of I-29 late in this period. A few thunderstorms could accompany this activity, along with scattered MVFR-IFR ceilings and/or visibility.

FSD WATCHES/WARNINGS/ADVISORIES

SD...None. MN...Dense Fog Advisory until 10 AM CDT this morning for MNZ072-081. IA...None. NE...None.