textproduct: Sioux Falls

KEY MESSAGES

- A band of showers and isolated thunderstorms will spread east across the region through today. Much of the region could see a quarter to half inch of rain by this evening, with highest amounts near to south of I-90.

- The forecast has trended drier for tonight into early Wednesday, but a second round of rain may lift back into areas south of a Vermillion SD to Spencer IA line Wednesday afternoon into early Thursday. If this round of heavy rain lifts far enough north, it will bring a potential for areal flooding and rises on local rivers in northwest Iowa.

- Impacts from heavy rain over the next few days are still focused across portions of northwest Iowa and northeast Nebraska. Rain totals of 1 to 2 inches are most likely by Thursday morning, but there is still a low (20%) probability for localized amounts up to 3 to 4 inches. A Flood Watch remains in effect.

- Drier weather prevails Thursday through Friday and again early next week.

UPDATE

Issued at 440 AM CDT Tue Sep 29 2026

Heavy rain potential remains the focus over the next 1-3 days, and the latest models have trended toward a drier solution for most of our forecast area. Today into early this evening is still on track to bring a band of showers and embedded non-severe thunderstorms east across the region. This portion of the system has trended a bit faster than was seen in previous days, with the rain moving east of Yankton to Sioux Falls to Marshall by early-mid afternoon. Still a chance that far southeast portions of the forecast area (south of Vermillion SD - Spencer IA) could see lingering showers tonight into early Wednesday. However, latest models have shifted the greater potential for heavy rain during this period closer to I-80 (in contrast to near Highway 20 as was seen at this time yesterday), which has decreased expected rainfall totals in our far southeast by an inch or more.

This band of rain is projected to return northward into the far southeast portions of our area Wednesday afternoon through early Thursday morning, though low confidence in whether it will remain south of Sioux City to Storm Lake, or lift farther north into our northwest Iowa counties. Given this uncertainty, decided to leave the existing Flood Watch as is for now. If the southern shift in the storm track persists, the risks of river flooding in our area will likewise decrease and the watch may be cancelled or adjusted accordingly.

DISCUSSION

Issued at 145 PM CDT Mon Sep 28 2026

LATE THIS AFTERNOON: Stratus is showing some breaks and lifting in the clouds through early this evening, more than previously anticipated. More sunshine filtering through may allow temperatures to rise into the low to mid 70s.

TONIGHT AND TUESDAY: Cloudy, mild, rainy weather takes hold tonight through at least early Wednesday across the region. Redeveloping and thickening evening cloud cover will keep temperatures mild overnight, only dropping to around 60 degrees. Broad warm air advection ahead of a northern stream shortwave will prompt scattered showers and isolated storms to slowly spread east from central SD toward the I-29 corridor overnight. A low level pool of moisture up to about 3000ft may bring drizzle ahead of the main area of rain, but it'll take the sfc low and associated low level jet tracking east overnight to saturate dry mid levels. Showers should hold off near and east of the I-29 corridor until Tuesday morning, spreading east through the day. Despite marginally supportive deep layer shear (25-40 kts), weak instability (<500 J/kg) should inhibit severe weather threat. Locally heavy rainfall is the main concern thanks to our prolonged wet pattern. High confidence (>80%) of most sites northwest of a Sioux City to Spencer line will see at least a quarter inch of rain Tuesday. Most higher resolution model guidance, including the HREF LPMM, suggests a few sites will see a 1+ inches of rain by Tuesday evening.

TUESDAY NIGHT THROUGH WEDNESDAY NIGHT: Mid week brings increased threat for heavy rain, localized ponding of water on roads and other surfaces, and rises on local rivers as an additional few rounds of rain progress through the region. Early Tuesday evening, rain should sag southeast further into IA, providing some breaks in the rain. Overnight through early Thursday, a broad region of widespread, persistent showers and isolated thunderstorms is expected to overspread much of NE, IA, and portions of far southeast SD and southern MN. During this period, stationary front stalls amidst broad ascent from the right entrance region of the upper jet and the remnants of Hurricane Polo track into the Central U.S. Of particular note is the low level jet feeding in moisture-rich, monsoonal air with precipitable water values of 1.75-2.00, near the max of climatology for this time of year.

From Tuesday through Thursday morning, an area near and south of a Norfolk to Sioux City to Spencer line has a high (>70%+) chance of receiving at least 2 inches of rain with potential for a few locations to see 4 to 5 inches. Recent model runs continue to wobble the heaviest axis of rain anywhere from the Hwy 20 to the I-80 corridor. This area with the heaviest, most persistent rains near/south of a Sioux City to Spencer line has the greater potential for any flooding impacts. There is definitely uncertainty in the northern extent of the rain as well. While areas near and south of Lake Andes to Madison and Pipestone line (including Yankton, Sioux Falls, and Worthington) have a high (>60%) chance of receiving over an inch of rain from tonight through Thursday morning, locations further northwest will be mostly done with rain by Tuesday night, missing the later rounds.

THURSDAY THROUGH THE WEEKEND: A strong northern stream cold front tracking through late Wednesday night kicks the Desert Southwest trough and associated stationary front eastward heading into Thursday. Dry air filtering in and surface high pressure passing to our north behind the departing system should bring a mostly dry Thursday and Friday with increasing sunshine. Thursday brings a breezy north wind with cooler air, but we'll see highs hovering around 60 degrees both days.

An early look at next weekend shows little to no notable weather impacts. Expect mostly dry conditions and a slight warm up for Saturday. A passing northern stream front on Saturday night brings a very low chance of rain north of I-90, but the main impact looks to be a cool down for Sunday.

AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/

Issued at 1242 AM CDT Tue Sep 29 2026

Mainly VFR conditions will transition to a mix of IFR to LIFR towards the end of the TAF period. Taking a look at satellite imagery, we're continuing to see cloud cover increase over our western-most counties mainly in response to the developing showers across central SD. Should see this activity gradually work its way eastwards ahead of a cold front through the morning hours of Tuesday. Pockets of MVFR to IFR vsbys will be possible at times as rainfall rates intensify. However, MVFR to IFR ceilings are also expected behind the developing precipitation shield and should persist into Tuesday night. Otherwise, increasing southerly winds will become more light and variable by Tuesday night leaving room for some widespread fog development. Locally dense patches could lead to IFR to LIFR vsbys at times.

HYDROLOGY

Issued at 440 AM CDT Tue Sep 29 2026

Precipitation thus far through September has been as much as 150-400% of normal, largely erasing the previously existing drought and resulting in wet soils across the broader region.

Projected rainfall amounts through Thursday have trended downward, with the most likely solution now remaining less than 2 inches. This has resulted in a lower probability for Minor flooding to less than 20% in most locations. However, uncertainty remains in where a secondary band of heavy rainfall will be focused late Wednesday into early Thursday, with a high-end rainfall potential (10% chance of exceedance) still in the 3 to 4 inch range across the existing Flood Watch area. With this in mind, will keep the Flood Watch in place for now. However, the potential for river flooding will ultimately depend on where this axis of heavier rain falls, with the more likely scenario resulting in ponding of water in low lying and poor drainage areas.

FSD WATCHES/WARNINGS/ADVISORIES

SD...None. MN...None. IA...Flood Watch from this evening through Thursday evening for IAZ014-021-022-031-032. NE...Flood Watch from this evening through Thursday evening for NEZ014.