textproduct: Sioux Falls

KEY MESSAGES

- Widespread showers and isolated thunderstorms will spread east through the region late tonight through Tuesday. While severe weather is not expected, much of the region will see a quarter to half inch of rain by Tuesday evening.

- From Tuesday night through early Thursday, additional rounds of rain will impact areas near and southeast of Lake Andes SD to Madison SD to Pipestone MN line. Prolonged rainfall on top of recent rainfall has potential to bring areal flooding and rises on local rivers.

- Impacts from heavy rain over the next few days are poised to be greatest in portions of northwest Iowa and northeast Nebraska, where 2 to 4 inches of rain is likely by Thursday morning. A Flood Watch has been issued for this area.

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

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 /18Z TAFS THROUGH 18Z TUESDAY/

Issued at 1258 PM CDT Mon Sep 28 2026

Early this afternoon, 700 to 1500 ft stratocumulus lingers across the region with growing breaks in the clouds. Most model guidance favors some lifting of ceilings to a scattered MVFR or low VFR deck late this afternoon but have low confidence in exactly how quickly that will occur.

VFR conditions are likely to prevail this evening. Showers with occasional thunder and MVFR to IFR stratus will spread eastward into the region overnight, mainly impacting Huron and Sioux Falls by 12z and lingering in the region through much of the morning. South winds will prevail through the period, gusting into the 20s through early this evening and weakening overnight.

HYDROLOGY

Issued at 205 PM CDT Mon Sep 28 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. With the forecast for an additional 2+ inches from eastern Nebraska into Iowa, at least some concerns for flooding begins to be introduced. HEFS guidance suggests between a 10-40% chance of Minor flooding along the lower Big Sioux, Floyd, and Little Sioux rivers. The NWM guidance is supportive of this idea as well with Annual Exceedance Probability (AEPs) of 20-50% (suggesting Action to Minor stage). While the more common impacts will likely be ponding of water in low lying and poor drainage areas, some potential for river flooding also exists but will ultimately be dependent on where the axis of heavier rain falls.

FSD WATCHES/WARNINGS/ADVISORIES

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