textproduct: Sioux Falls

KEY MESSAGES

- Light showers will continue this afternoon mainly south of U.S. Highway 18 with up to an additional quarter of an inch of rain. There is a 20 to 30 percent chance for additional showers and isolated non-severe thunderstorms into this evening across far southeast South Dakota, far northeast Nebraska, and northwest Iowa.

- Gradually clearing skies overnight may lead to the rapid development of dense fog. Be prepared to allow for extra time during your Wednesday morning commute if fog develops.

- A moisture rich storm system will move across the Central Plains on Wednesday. A tight rainfall gradient will be in place, with the highest rainfall amounts south of the forecast area. Areas along the Highway 20 corridor may still see one half to one whole inch of additional rainfall. A Flood Watch is still in effect for parts of northwest Iowa if the higher rainfall amounts shift north.

- Expect drier and cooler conditions Thursday and Friday with the next chance for light rain north of Interstate 90 on Saturday.

DISCUSSION

Issued at 238 PM CDT Tue Sep 29 2026

Regional radar as of 29.1930Z shows the majority of warm advective rain showers have moved into areas south of U.S. Hwy 18 across southeast SD and northwest IA. These showers will continue to move eastward into the late afternoon/early evening with most areas drying out by 00Z. That said, some CAMs show scattered convective activity re-developing along a trailing surface boundary/front late this afternoon and evening mainly along and south of a Yankton to Sioux Falls to Windom line. Area soundings show some weak instability, so will keep chance POPs/isolated thunder into the evening across far southeast SD into northwest IA. Any lingering activity will diminish overnight, although clouds will be slow to move out. Some clearing across southeast SD will allow lows to drop into the low to mid 50s, whereas temperatures will bottom out closer to 60 degrees for areas under cloud cover across northwest IA.

In addition, will need to watch for fog overnight. Given weak near surface flow and ample boundary layer moisture from rain today, could see rapid fog development if the broader cloud shield dissipates or moves far enough east.

For Wednesday, a strong Four Corners closed low will begin to eject east-northeastward, and with it will be remnant moisture from Hurricane Polo. This is a very wet system for late September with NAEFS PWATS above the 99th percentile. However, our southernmost forecast area will be on the northern periphery of the highest moisture as the surface low moves across the Central Plains and the stationary front sets up across NE/IA. 00/06Z model runs last night and now the 12Z suite this morning have trended south with the highest QPF. There is likely to be a sharp gradient in rainfall, ranging from amounts less than an inch to higher than 2-3 inches. Areas north of U.S. Hwy 18 will likely remain dry all day. Current trends suggest our forecast area will be on the lower end, but given some lingering concern about a late model shift north, will keep current Flood Watch in place and adjust as needed on later shifts. Otherwise, Wednesday will be warm and mostly cloudy with high temperatures in the low to mid 70s.

Some lingering showers are possible Thursday across northwest IA, but the majority of the area will be dry with much needed dry conditions continuing into Friday as surface high pressure moves across the region. A cold frontal passage Wednesday night will also usher in cooler air, keeping high temperatures Thurs/Fri mainly in the 60s. As long as skies remain mostly clear, overnight lows will dip into the mid to upper 40s.

Another weak wave moves through northwest flow aloft on Saturday, bringing a 20-40 percent chance of showers to areas mainly north of I-90. Synoptic 500 hPa flow begins to diverge between the major models Sunday into early next week, but the consensus now is for dry conditions with seasonable early October temperatures.

AVIATION /00Z TAFS THROUGH 00Z THURSDAY/

Issued at 703 PM CDT Tue Sep 29 2026

VFR Conditions for much of the CWA to remain in place until around 5z and will then deteriorate all the way down to LIFR for many areas east of highway 81 by approximately 9/10z. With calming winds into the evening and clearing out of the clouds in much of the CWA, fog potential stays high for the late overnight hours into the morning. Higher confidence for areas of more dense fog remain around KFSD and between highways 81 and 75 stretching up to the northernmost edges of the CWA and down to KSUX. Confidence for fog around KHON and west of highway 81 remains low and will most likely at least settle into half of Beadle county and southward. Winds to remain light and variable for the CWA through the period. VFR conditions are expected to return to most of the area by 19z, excluding much of northwest IA.

FSD WATCHES/WARNINGS/ADVISORIES

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