textproduct: Sioux Falls

KEY MESSAGES

- Rainfall and areas of drizzle continues early this morning. Although rain coverage will gradually diminish through today, areas of light drizzle may linger through tonight and perhaps into early Tuesday.

- Below normal temperatures will continue through mid-week, then gradually become more mild late in the week.

- Additional rain chances return Wednesday night into Friday, but severe weather risks remain low.

UPDATE

Issued at 450 AM CDT Mon Sep 21 2026

Main area of rain has shifted north of I-90 through the early morning hours as a mid-level dry slot wraps into the southern half of the forecast area. Thick low cloud persist beneath this dry slot, however, and weak lift atop the moist layer will aid in continued drizzle/light rain chances today.

The upper low is slow to move east and while rainfall amounts will taper off, models are trending a little slower in bringing rain/drizzle to an end altogether. Have extended low (20-30%) pops through tonight and later shifts may need to consider if drizzle will linger into Tuesday in some areas as low clouds persist.

The persistent cloud cover and occasional drizzle/rain chances will keep temperatures on the cool side for these last couple of days of astronomical summer with highs in the 50s today and near 60 on Tuesday. Autumn begins at 7:04 PM CDT Tuesday.

DISCUSSION

Issued at 141 PM CDT Sun Sep 20 2026

Patchy drizzle remains possible late this afternoon into the evening ahead of the incoming rainfall. The rain will move into the area through the afternoon into tonight. Low pressure to the southwest will swing saturation and lift aloft into the area this afternoon with a dry slot working north towards the Missouri River. This seems to suggest that the better potential for a more steady rainfall will be closer to I-90 as the dendritic layer dries out south of I-90 during the evening, but remains saturated near and north of I-90. Model soundings do hint at some weak instability near the Missouri River which may help overcome the lack of saturation in the dendritic layer during the evening and overnight hours, but the rainfall would be more scattered in nature. If this weak instability does not come to fruition, drizzle will be more likely towards the Missouri River. Given how weak any expected instability will be, thunderstorms are not expected.

As for amounts through tomorrow, model agreement is fairly high in a swath of a half an inch to an inch and a half, focused near I-90 in southeast SD, then east into northern parts of northwest IA. These rainfall amounts are not expected to cause any flash flooding or rapid rises along rivers.

Monday will see lingering showers across the area, with a more focused area over northwest IA as the upper level low slowly moves out of the area. Amounts will be lower through the day on Monday, but parts of northwest IA could see a quarter inch of additional rainfall. Otherwise Monday will be cool and cloudy with highs from the upper 50s to mid 60s.

Monday night into Tuesday morning could see some fog with light southeast flow and a fairly moist boundary layer.

Tuesday and Wednesday should both be dry and cool with highs in the 60s both days. Winds will increase on Wednesday with gusts around 30 mph possible near and west of the James RIver.

Wednesday night into Thursday will be the next better chance for showers and thunderstorms. At this time severe weather and heavy rain look unlikely.

Temperatures will rebound back to around normal Friday into next weekend as westerly flow aloft develops. Periodic chances for showers and thunderstorms are expected during this time.

AVIATION /06Z TAFS THROUGH 06Z TUESDAY/

Issued at 1250 AM CDT Mon Sep 21 2026

MVFR to IFR stratus continues to dominate the area to start the period, the exception being along the Highway 14 corridor where the lower stratus has been able to break away at times. Should see some lower stratus fill back into these areas over the next couple of hours. Heading into the daytime hours today, MVFR to IFR stratus will continue, though improvements are possible this afternoon once again along the Highway 14 corridor. Confidence was not high enough to bring back VFR conditions in the afternoon to KHON at this time, but trends will be monitored.

Lastly, showers and drizzle will also continue to impact the area, with drizzle most prominent in areas closer to the Missouri River Valley. Areas of drizzle will diminish during the afternoon as cigs do rise a bit, but additional drizzle may return to portions of the I-29 corridor this evening. Winds throughout the period will be out of the east-northeast around 5-12 kts.

FSD WATCHES/WARNINGS/ADVISORIES

SD...None. MN...None. IA...None. NE...None.