textproduct: Sioux Falls

KEY MESSAGES

- Patchy fog continues this morning. Dense fog continues through mid morning along US Highway 20 into far southern South Dakota, with visibility below half a mile at times.

- Isolated to widely scattered thunderstorms may develop this afternoon and evening. Two inch hail and 75 mph winds are the main hazards the strongest storms.

- Warmer temperatures continue on Friday, with another very conditional risk for thunderstorms in the afternoon and evening.

- Slightly cooler weather for the weekend. Saturday should remain dry with rain chances returning to portions of the area Sunday morning.

UPDATE

Issued at 327 AM CDT Thu Aug 20 2026

Patchy fog, including dense fog, has developed across the southern portions of our forecast area. More isolated dense fog has developed elsewhere. Issued a Dense Fog Advisory for the US Hwy 20 corridor into far southern SD through 9 AM CDT, although may need to extend in time or expand in area before then.

Focus remains on the chances for thunderstorms later today into tonight. May see some showers this morning, mainly for southwestern MN, as WAA and a weak mid level vort lobe swing south ahead of the front. Soundings continue to show a robust dry layer, so expect sprinkles to very light showers at best. Cold front pushes through the area today/tonight with 20.00z and later guidance quite varied in the timing. Again, better forcing is to our north and east across central/northern MN with the main wave but the proximity to the forcing should keep more scattered activity in southwestern MN vs more isolated convection elsewhere.

However, one change in guidance over the last 24 hours has been the upward trend in instability, with a majority of guidance showing 2500 J/kg or more of MUCAPE just ahead of the front - focused south of I-90 into NE. Have seen a small trend downward in the extent of the strongest cap, but there still will be around or more than 100 J/kg of CIN. Shear values remain supportive of rotating updrafts, and may be strong enough to support hail around 2" (or more) despite mid level lapse rates around 6.5 deg C/km. Drier low levels support strong wind gusts to 75 mph. Tornado threat is low, but will have to watch if a stronger storm is able to be more surface based and along the front this afternoon. Storms will be progressive enough to limit flash flooding potential, although could see some brief heavy rain. No changes to the timing of the severe weather, with the greatest chances between 4 PM and midnight. Stay weather aware through the day today and into tonight.

Storms exit the area by early overnight. May see patchy and locally dense fog develop again for the Friday morning commute.

DISCUSSION

Issued at 1256 PM CDT Wed Aug 19 2026

THIS AFTERNOON: Rain continues to move southeast this afternoon, with gradual subsidence setting in behind the passing mid-lvl vorticity. There remain a very low risks for a few elevated storms to form along the MO river this afternoon, and then further northwest into central South Dakota where a narrow tongue of increased boundary layer moisture remains.

TONIGHT: Much of the evening and overnight hours is expected to remain quiet and dry. While a few showers and thunderstorms may form over central and eastern ND and perhaps extreme northern South Dakota. Winds at the surface will turn light and variable, and with recent rain and mostly clear skies some areas of fog may develop by daybreak. The lowest visibilities may form along the I-29 corridor with locally dense fog possible at times.

THURSDAY: Another embedded wave within the northwesterly flow pattern is expected to drop through the Plains and into the Upper Midwest on Thursday. This wave will push a surface front into the Tri-State are in the afternoon with breezy southwesterly winds ahead of the front in the morning. Soundings ahead of the boundary by afternoon show moderate instability upwards of 2500 J/KG in a few locations. However they also show at least 100 J/KG CIN, which could serve as a big limiting factor for convective development in the afternoon. The greatest forcing along the boundary remains focused from northern Minnesota into central Minnesota, with much weaker surface convergence along the boundary further southwest into northern Nebraska. This is likely why, outside of any significant moisture/thermal advection in the boundary layer, CAMs remain very isolated in convective development by late afternoon and early evening. That said, given effective shear of 35-40 knots, any updraft could rotate and increase the hail size towards the 1.5-2" mark.

FRIDAY: We may see a quick warmup on Friday into the middle and upper 80s as brief mid-lvl ridging moves into the Tri-state area. By the afternoon however, model guidance is all over the place suggesting perhaps another weak trough passes through the Plains/Midwest late in the day. Should this wave materialize, it could drive a front and some associated convection through the eastern Dakotas Friday afternoon and evening. This remains a very low-probability event, so will keep PoPs on the lower end of guidance.

SATURDAY-SUNDAY: Much of the upcoming weekend will be dry as Canadian high pressure moves through the region. Light northerly winds and weak CAA aloft will promote a very pleasant afternoon with northerly surface winds and lower humidity. Some modest return flow and weak warm advection may develop over the western Dakotas and Nebraska panhandle into Sunday. Medium range guidance has slowed down this eastward progression over the past 2 days, now indicating that precipitation chances should remain west of the CWA until Sunday night into Monday. High temperatures Sunday will be very similar to Saturday, in the upper 70s to lower 80s.

MONDAY-TUESDAY: Precipitation chances increase early next week as a shortwave crests the top of the growing mid-lvl ridge over the central CONUS. Model spread in both timing and location of precipitation will keep these PoPs only around the 20-30% range. Temperatures do show a warming trend into next week, with growing probabilities of highs reaching or exceeding the 85 mark at times into mid-week.

AVIATION /12Z TAFS THROUGH 12Z FRIDAY/

Issued at 631 AM CDT Thu Aug 20 2026

Patchy and dense fog, mainly in valleys continues through mid morning, with visibility in some locations below 1/2SM. Although conditions at the terminal have improved for KSUX, there is still enough dense fog around the area to keep in the TAF and think conditions will drop again. A few showers/sprinkles are possible this morning in southwestern MN and along US Hwy 14. Winds increase with some locations gusting around 20 knots this afternoon, with a wind shift behind the front.

Isolated to widely scattered storms may develop this afternoon and evening along a cold front. If storms develop, they may quickly become severe with wind gusts to 65 knots and 2 inch hail between 20.22z and 21.05z. With low confidence in development and coverage of storms, have omitted mention for KFSD and KSUX for now.

Winds become light and variable again late tonight, leading to additional patchy fog development across the area.

FSD WATCHES/WARNINGS/ADVISORIES

SD...Dense Fog Advisory until 9 AM CDT this morning for SDZ068>071. MN...None. IA...Dense Fog Advisory until 9 AM CDT this morning for IAZ020-031. NE...Dense Fog Advisory until 9 AM CDT this morning for NEZ013-014.