textproduct: Sioux Falls

KEY MESSAGES

- Fog may develop after midnight. Locally dense fog could be possible with visibilities less than 1 mile especially along the Highway 20 corridor in northwest Iowa.

- Isolated to widely scattered thunderstorms possible late Thursday afternoon and evening. Golf ball size hail and 60 mph winds will be the primary risk.

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

- Slightly cooler weather, with mostly dry conditions can be expected for the upcoming weekend, with rain chances returning by Monday morning.

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 /00Z TAFS THROUGH 00Z FRIDAY/

Issued at 656 PM CDT Wed Aug 19 2026

Areas of fog look to develop near and south of I-90 after about 20.07Z , especially from the I-29 corridor eastward. These areas will generally see light and variable winds overnight. Low confidence in the coverage and degree of visibility reduction in any fog, but the best chance of less than a mile visibility will be along the Highway 20 corridor of northwestern Iowa. Any fog should lift quickly by the mid-morning hours Thursday.

A few light showers will be possible mainly along and north of the Highway 14 corridor by daybreak. The primary concern comes during the afternoon when guidance is in pretty good agreement that a few thunderstorms develop, the main question is how widespread will development be and where does it develops. At this time, looking at the better chances over southwest Minnesota.

Southerly winds increase by the daytime morning hours Thursday, with a cool front moving in from central South Dakota during the day. This front will shift winds to the west-northwest as it moves towards the I-29 corridor to end the period.

FSD WATCHES/WARNINGS/ADVISORIES

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