textproduct: Sioux Falls

KEY MESSAGES

- Patchy fog continues to develop early this morning. Dense fog is be possible with visibilities less than 1 mile especially along the Highway 20 corridor and into northwest Iowa.

- Isolated to widely scattered thunderstorms possible late Thursday afternoon and evening. 2-inch hail and 70 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.

UPDATE

Issued at 901 PM CDT Wed Aug 19 2026

Patchy fog looks to develop after about 2-3 am tonight mainly along the I-29 corridor near and south of I-90 down into northwest Iowa and the Highway 20 corridor given light winds and clearing skies expected tonight in those aforementioned locations. That said, there is lower confidence in the northern extent of any fog up to I-90 since winds may be able to tick up just enough closer to daybreak to prevent it from developing. The better coverage and the better potential for fog with visibilities a mile or less will be over the Highway 20 corridor of northwest Iowa. Any fog should lift by 9 am, with perhaps one or two low lying areas having fog linger through 9:30/10 am.

A few showers may creep into areas along and north of Highway 14 Thursday morning, though dry low level air should prevent most of this from reaching the ground outside of some sprinkles. The main concern comes in the mid-to-late afternoon as a front moves into the area and helps provide some lift for a very conditional thunderstorm threat. This is very conditional because the better upper level support will be north and east of the area and so uncertainty remains high if storms can develop at all. But if a storm is able to develop and sustain itself, it will be capable of severe weather with the potential for supercellular storm modes given effective shear values around 40 kts.

Southwest Minnesota will be closer to the better upper forcing and so the best chances of storm development will be there (25-35% chance). The highest instability will be to the south and west, but given the storm mode, large hail upwards of golf ball size will be the main concern. Secondary threat is wind gusts up to 65 mph due to fairly steep low level lapse rates. This wind threat will diminish after sunset as storms lose touch with the surface, though the hail threat would continue. Farther south and west into the Missouri River Valley of southeast South Dakota, storm chances diminish even further due to the weaker upper level support (15-25% chance). However, there will still be a threat for a very isolated supercell to develop. This is even more of a highly conditional threat than over southwest Minnesota, but these storms if they were to develop would have a higher ceiling and could produce hail to 2 inches or perhaps slightly larger as the main threat. Wind gusts to around 70 mph will be the secondary concern. No matter what ends up happening, most areas will likely stay dry Thursday afternoon and evening, but still keep that eye to the sky if you have any outdoor plans! Main time frame for severe weather chances will be 4 pm to midnight.

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 /06Z TAFS THROUGH 06Z FRIDAY/

Issued at 101 AM CDT Thu Aug 20 2026

Areas of fog, some locally dense, will be the main aviation concern through the mid-morning hours today. Confidence has increased on this potential up to the I-90 corridor, though the best chance of dense fog is along the Highway 20 corridor into northwest Iowa. Any fog should lift by 20.14-20.15Z.

Though there is a good chance at least one thunderstorm will develop somewhere over the area late this afternoon and evening, confidence remains low on where it develops and the overall coverage of storms. For this reason, left out -TSRA from the TAFs. There is still a 15-25% chance of storm at KFSD and KSUX; however the slightly better chances (25-35% chance) remain over southwest Minnesota.

A cool front will bring a shift in winds from the south-southwest to the northwest as it moves through areas west of I-29 during the day today. This front may stall in between the James River and I-29 by the late afternoon and so a wind shift is unlikely to occur east of I-29. Winds will become light and variable again anyway to close out the period.

FSD WATCHES/WARNINGS/ADVISORIES

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