textproduct: Sioux Falls

KEY MESSAGES

- Showers and very isolated thunderstorms will diminish and shift southeast through daybreak.

- Patchy fog is likely over parts of northwest Iowa and nearby areas of southeastern South Dakota through the mid-morning hours. There is a 20-50% chance of locally dense fog with this activity. - Milder temperatures through Wednesday with above normal temperatures Friday into the weekend. Although details are uncertain, showers and thunderstorms may return to the region this weekend.

UPDATE

Issued at 852 PM CDT Tue Aug 4 2026

No major changes to the forecast for tonight and tomorrow. Scattered showers and very isolated thunderstorms continue mainly south of I-90 this evening and will gradually push east- southeast through tonight. Severe weather is not expected due to very limited instability. Clouds will gradually clear as well from northwest to southeast (though not as far southeast as a Sioux City to Storm Lake line), and this should allow for some patchy fog to develop. The best chance for this will be over portions of northwest Iowa north and west of a Sioux City to Storm Lake line and adjacent areas just over the border into far southeastern South Dakota. Locally dense fog cannot be entirely ruled out, with the 04.18Z REFS ensemble showing a roughly 15-25% chance of visibility less than a quarter mile in these areas. Any fog looks to develop by 3-5 am and erode by mid- morning.

Otherwise, it will be quite the cool night, especially north of I-90. This where we could see lows in the upper-40s by daybreak! This will ultimately depend on how quickly the clouds clear out, but the 04.19Z NBM has around a 60% chance of lows less than 50 degrees, mainly over portions of southwest Minnesota north of I-90 and into adjacent areas of east-central South Dakota. Outside of a lingering light shower/sprinkle near and south of Highway 20 through the early afternoon as an upper level wave rounds the base of a trough near that area, a quiet day is expected for Wednesday. Expect mostly sunny skies overall, though look for more cloud cover over the aforementioned Highway 20 corridor through the early afternoon. This will lead to highs in the upper 70s to lower 80s for most of the area, slightly cooler in the middle to upper 70s near and south of Highway 20.

DISCUSSION

Issued at 124 PM CDT Tue Aug 4 2026

A weak trough with mid level moisture and weak elevated instability will exit by about early evening, which will end the showers and thunderstorms across the area. Behind this wave skies will clear a bit and with light wind some fairly chilly morning lows are expected. Expect lows in most locations from the upper 40s to mid 50s. With lighter winds and clear skies some patchy low land fog will be possible.

Low pressure moves along the Canadian border on Thursday, with a trailing trough expected to move through the area Thursday night. For now the atmosphere looks a little too dry for much in the way of showers or thunderstorms, so will keep chances at 20 percent or less.

Friday is a tough call as instability increases across the area, but capping looks to limit the convective potential. Also of note is some disagreement on timing of the weak wave moving through Thursday night. If the faster runs are more accurate, then capping is likely to be even more difficult to overcome. A slower trough could allow for a little additional heating which would support a few thunderstorms.

Models continue to point towards faster westerly flow aloft Saturday and Sunday which may bring some scattered showers and thunderstorms to the area this weekend. Currently looking like Sunday night will be the better chance for showers and storms.

Upper level flow will turn light northerly early next week which will bring temperatures back to below normal with limited chances for precipitation.

AVIATION /06Z TAFS THROUGH 06Z THURSDAY/

Issued at 1243 AM CDT Wed Aug 5 2026

Isolated showers continue to develop and move across the Highway 20 corridor and into the rest of the northwest Iowa to start the period. Thunder is unlikely, but a rogue lightning strike can't be ruled out. This activity will continue to move southeast and out of the area by daybreak; however, an additional sprinkle or two is possible along the Highway 20 corridor through the early afternoon today.

The main concern tonight will be the potential for fog. Guidance was fairly split between having fog in around KSUX or having no fog by sunrise, but currently think clouds will linger long enough to prevent KSUX from dropping to or very close to the dew point and thus left out any fog from the TAF at this time. Otherwise, fog is likely to develop over northwest Iowa north and west of a KSUX to KSLB line and into adjacent areas of southeastern South Dakota where clouds do clear and where there is sufficient moisture. The 05.00Z HREF indicates a 20-50% chance of locally dense fog, which seems reasonable given the light winds, clearing skies, and recent rainfall.

Outside of any fog through the mid-morning, VFR conditions are expected through the period. Winds will generally remain light through the day today, but south/southwesterly winds will tick up near and west of the James River late this afternoon and this evening with wind gusts up to 15-20 kts possible.

FSD WATCHES/WARNINGS/ADVISORIES

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