textproduct: Sioux Falls

KEY MESSAGES

- Patchy to areas of fog look to develop tonight/early tomorrow morning mainly over northwest Iowa and nearby portions of northeast Nebraska and southeast South Dakota. Be prepared for changing visibility on your morning commute in these areas, with any fog lifting by the mid-morning hours.

- Temperature will continue to trend near to above normal into the weekend with highs mainly in the 80s to low 90s.

- Besides some smaller periodic rain chances (20% or less), mostly dry conditions are expected through Sunday.

UPDATE

Issued at 929 PM CDT Wed Aug 5 2026

No major changes to the forecast for tonight and tomorrow. The main concern into tomorrow morning will be patchy to areas of fog that look to develop mainly over northwest Iowa, far northeast Nebraska, and adjacent areas of far southeastern South Dakota. Some dense fog is likely, with 05.18Z REFS and 05.12Z HREF probabilities of less than a quarter mile visibility around 40-60%, highest near and southeast of a Sioux City to Storm Lake line. Uncertainty remains on how widespread any dense fog would be, so current plan is to hold off on issuing any headlines at this time. Still be prepared to take extra precautions while heading out on the roads during the Thursday morning commute. Fog looks to form by 1-3 am tonight and then erode by the mid-morning hours tomorrow. Cooler than normal temperatures continue tonight, though it won't be nearly as cool as last night with lows by Thursday morning mainly in the 50s, warmest northwest and gradually cooler with southeastward extent.

High temperatures will continue to warm into the day on Thursday as southerly flow increases and also brings in higher dew points to the area. Still won't be that humid, but dew points in the 60s west of I- 29 could push the heat index slightly above the air temperature tomorrow afternoon. Speaking of air temperatures, look for highs to range from the middle 80s to around 90 near and west of the James River, with mainly lower 80s elsewhere. Instability looks to surge into areas near and west of the James River tomorrow afternoon ahead of a boundary that will push towards central South Dakota. Soundings show a stout cap in place, so thunderstorms are unlikely (less than 15% chance). IF a storm can break the cap in the late afternoon as shown in the typically aggressive HiResW-FV3 model, then severe weather will be a possibility in south-central South Dakota where the Storm Prediction Center currently depicts a Marginal (Level 1 of 5) Risk. Main severe weather hazards in the very low chance a storm develops will be wind gusts to 65 mph and hail to the size of ping pong balls.

DISCUSSION

Issued at 137 PM CDT Wed Aug 5 2026

Today will be mostly sunny and quiet under high pressure with light winds and highs in the upper 70s to low 80s. Northwest flow aloft will bring smoke into the region from wildfires out west. The smoke is expected to remain mostly aloft and currently throughout our CWA the air quality is good. For more on air quality and to get an air quality forecast for your location please see AirNow.gov

Patchy valley fog is possible early Thursday morning as winds become light to calm. Fog is most likely east of I-29 over northwest Iowa. Any fog that forms should burn off by mid- morning. As the high pressure moves off to the east this afternoon and evening winds will become southerly. At the same time from the west an upper shortwave will approach the region, strengthening the LLJ. Overnight winds will begin to increase from the west to the east. During the day Thursday good mixing up through the 850 mb level will bring down stronger gusts of 20 to 30 mph for much of the region. Portions of northwest Iowa will remain lighter at 15 to 20 mph gusts. Conditions are expected to be dry and warmer thanks to modest WAA. Highs will range from the low to upper 80s with warmest conditions over central South Dakota.

Thursday night another subtle upper wave will pivot over the region bringing chances for showers and thunderstorms. There are a few trends we will need to monitor over the next 24 to 36 hours. A narrow band of modest moisture return looks to set up over central South Dakota, but areas to the east look to remain fairly dry. Moderate surface based instability of around 1000 J/kg of CAPE will build through Thursday afternoon over central South Dakota. However, as you move east there is an increasingly strong cap in place. Instability above the cap is lacking, so elevated convection would struggle to initiate. So while an isolated strong to severe storm is possible over central South Dakota, confidence in any strong storms making it farther east than US Highway 281 is low. If storms should form it would between roughly 6 pm and midnight.

Friday sees the return of higher dew points in the upper 60s to low 70s. With highs in the 70s to 80s it will be a bit of a muggy day. A series of subtle waves will keep low PoPs in the forecast for much of the day. A more defined wave pivots through Friday evening into Saturday morning bringing chances for more organized convection, mainly east of I-29. Periodic storm chances continue through the rest of the weekend and into the beginning of next week. However, confidence in timing, coverage, and severity of storms is low as this is the point where mid-range guidance begins to diverge.

AVIATION /00Z TAFS THROUGH 00Z FRIDAY/

Issued at 621 PM CDT Wed Aug 5 2026

VFR conditions begin the TAF period. Light easterly winds begin the period but the wind will go light and variable later this evening. The main uncertainty for the period revolves around fog development tonight. Probabilities for fog are modestly high around 60-70%. However, latest guidance varies on how expansive the fog will be. Northwest Iowa has the highest chances for seeing fog but left out of KSUX's TAF due to this uncertainty. Will monitor trends in the fog potential through the evening and night. Any fog that does develop will burn off tomorrow morning, leaving gusty southerly winds for the rest of the period, with gusts up to 15 to 25 knots.

FSD WATCHES/WARNINGS/ADVISORIES

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