Sioux City, IA 51101 Weather

Key Messages

- Elevated fire weather is expected on Thursday with the potential for critical fire weather conditions on Friday.

- Patchy valley fog is possible through mid Wednesday morning. Patchy valley fog may once again develop Thursday morning.

- Temperatures through Friday remain 10 to 20 degrees above the seasonal normals for mid/late October.

- Dry conditions are expected through Thursday evening. Low (30% or less) rain chances Friday morning with better chances (around 50-60%) this weekend.

Update

Issued at 605 AM CDT Wed Oct 21 2026

Surface boundary has been a little bit slower to move into the forecast area this morning, keeping southerly winds a couple mph stronger than expected. This doesn't sound like much, but has kept fog development to a minimum for our area. Can't rule out isolated river valley fog over the next couple hours, but decreasing confidence in development.

Previous Update: Issued at 311 AM CDT Wed Oct 21 2026

Another warm start to this mid October morning, with temperatures through 3 AM in the upper 40s to mid 50s - closer to average highs for this time of year. Winds are still a bit breezy as the surface boundary stretches across central SD, so haven't seen any fog develop yet. Still expect winds to weaken though, and guidance continues to support at least patchy valley fog. Moderate (40-50%) chances of below 2 mile visibility this morning in the James Valley. Any fog that develops should dissipate by mid morning. For Thursday morning, HREF/REFS and other guidance show more widespread fog (increased/expanded probability of visibility below 2 miles) with calm to light winds.

NBM high temperatures continue to under perform, so raised highs with the 90th percentile through Friday (still increasing 2-4 degrees or more). Used a blend as a more southeasterly wind may temper mixing (and high temperature) potential somewhat late week. Still, highs today in the 70s with mid/upper 70s to lower 80s Thursday and Friday. Not approaching any record highs, but some record warm low temperatures are possible late week. Although humidity values are expected to remain above 35% Friday, strong winds coupled with dry grasses and crops may be enough to overcome these slightly higher values. This would lead to widespread High to Very High fire danger. Use caution to prevent sparks, especially with harvest activities.

Still very low rain chances early Friday morning (less than 30%), with better rain chances late Friday night through the weekend.

Discussion

Issued at 313 PM CDT Tue Oct 20 2026

Breezy southerly winds will gradually decrease this afternoon and evening with mostly clear skies. Lows will fall to the mid to upper 40s. For reference average lows this time of year are in the mid 30s. Early Wednesday morning winds will become light, which may allow for patchy fog development along river valleys and low areas. Especially along the northern James River Valley where probability of visibility falling to 2 miles or less is around 50%. Any fog development will burn off by mid-Wednesday morning.

As for the rest of Wednesday an upper trough and surface low located over eastern Nunavut and Manitoba will drag a cold front into the region early Wednesday morning, connected to a secondary weak mid- level shortwave and surface low over western South Dakota. At the same time moderate WAA on southerly winds will begin to amplify a shallow upper ridge located over the south central US. As this ridge continues to amplify through the day it will push back against the cold front causing it to stall out, eventually washing out the front and secondary low pressure. This will result in a diffuse weak surface boundary. Winds may vary along this boundary for a short time before becoming southerly once again as a surface high pressure strengthens over the central Midwestern states. By Friday 200 mb heights will be near to or at maximum for climatology, as will 700- 500 mb temperatures. What this means is at the surface we will be experiencing abnormally warm October days. The NBM continued showing a cold bias this run and so coordinated with neighbors to boost highs using a blend of NBM/NBM90th. Highs will rise into the low to mid 70s Wednesday, and the mid to upper 70s Thursday. There may even be a few 80s! While this is 10 to 20 degrees F warmer than average, we are still more than 5 degrees away from tying or breaking any record highs.

Looking now to the west we see a deep upper trough moving on shore stretching from British Columbia down to southern California. As this and a series of embedded shortwaves on the west side of the ridge move east the SPG will begin to tighten. Southerly winds will increase, gusting 15-25 mph by Thursday afternoon for areas along and west of I-29. East of I-29 gusts will 15 mph or less. As this entire pattern shifts east overnight gusty winds will spread east. Friday winds will continue to increase, with widespread gusts of 30 to 40 mph. Higher gusts are expected as you move west toward central South Dakota. Gusts west of the James River could reach 45+ mph. While minimum relative humidity values will remain above 40%, strong winds and drying grasses will lead to locally elevated fire risk at times through the remainder of the week.

In addition, early Friday morning WAA along the nose of the LLJ may result in light, scattered showers over south central South Dakota early Friday morning moving northeast into southwest Minnesota around noon. These showers are likely to be hit and miss, meaning not everyone will get rain, and those that do will probably only see a trace to a hundredth or two. Friday night into Saturday as the previously mentioned upper trough moves into the central plains of Canada and the US, draping a cold front down into eastern Colorado. Showers will form along the front and move southeast from early Saturday morning through Saturday evening. Once again showers are expected to be light, and could be spotty at times. A brief break overnight before a southern stream upper trough moves northeast into the region bringing more chances for rain Sunday afternoon into Monday morning. Overall totals are expected to be light, with areas of southeast South Dakota, northeast Nebraska, and northwest Iowa seeing around a 60% probability of 0.25 inches. Probabilities fall off quickly from there, and most will only see a few hundredths to a tenth or so.

Aviation

Issued at 605 AM CDT Wed Oct 21 2026

An area of VFR clouds over northwestern IA (ceilings around 5kft); otherwise, clear skies over the area. Fog has really struggled this morning, but can't rule out some very patchy river valley fog through 14z or so. Winds remain light and somewhat variable today into tonight.

With light winds, patchy valley fog is again possible Thursday morning. Outside of fog, VFR conditions prevail.