textproduct: Sioux Falls

KEY MESSAGES

- Widespread thunderstorm risks remain low tonight, but it's possible one or two stronger storms or clusters of storms could pass through the MO River Valley late tonight. These storms could pose a wind and marginal severe hail threat if they survive.

- Dense fog and widespread low stratus may form again into Wednesday morning.

- Following a warm and dry Wednesday, an influx of moisture will bring a chance of thunderstorms into Thursday. Very uncertain, but a highly localized heavy rain risk could develop.

- Cloud cover and cooler temperatures move in for Thursday and Friday, along with another risk of rain Thursday night.

DISCUSSION

Issued at 115 PM CDT Tue Aug 11 2026

THIS AFTERNOON: VERY humid conditions continue early this afternoon, but a wide array of temperatures in the region. Early morning stratus and fog has kept a lid on temperatures north of I-90, but south of the Interstate we've seen highs climb towards the 90s. Heat index in a few areas may reach the 100 degree mark. A couple patches of ACCAS based AOA 600 mb continue to move along I-90 this afternoon, but development potential remains low as this moves into NW Iowa. Will continue to monitor.

THIS EVENING/TONIGHT: CAMS continue to struggle resolving the myriad of weak forcing waves moving through the Plains, and these trends continue today. The result is sporadic convection developing over the western High Plains in the late afternoon and evening and then gradual tracks southeast overnight. Despite the high level of instability across the Dakotas, very few strong focusing mechanisms for convection remain apparent this afternoon. What may happen is gradual upslope flow over the western Dakotas combined with increasing SE 850mb flow could induce a cluster or two of storms. This activity will tend to follow the mean flow and push east southeast into the overnight hours. However, a closer look at soundings continues to show fairly strong inhibition both to surface based parcels but also to elevated parcels. So with that in mind, planning on keeping PoPs low, with some focus along I-90 and areas moreso over the Missouri River valley. Continue to monitor trends this evening, as strong storms could bring some higher wind gusts if they survive.

Higher confidence again in the development of stratus and fog given the continued light northeast wind and trapped boundary layer moisture. Much like this morning, areas of locally dense fog may form with widespread stratus potential possible.

WEDNESDAY-WENDESDAY NIGHT: Assuming stratus/fog does not hinder the warmup tomorrow significantly, we'll continue to see another warm afternoon on Wednesday with dry conditions. Dew points will remain in the 70s, which should cap high temperatures in to the middle 80s. A slight shift in the mid-lvl flow begins Wednesday, with persistent ridging over the Four Corners translating eastward into the Southern Plains through the day. This will result in pulling a sub-tropical moisture plume with embedded vorticity northward through the Rockies and into the Plains late in the day and overnight. Slightly better signals for convection tied to this wave will begin to focus over western SD/NE late in the afternoon, with rain gradually shifting eastward into Thursday morning. The severe weather risks remain on the lower side but not non-zero as some lingering wind threat could present itself along the MO river valley. Mid-lvl lapse rates quickly worsen as you move into the overnight hours however. One noticeable thing to watch is the increasing PWAT values which approach 1.75" by Thursday morning. This could signal at least the potential for highly localized heavier rain amounts, which the HREF PMM signals in the 2-4" range. The key to watch will be where the residual surface boundary falls by late Wednesday, as this could pull better convection risks south of the CWA.

THURSDAY-FRIDAY: Quite a bit of cloud cover may be in the area for Thursday as cooler northeasterly flow returns and we'll be on the edge of the baroclinic zone to our south. At this point, highs in the 70s are likely, with a gradually increasing rain chance into Friday as mid-lvl flow shifts back to the southwest, mid-lvl vorticity slides through the Central Plains, and the baroclinic zone lifts northward. The highest PoPs will be focused Thursday night into Friday.

SATURDAY-MONDAY: A flattened mid-lvl ridge remains over the central CONUS into the upcoming weekend, bringing another risk for rain by Saturday as energy tracks back northward into the Upper Midwest. Confidence is rising that we'll see a dry Sunday and Monday as high pressure arrives, mid-lvl ridging shifts back westward, and quasi- northwest flow moves back in. Temperatures may begin to rise back into the 80s by Sunday and Monday.

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 1214 PM CDT Tue Aug 11 2026

MVFR stratus mostly north of I-90 will continue to mix upwards this afternoon. Leaving a light and variable to northeast flow in place into the evening.

Convection risks continue to remain low into the overnight hours, with a few storms far west river attempting to move through the MO river valley overnight. Probabilities remain too low to indicate in current TAF.

More concerning will again be the potential for low stratus and fog into the mid morning hours. As long as light winds and even a cooler northeast wind prevail, given the soupy boundary layer fog should again develop. Potentially dense fog is possible, and future TAFs could trend conditions lower.

FSD WATCHES/WARNINGS/ADVISORIES

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