textproduct: Sioux Falls

KEY MESSAGES

- Fog is likely again in areas along and south of I-90 and especially northwest Iowa into Friday morning. Locally dense fog will be possible mainly near and southeast of a Sioux City to Spencer, IA line. - A few thunderstorms look to develop mainly near and east of I-29 by daybreak Friday and move southeast. Isolated severe storms may be possible, with the potential for golf ball size hail and localized 60 mph gusts.

- A somewhat unsettled weather pattern with low predictability will begin to bring daily risks for thunderstorms into next week. A few of these may be strong to severe.

UPDATE

Issued at 941 PM CDT Thu Aug 6 2026

Areas of fog look to develop late tonight/early this morning, mainly over northwest Iowa and adjacent areas of northeast Nebraska, southeast South Dakota, and southwest Minnesota. Some dense fog appears likely mainly near and southeast of a Sioux City to Spencer, IA line; however, any dense fog is likely to remain very patchy as winds should be a tad bit higher than last night. Temperatures will be a lot warmer tonight due to southerly flow in place, with lows in the 60s and warmest in south-central South Dakota.

As for storm chances very late tonight into tomorrow, did increase PoPs to around a 30-55% chance starting around daybreak mainly near and east of I-29 as guidance is getting into better agreement that we'll get at least some shower and storm development early tomorrow morning. Isolated strong to severe storms will be possible with this activity mainly over southwest Minnesota and northwest Iowa, with golf ball sized hail as the main threat and wind gusts to 60 mph the secondary threat if stronger winds can mix down to the surface. Showers and storms look to develop between 4 and 6 am in either east-central South Dakota or southwest Minnesota and move southeastward while also spreading southward into northwest Iowa through the morning hours and exiting the area by the early afternoon.

DISCUSSION

Issued at 202 PM CDT Thu Aug 6 2026

THIS AFTERNOON: Weak troughing sliding through the region this afternoon has kept a narrow corridor of mid-lvl moisture stretching from northern Nebraska into western Minnesota. This moisture, confined within an area where H850 mb flow 30-40 knots, may eventually lead to isolated high based showers or thunderstorms later this afternoon. Confidence on development remains low, and will keep PoPs below 20% ATTM. Elsewhere, southerly winds are increasing, finally mixing out low-stratus and fog. High temperatures climb into the upper 80s west of the James River to the lower 80s further east.

TONIGHT: Two concerns in the overnight period. First of all, southerly winds should continue to transport low-lvl moisture northward, pushing dew points into the mid-60s. Should winds weaken sufficiently and temperatures drops not be overly impacted by widespread wildfire smoke aloft, then areas of fog may once again develop along and south of I-90. HREF probabilities focus a 70% probability of <1 mile visibility along and southeast of a line from Yankton to Windom. However, if the second concern we'll discuss next takes place, then those fog probability may drop significantly.

That second concern is a risk for elevated convection by daybreak. The pesky quasi-northwest flow has the potential to bring subtle mid- lvl vorticity into the area by daybreak. Guidance is all over the place, which is typical in this setup, suggesting either minimal to no convection, or fairly widespread storm development. From a parameter perspective incoming CAA AOA 850:700 mb will create a steepened mid-lvl lapse rate, but also push MUCAPE over 2000 J/KG. Shear remains a bit marginal AOA 20 knots, suggesting convection may stay a bit less organized. However analog soundings indicate potential for golf ball hail with freezing levels near 11K ft AGL, with some modest mid-lvl dry air also suggesting a few stronger wind gusts that may make it through the stable boundary layer. If this threat develops storms would push southeast with the mean flow and exit late morning.

FRIDAY: The initial morning convection will precede a cold front that will push southeast during the day. Temperatures behind this front will not be all that cool, with highs again climbing into the lower to middle 80s. The rest of the day and evening will be quiet. After midnight however, some guidance is showing another wave moving through central SD and into NE. Should this wave develop, there may be sufficient convergence on the LLJ to induce scattered convection in central and south central South Dakota after 3pm. Isolated hail will be the most likely risk with these storms.

SATURDAY/SUNDAY: There remains quite a bit of uncertainty heading into the upcoming weekend given the synoptic flow pattern. Saturday morning convection should continue to push away from the CWA in the morning hours, which would suggest stabilization in the Tri-State area for the rest of the day. However the nocturnal ignition of the LLJ would pose some risk for renewed elevated storms into Sunday morning south of I-90. There seems to be a bit more agreement by late Sunday into Monday as a stronger wave moves through the Dakotas late in the evening. Depending on the degree of instability in the region, this could serve as a reasonable setup for a nocturnal MCS given the intensifying 850mb southerly flow and increased mid-lvl westerlies. Continue to monitor severe weather outlook risks into Monday morning.

MONDAY-WEDNESDAY: There will really be no change in the forecast into the middle of the week as the region remains under slight northwesterly flow aloft. This flow will keep a persistent risk for daily convection, though with extremely low confidence and predictability until a day or so before. Temperatures may fall back slightly into the lower to middle 80s near the seasonal normals.

AVIATION /00Z TAFS THROUGH 00Z SATURDAY/

Issued at 633 PM CDT Thu Aug 6 2026

VFR conditions begin the TAF period. A lone shower has developed near KPIR. This shower may strengthen to a weak storm but should weaken before it reaches the forecast area. Fog is on the table for tonight for locations generally along and south of I-90. Locations near the interstate should still see a small period of time with lowered ceilings. Showers and some thunderstorms look to develop late tonight into tomorrow morning. Any chance for rain looks to be mainly in southwest Minnesota but could develop back to near KFSD. The fog will burn off during the morning hours while chances for rain wane through the same period of time. This will leave marginally breezy northerly flow for the afternoon hours.

FSD WATCHES/WARNINGS/ADVISORIES

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