textproduct: Sioux Falls

KEY MESSAGES

- Fog again possible in areas along and south of I-90 into Friday morning. Locally dense fog will be possible.

- A few thunderstorms may develop by daybreak Friday and move southeast. Isolated to scattered severe storms may be possible, with potential for golf ball size hail and localized 60 mph gusts. Confidence on development remains low.

- 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.

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 /18Z TAFS THROUGH 18Z FRIDAY/

Issued at 1238 PM CDT Thu Aug 6 2026

VFR conditions will be the rule at KFSD/KHON with scattered cirrus and some wildfire smoke aloft through this evening. Surface winds will be from the south with occasional gusts between 20 and 25 kts. Attention then turns to the potential for scattered thunderstorms overnight and into the early morning hours on Friday with the passage of a cold front. Current guidance suggests storms will first develop east of KHON, so will keep airfield dry. That said, this activity could develop near or just west of KFSD, so added this potential with a PROB30 group from 07.09-07.13Z. Surface winds will then shift to the west-northwest with passage of the front, gusting at times to near 20 kts.

After morning fog/stratus, expect MVFR ceilings at KSUX to dissipate from 06.18-06.20Z with VFR conditions thereafter into the evening and early overnight. Forecast guidance then suggests a resurgence of fog tonight, so introduced this potential with a TEMPO group from 07.09-07.13Z, but will need adjustments for the 00Z and 06Z TAFs. Surface winds at KSUX will be light from the southeast through tonight, then veering to the southwest and west by mid-Friday morning with the aforementioned cold front. Similar to KFSD, could see some scattered thunderstorms with this frontal passage, but not enough confidence in coverage/timing farther south to include at KSUX at this time.

FSD WATCHES/WARNINGS/ADVISORIES

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