textproduct: Sioux Falls

KEY MESSAGES

- Expect reoccurring morning fog and potentially low stratus through the area until at least Tuesday morning.

- Isolated overnight storm clusters are possible each night through early Wednesday. Risks tonight in any storm that develops will be golf ball size hail, mostly west of the James River.

- Highs will remain in the 80s through Wednesday, with daytime heating impacted by morning clouds and high level smoke. Highs in the 70s are possible by the end of the week/weekend.

- Forecast confidence drops from Thursday to Saturday, with a daily 20 to 30% rain chance, meaning rain is far from guaranteed.

DISCUSSION

Issued at 122 PM CDT Sun Aug 9 2026

THIS AFTERNOON: Cool northeasterly surface and low level flow has brought widespread stratus into the region this afternoon. We're gradually seeing this cloud layer mix-out, which should still result in a brief window of highs in the lower 80s. Further west, lots of mid-high level AC moving eastward towards the central Dakotas. These clouds, tied to weak dPVA at the 600:500 mb layer, will continue to push eastward this evening. Below this cloud layer, soundings indicate a very stout and deep dry layer which should prevent anything more than a few sprinkles reaching the ground later this afternoon/evening.

TONIGHT: A very tricky and uncertain forecast continues into Monday, as no clear forcing mechanisms to develop convection are present at this point. Despite the pesky northwest flow pattern, most CAMs and global models have backed off on surface based convection over the western Dakotas this afternoon and evening. The thick layer of smoke aloft could potentially cause some limitation on convective development, but also the lack of any clear defined shortwave in the flow. Now with that said, there are a few hints at some weak 700 mb warm advection trying to develop after midnight (perhaps closer to 3-4am) into central South Dakota. A parcel lifted off the 700 mb layer should have upwards of 2000 J/KG MUCAPE with upwards of 25 knots of shear. Thus if a storm develops, it could briefly produce golf ball size hail and some modest wind gusts. The eastward movement of this activity beyond the James River should be limited by lingering low-lvl dry air. Elsewhere, given low-lvl subsidence continuing into the overnight hours, along with boundary layer dew points in the 70s, there will also be concern for fog/stratus development.

MONDAY: Any morning convection should again weaken by mid-morning, leaving potential stratus and fog in place through the morning in a light east southeast wind. Eventually temperatures do rise into the low to middle 80s, but smoke aloft could again limit the rise of temperatures. There still appears to be a better signal for a passing shortwave Monday night into Tuesday capable of producing evening convection over the western Dakotas/Nebraska. The residual moderate elevated instability, and steepening mid-lvl lapse rates, combining with an increasing nocturnal LLJ should be able to produce one or more clusters of stronger storms moving eastward towards central SD into the overnight hours. However soundings present more and more inhibition as storms move into the CWA.

TUESDAY: A bit of a rinse and repeat forecast for Tuesday as we may again start the day with stratus and some fog and even a few lingering elevated thunderstorms. Gradual upslope flow on the Plains Tuesday afternoon/evening may again develop one or more clusters of convection that may move towards the CWA into Wednesday morning.

WEDNESDAY: Wednesday continues to look like a quiet day in the region with more influence from minor ridging in the Plains. Temperatures with a light wind will rise towards the lower to middle 80s. By the overnight hours, some potential for MCS activity in KS/NE will attempt to move eastward and could bring some thunderstorm risks to the MO river valley.

THURSDAY-SATURDAY: Confidence remains rather low in the extended as the mid-lvl pattern shifts from a quasi-northwest flow to zonal by the end of the week. Several waves are likely to traverse these westerlies in the weekend, but a wide array of solutions continues to keep lower PoPs (20-30%) for multiple days. In reality, chances are likely going to be more limited than the NBM would suggest, however model spread will need to tighten before a better timing could be had. Confidence is a bit higher on the slight cooldown of temperatures into the 70s into next week.

AVIATION /18Z TAFS THROUGH 18Z MONDAY/

Issued at 1204 PM CDT Sun Aug 9 2026

MVFR ceilings will persist into the early and middle afternoon hours. We'll continue to see light winds, with ceilings gradually mixing upwards to VFR levels by mid-afternoon. Further west, very high based AC field AOA 15K ft AGL will move eastward. There could be enough lift in this layer for a few sprinkles, but a really dry airmass continues between this layer and the low strato-cu layer suggesting nothing will reach the ground.

Into the overnight hours, the only really strong potential for convection will be in western SD and perhaps central SD after midnight. Though significant uncertainty remains on if storms will actually develop. Further east, residual moisture could lead to some low stratus or fog development.

FSD WATCHES/WARNINGS/ADVISORIES

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