textproduct: Sioux Falls

KEY MESSAGES

- Mostly dry this afternoon, but isolated thunderstorms may develop over SW MN and NW IA this evening. Marginally severe hail and 60 mph could be possible, but risks remain fairly low.

- Scattered showers and non-severe thunderstorms slide through the MO river valley early Wednesday. The rest of the day looks dry with highs in the 70s/80s.

- Late Thursday afternoon and evening thunderstorms possible along an approaching front. Best chances in Northern and Central MN, but isolated risks also further southwest. Large hail and localized strong winds main risks.

- Mostly dry conditions heading into the weekend with highs either side of 80 degrees. Morning rain on Sunday is possible.

DISCUSSION

Issued at 106 PM CDT Tue Aug 18 2026

THIS AFTERNOON: A weak frontal boundary continues to sink southward early this afternoon, with noticeable strato-cu field developing along the Highway 20 corridor. The remainder of the afternoon is likely to remain dry in most locations, outside of very isolated development along this aforementioned boundary.

THIS EVENING/TONIGHT: Some guidance shows isolated development this evening along a weak convergence zone in SW Minnesota and northern Iowa. This convection may also be sparked by a weak area of vorticity dropping through the Dakotas from the northwest. Any storm that develops will be in an area of MLCAPE AOA 1000-1500 J/KG, with sufficient shear to promote perhaps a rotating storm. Anticipating the severe weather risks to remain low.

Further west, additional convection is expected to form over the Black Hill region this evening as a stronger wave enters the area. The track of this wave, along with how the LLJ propagates overnight, should push some of this activity into south central SD after 1am. MUCAPE remains quite low, with marginal mid-lvl lapse rates, so widespread severe weather is not anticipated. Though, a few gusty wind gusts could remain possible through 3am. There remains reasonable agreement that this convection could track along the MO river valley into eastern Nebraska by daybreak. This is an area where higher PoPs will be focused.

WEDNESDAY: The departure of this shortwave should lead to a period of subsidence and quiet weather well into Wednesday. Cloud cover may be more substancial over the southern half of the CWA thanks to mid- upr clouds from convection. By the afternoon southwesterly surface winds will return, allowing high temperatures to rise into the upper 70s to lower 80s. Convection risks in the afternoon and evening may be quite narrowed to a zone ahead of an approaching surface front in north central SD. Further southeast, there doesn't seem to be any strong focusing mechanism, with weak convective inhibition likely keeping the area free of any isolated larger storm growth. While surface and low-lvl focus remains absent, we'll need to watch the presence of any weak vorticity in the NW flow. Further northwest, any convection that develops along the frontal boundary may try to track southeast, though lessening instability should limit the overall severe risks.

THURSDAY: Any precipitation risk continues to look minimal heading into Thursday morning as a stronger EML begins to build into the region. Southwesterly surface winds overnight may keep overnight low temperatures from falling significantly. The aforementioned surface boundary from Wednesday will sink southeast through the day on Thursday providing a focus mechanism for storms by late afternoon and evening. MLCAPE approaching 2000-3000 J/KG may develop along this boundary as dew points try to quickly surge northward. The lingering questions for convective development are focused on the track of mid-lvl vorticity in the late afternoon, along with convective inhibition ahead of the boundary. The track of shortwave energy could favor more development in central and northern Minnesota, with only isolated storms further southwest along the boundary. While instability is greater, shear is a bit more marginal suggesting less organization to anything that develops.

FRIDAY-MONDAY: Mid-lvl heights begin to rise over the Rockies as the ridge amplifies. The lingering, but sharper, NW flow pattern over the Dakotas may bring one more wave into the region Friday. However the southward passage of Thursdays front should push the best boundary layer moisture south of the area. The ridge axis begins to push eastward as we move into the upcoming weekend. A pleasant Saturday is likely in the region with highs either side of 80. Increasing low-lvl warm advection into Sunday could bring elevated showers and thunderstorms eastward into Sunday morning, along with more clouds and slightly warmer highs into the lower to middle 80s. At this time, rain coverage may be limited with only a 20-30% PoP.

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 1212 PM CDT Tue Aug 18 2026

Scattered CU will continue to develop this afternoon along a couple weak frontal boundaries. Some greater concentration of MVFR CU will be near Highway 20 into this evening. Winds will remain light from the west to northwest, though considerable variability will be expected this evening. A rogue thunderstorm could be possible along the Ridge of SW Minnesota this evening.

Elsewhere, VFR conditions persist as scattered convection forms in the West River areas overnight. While uncertainty persist in the track of convection overnight, the greatest potential will be along an west of the James River valley into daybreak.

FSD WATCHES/WARNINGS/ADVISORIES

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