A small mesoscale convective cluster of storms developed in an area of steep mid-level lapse rates and marginal deep-layer shear. The northern edge of the cluster brushed into far southern South Dakota, with an isolated incident of large hail near the Missouri River.
Every event with a place to put it. Tap one for its entry below.
Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS FSD wrote it up.
A small mesoscale convective cluster of storms developed in an area of steep mid-level lapse rates and marginal deep-layer shear. The northern edge of the cluster brushed into far southern South Dakota, with an isolated incident of large hail near the Missouri River.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Sioux Falls, SD wrote its last area forecast discussion before the first event at Aug 25, 2:24 PM CST, 32 minutes earlier.
AREA FORECAST DISCUSSION National Weather Service Sioux Falls SD 324 PM CDT Wed Aug 25 2021 .SHORT TERM...(This Evening through Thursday Night) Issued at 317 PM CDT Wed Aug 25 2021 This afternoon, a weaker MCS has formed oriented N-S as a weak wave transversed the upper ridge, with one stronger cell on the southern end tracking east across the MO River Valley. See no reason why this cell wouldn't continue to track east along the instability gradient into the early evening. Given favorable deep layer shear, weak instability, and strong mid level lapse rates present, could see isolated large hail up to 1 inch in diameter and 60 mph wind gusts possible. Early this evening, most model guidance supports develop of scattered elevated showers or storms lingering central to eastern SD as the right entrance region of the northern SW-NE oriented jet builds east into the Northern Plains. While a stray strong to severe storm cannot be entirely ruled out amidst broad weaker ascent aloft, most model soundings keep the environment capped further north. The more mixy GFS and HRRR solutions continue to suggest that areas of evening convection are possible if the instability gradient can continue to shift northward this evening, but exactly where is uncertain with the GFS/HRRR taking a more southerly solution and the NAM much further north near the Hwy 14 corridor. The latest deterministic runs have somewhat better agreement on where the best chance for severe weather resides overnight and point to two areas of focus: northern and east central SD to western MN, and further south near the MO River Valley into northwest IA. This is dependent on how far north the LLJ, and thus the instability axis, extends and when a weak wave to the north tracks into the region (overnight vs early in the morning). There is a signal for both areas, or perhaps one and not the other, to materialize, depending on how evening activity and the LLJ evolve. Ultimately, the greatest severe weather threat likely resides from 4-10 AM Thursday morning, as favored by the 12z HREF UH tracks. Anticipate more isolated severe weather with a southern MCS (MO River Valley/Hwy 20 or further south) and potentially scattered to numerous strong to severe storms with an elevated MCS further north closer to the Hwy 14 corridor and extending into southwest MN. Storms should initially develop in clusters, merging into an MCS as motion would be more parallel to the front/instability gradient, with the main threats being large hail and damaging wind gusts. Pwat values of 1.5-2.0+ and storm training will support locally heavy downpours with isolated amounts up to 2-3 inches possible, especially in southwest MN. While there is some nice low level hodograph curvature, 0-3 km shear vectors remain parallel to the line so deem tornado threat unlikely at this time. The progression of one or several morning MCSs will greatly impact the evolution of the warm front on Thursday with the deterministic guidance still very split on solutions. Should see a lull in activity with some instability recovery around mid day to early afternoon, despite a cloudy, muggy day unfolding. There is high uncertainty in the timing/location of the next round of severe weather on Thursday. For example, the 12z NAM keeps convection going on and off during the day, preventing any organized stronger complex from forming until later Thursday evening, and the 12z GFS holds strong capping for a quiet day and early evening initiation. As mentioned in previous discussions, the environment remains favorable for severe weather throughout Thursday and Thursday night as strong deep layer shear, steep mid level lapse rates, and moderate instability linger in the warm sector across the region. Be prepared for more isolated severe threat during the day Thursday after the initial morning MCS, with more widespread severe threat with any complex developing late afternoon or through Thursday night. All threats will be possible with activity Thursday afternoon and night, including isolated tornadoes and localized flash flooding. Most model guidance supports locally heavy downpours and isolated 1- 3 inch amounts again Thursday night, though this time the atmosphere is even juicier with 1.5-2.5 inch Pwats and the MCS would likely have a longer residence time in the region, as highlighted by the Day 2 WPC Excessive Rainfall Outlook. If some locations, especially in southwest MN, get multiple rounds of 1-3 inches of rain over the next 2 days, wouldn't be surprised to have some flash flooding issues. However, given the uncertainty in any MCS tracks over the next few days, held off on a Watch issuance for now. .LONG TERM...(Friday through Wednesday) Issued at 317 PM CDT Wed Aug 25 2021 Main concern through extended period continues to be several periods of potential for severe weather with the greatest threat developing Saturday/Saturday night. Friday looks a bit quieter for the region behind the departing system. The unstable warm sector should recover and build further north through the region during the day, leading to another warm muggy day. Despite enhanced 500mb jet and moderate instability, latest model soundings maintain capping in the forecast area and slide a strong shortwave to the north across ND and northern SD/MN. This would likely keep severe weather chances north of the region (unless the wave tracks further south). On Saturday, the upper trough meandering along/north of the International Border finally digs into the Northern Plains. Height falls aloft combined with upper jet right entrance ascent, moderate to strong instability in the warm sector ahead of an approaching sfc cold front, and increasing mid level winds support the better chance of severe weather through this active week, as highlighted by the SPC Day 4 threat area. There is low confidence in details such as timing or sub-region with highest threat at this range, but the Saturday afternoon/night period certainly bears watching for severe weather threat in the coming days given the strong forcing for ascent. Behind Saturday's departing system, Sunday is poised to be cooler and dry/quiet. Early next week, zonal flow aloft transitions to ridging building over the Rockies, which favors some periods of rain chances, but progression of the upper ridging and resultant warming is very uncertain as the Gulf is poised to become more active again. && .AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon) Issued at 1242 PM CDT Wed Aug 25 2021 High based showers and storms with erratic winds may impact the Huron to Chamberlain this afternoon, but isolated coverage means any impacts should be short lived. MVFR stratocumulus will linger near SUX early this afternoon as well, with mainly VFR conditions prevailing elsewhere. Expect northeast winds with gusts up to 25 kts this afternoon, weakening some and turning more easterly overnight. Main concern for tonight/Thursday is convective development and coverage. May see isolated showers and storms continue this evening in SD, but better chance of more widespread storms developing or moving in will be from 09-15z. During this early morning period, storms may become strong to severe with gusty erratic and perhaps damaging winds, large hail, downpours, and frequent lightning. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...NONE. MN...NONE. IA...NONE. NE...NONE. && SHORT TERM...BP LONG TERM...BP AVIATION...BP
Hazardous Weather Outlook National Weather Service Sioux Falls SD 528 AM CDT Wed Aug 25 2021 IAZ001>003-012>014-020>022-031-032-MNZ071-072-080-081-089-090-097- 098-NEZ013-014-SDZ038>040-050-052>071-261030- Lyon-Osceola-Dickinson-Sioux-O'Brien-Clay-Plymouth-Cherokee- Buena Vista-Woodbury-Ida-Lincoln-Murray-Cottonwood-Nobles-Jackson- Pipestone-Rock-Dixon-Dakota-Beadle-Kingsbury-Brookings-Gregory- Jerauld-Sanborn-Miner-Lake-Moody-Brule-Aurora-Davison-Hanson-McCook- Minnehaha-Charles Mix-Douglas-Hutchinson-Turner-Bon Homme-Yankton- Union- 528 AM CDT Wed Aug 25 2021 This Hazardous Weather Outlook is for northwest Iowa, west central Iowa, southwest Minnesota, northeast Nebraska, central South Dakota, east central South Dakota, south central South Dakota and southeast South Dakota. .DAY ONE...Today and tonight. Isolated thunderstorms will be possible north of Interstate 90 during late afternoon into early evening, with only a very remote chance of a marginally severe storm with large hail and damaging wind. Scattered thunderstorms will develop across the area, mainly after midnight. Isolated storms could become severe with hail up to ping pong ball size and damaging wind gusts to 65 mph. Some areas of heavy rainfall are also possible that could result in ponding in urban and poor drainage areas. .DAYS TWO THROUGH SEVEN...Thursday through Tuesday. Thunderstorms will continue into Thursday before tapering off during the day. Some areas of heavy rainfall will remain possible in the morning. Storms will redevelop Thursday night, with another threat for severe storms with large hail and damaging wind. Additional storms on Friday night will be mainly focused north of Interstate 90, and a few of these could also be severe. Finally, thunderstorms are likely to develop along a cold front later Saturday afternoon and Saturday evening, some of which could again be severe. .SPOTTER INFORMATION STATEMENT... Spotter activation is not expected at this time, as the threat of severe storms is mainly after midnight.
The Sioux Falls radar (KFSD), 125 km away, was scanning. The volume nearest the first event began at 20:58 UTC.
1 hail. Local standard time, as filed.
Dime size hail was reported with occasional ping-pong size hail. Crop damage is an estimate from insured losses. Information provided by the United States Department of Agriculture.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in South Dakota in 2021Every state in 2021This day in weather historyWarnings in South Dakota, August 2021
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 982305.