An elevated supercell storm developed along a mid-level frontal boundary as a weak wave helped to increase and focus the low-level jet. With fairly weak low-level stability, severe winds easily reached the surface along the track of the storm as it transitioned to a linear structure.
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The episode as NWS FSD wrote it up.
An elevated supercell storm developed along a mid-level frontal boundary as a weak wave helped to increase and focus the low-level jet. With fairly weak low-level stability, severe winds easily reached the surface along the track of the storm as it transitioned to a linear structure.
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 26, 5:45 AM CST, 4 hours 12 minutes earlier.
AREA FORECAST DISCUSSION National Weather Service Sioux Falls SD 645 AM CDT Thu Aug 26 2021 .SHORT TERM...(Today and Tonight) Issued at 500 AM CDT Thu Aug 26 2021 Greatest concern in the short term surrounds the severe weather potential and flooding threat. Starting to see uptick in convection across northeast Nebraska as low-level jet begins to increase atop a fairly shallowly sloping frontal boundary, much to the dismay of CAMS. Environment is such that there is up to ~1500 J/kg of elevated CAPE with 1-6km shear on the order of 25-35 kts. Storms will be tough to organize past the cluster they have become, but enough for some briefly stronger updrafts (noted by strong cooling on IR tops) which could produce some hail up to half dollar size and perhaps even a rogue wind gust to 60-65 mph moving across far southeast SD, northeast NE and into western portions of northwest IA. Airmass progressively to the northeast is less conducive to maintaining strength, so should see a downtrend starting in a couple hours in intensity. Otherwise, a more elevated frontal boundary lies along the far northern portion of the area, and back across western/central SD where the low-level jet is currently fully focused ahead of the leading mid-level wave, have seen development of elevated supercells. A very strong consensus on mode (not timing) that storms will become bowing segments and make the turn to the east/southeast sometime early morning moving through north central SD, which then track through areas east of De Smet southeast through the Iowa Great Lakes area later morning through early afternoon. While airmass is not as potent further east in terms of both shear and elevated instability, the very organized storms should be capable of damaging wind and perhaps some isolated hail. Of great importance to the forecast will be the inevitable outflow pool which is handled very differently by various CAMS, but have tried to go more with the more intrusive solutions, which drive the boundary as far south as areas between I-90 and the Mo River, and further south in northwest Iowa. Likely a tail of convection will remain elevated above the cold pool into the midday as the leading wave moves past, and if not for the continued warm advection would have thought a brief down period would be possible most area. Storms near the boundary, especially if rooted near surface, could be quite feisty mid afternoon into the evening hours, especially as the low-level jet once again begins to enhance with an even stronger wave moving into the western plains this evening. If storms remain isolated near/south of I-90, especially with the enhanced 0-1km shear near the boundary, all modes of severe would be possible, from hail up to golf ball and a possible tornado, and then organization into short bowing structures would lead to winds perhaps as high as 75 mph. Deep-layer shear should be on the order of 35 to 45 knots, locally enhanced near the boundary, with potential instability around 2000-2500 J/kg in a reservoir to the south. If this is not enough, it is likely that storms will develop back toward central SD this evening near the wave and become very organized with the stronger winds aloft enhancing the deep-layer shear. These storms then march eastward/northeastward during the overnight as the low-level jet remains burly. These storms would have damaging wind as the greatest threat, with some threat for large hail on the exposed trailing southern flank. Repeated storms would set up potential for several inches of rainfall in spotty locations, especially heading toward southwest MN and northwest IA, perhaps even extending near I-90 in southeast SD. Excessive rain outlook remains in the slight category, but given the potential rain rates with the warm thermal profiles, would expect that at a minimum urban areas and small drainages will have challenges dealing with rainfall. Through Friday morning, HREF PMM precipitation shows a widespread 2-4 inches east of I-29, However, the location of likely heaviest precipitation remains at odds with confidence in the north/south location of the important focusing boundaries. .LONG TERM...(Friday through Wednesday) Issued at 500 AM CDT Thu Aug 26 2021 On Friday, appears there will still be some early day storms pulling away mainly through areas north of I-90/east of I-29. The boundary will likely be suppressed well south into NE/IA, but appears through the day as if it will surge northward toward the lower Missouri River valley into northwest Iowa. If so, will destabilize greatly to a near-surface condition with cap weakening late day as temps reach around 90 near the boundary and dew points are in the lower 70s. Likely around 25-35 kts of deep-layer shear and a festering axis of instability in place, so some loosely organized storms may develop in wake of the exiting wave, but coverage would likely be fairly sparse. All modes again possible during the afternoon and evening for northwest Iowa and adjacent areas. More storms possible later at night further northwest as the next wave moving across MT redirects low-level jet back into the western Plains. Potential that an organized cluster could brush into the highway 14 corridor later in the night, but should have a little more east northeast track to system. With as much convection as will be present in the intervening time, putting too much faith in details of instability and shear is a bit dubious beyond the gross estimates, and mainly looking at potential. The last threat of severe storms in this sequence looks to arrive during the later afternoon on Saturday into Saturday night, as the strongest wave of the period pushes into the ridge and drives a cold front through the region. Boundary should lift north of the area overnight Friday or early Saturday, and as long as the axis of likely 8-8.5C/km lapse rates does not keep accas going well into the day, should remain capped waiting for late day mid-level cooling and wind increase associated with the approaching wave. Day 3 outlook maintains a slight risk from around the KFSD area north/east, and this seems quite appropriate with the strong forcing, potential instability and degree of deep-layer shear. Sunday should provide a quiet period, cooler with reduced humidity levels behind the front. However, the larger scale will revert toward a more zonal pattern in transition to ridging once again early next week, and will likely see a period of warm advective thunder threat a couple times between Sunday night and Wednesday. && .AVIATION...(For the 12Z TAFS through 12Z Friday morning) Issued at 644 AM CDT Thu Aug 26 2021 Patches of MVFR ceilings will wander the area today, both in advance of convection early this morning, and forming within the cold pool expanding from a cluster of storms near KSUX early this morning, and moving in from the north/west toward KHON and KFSD during the later morning/early afternoon. Timing of thunderstorms will be very difficult, and have attempted to minimize mention in TAFs to the most probable periods, although there will be chances with persistent weak forcing throughout the period. Severe storms will be most probable for the afternoon and into the overnight hours, especially for KFSD/KSUX and eastward. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...NONE. MN...NONE. IA...NONE. NE...NONE. && SHORT TERM...Chapman LONG TERM...Chapman AVIATION...Chapman
Hazardous Weather Outlook National Weather Service Sioux Falls SD 553 AM CDT Thu Aug 26 2021 IAZ001>003-012>014-020>022-031-032-MNZ071-072-080-081-089-090-097- 098-NEZ013-014-SDZ038>040-050-052>071-271100- 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- 553 AM CDT Thu Aug 26 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. Several periods of thunderstorms will impact the area today and tonight. An isolated thunderstorm this morning from northeast Nebraska and extreme southeast South Dakota into northwest Iowa could yield quarter size hail or winds gusts near 60 mph. A second line of storms will drop southeast into east central South Dakota, southwest Minnesota and into northwest Iowa during the later morning through mid afternoon. A few storms could be severe with hail to ping-pong ball size and winds to 60 mph. Finally, thunderstorms are expected to redevelop later this afternoon and evening along the outflow from earlier convection near and south of Interstate 90, and progress eastward through the overnight. These storms would likely be the strongest of the events through tonight, and could produce hail up to golf ball size and damaging winds to 75 mph. An isolated tornado will also be possible near and south of Interstate 90. Rainfall could be locally heavy in areas east of the James River valley, especially east of Interstate 29. With potential for locally a few inches of rainfall through early Friday. At a minimum, urban areas and some lower spots are likely to see ponding in poor drainage areas. .DAYS TWO THROUGH SEVEN...Friday through Wednesday. Additional chances for severe storms will be present on Friday and Saturday. .SPOTTER INFORMATION STATEMENT... Spotter activation may be needed at times later today and tonight.
PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SIOUX FALLS SD
927 AM CDT THU AUG 26 2021
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0909 AM TSTM WND GST 2 NNW HURON 44.39N 98.23W
08/26/2021 M73 MPH BEADLE SD ASOS
ASOS STATION KHON HURON REGIONAL AIRPORT.
&&
AJPPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SIOUX FALLS SD
1008 AM CDT THU AUG 26 2021
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0950 AM TSTM WND DMG DE SMET 44.39N 97.55W
08/26/2021 KINGSBURY SD EMERGENCY MNGR
SPOTTER REPORTED DARK ROLL CLOUDS, SHORT
LIVED STRONG WINDS. REPORTED TREES DOWN AND
A SEMI ROLLED OVER OFF HWY 14, WEST OF TOWN.
STILL INVESTIGATING IF THAT WAS WIND
RELATED.
&&
AJPPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SIOUX FALLS SD
1032 AM CDT THU AUG 26 2021
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
1015 AM TSTM WND DMG ARLINGTON 44.37N 97.13W
08/26/2021 KINGSBURY SD LAW ENFORCEMENT
TREE FELL ON CAR IN ARLINGTON, SPOTTER ALSO
ESTIMATED WINDS TO BE 50-60 MPH. SHORT LIVED
EVENT.
&&
AJPThe Sioux Falls radar (KFSD), 104 km away, was scanning. The volume nearest the first event began at 15:57 UTC.
2 thunderstorm wind. Local standard time, as filed.
Thunderstorm wind gusts downed a few trees in Ivanhoe. Property damage costs are a rough estimate.
Thunderstorm wind gust was measured at automated weather station KMML. 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 Minnesota in 2021Every state in 2021This day in weather historyWarnings in Minnesota, 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: 982429, 982430.