A mid-level disturbance moving from Wyoming into South Dakota initiated late evening convection in an environment with around 2000 J/kg MLCAPE and deep-layer shear of around 50 knots. Storms expanded and rapidly evolved into a large-scale bow echo.
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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 mid-level disturbance moving from Wyoming into South Dakota initiated late evening convection in an environment with around 2000 J/kg MLCAPE and deep-layer shear of around 50 knots. Storms expanded and rapidly evolved into a large-scale bow echo.
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 27, 11:00 PM CST, 54 minutes earlier.
AREA FORECAST DISCUSSION National Weather Service Sioux Falls SD 1200 AM CDT Fri Aug 28 2020 .UPDATE... Issued at 806 PM CDT Thu Aug 27 2020 No significant changes to the forecast thinking for the overnight severe weather potential. Lift is now ejecting off the Rockies into the Plains with thunderstorms erupting ahead of it across the western Dakotas and northwest Nebraska. Expectation is still for this activity to eventually congeal into one or more bowing segments as it approaches central and eastern South Dakota and then move east into southwest Minnesota into the early morning hours Friday. Timing still appears to very much to be an overnight event for our coverage area with storms making into toward a Chamberlain to Huron line closer to midnight and the I-29 corridor likely toward the 2-4am timeframe. Severe threat is looking to come into perhaps a bit clearer focus with mid level temps favoring the strongest storms to ride near and especially north of I-90. This would also coincide with the best convergence on the nose of the LLJ. SPC's latest outlooks has peeled back the Enhanced risk to these areas and matches local thinking relatively closely for the greatest risk area. Damaging wind gusts to around 80 mph continues to be the main concern with DCAPE values in excess of 1000 J/kg. Also of concern would be the potential for a QLCS tornado or two within any organized bowing segment with 0-3 km shear vectors at 30 to 35 kts perpendicular to any expected linear segment orientation along the Hwy 14 corridor. Latest high-res guidance would put the greatest concern from Huron to Brookings to Marshall to Windom for perhaps the strongest activity. Trailing showers and weaker thunderstorms may very well linger into mid morning Friday in some areas. && .SHORT TERM...(This Evening through Friday) Issued at 330 PM CDT Thu Aug 27 2020 At mid afternoon, frontal boundary has slowed along a line from near the Iowa Great Lakes to Vermillion. Warm and humid air either side of the boundary has brought some heat indices well into the 90s, but the greater concern in the short term is the convective threat overnight. Has been some random mid-level convection ongoing much of the day from western into central SD, so far not amounting to much with perhaps 1000-1500 J/kg available to a mixed-layer rooted cell, so elevated nature speaks for less than that at this point. This cluster between K9V9 and KPIR should move into the mid-James River valley later afternoon within the warm advection zone/along edge of the steeper mid-level lapse rates. Not out of the question that rooting into the boundary layer could give a couple of cells a chance for an early start to severe storms, but this is an unlikely scenario at this point. Much more likely that wave moving across northern WY is likely to initiate some high plains convection very late afternoon into early evening. Storms will have environment of decent instability (1500- 2000 J/kg MLCAPE) and appreciable shear with strong mid-level westerlies, with a solid 45-55 kts in the 0-6km layer and not a great deal less cutting off the lowest km. Low-level moisture transport solidly back toward central and western SD, and initiation of more significant convection likely to feed on this as the low- level jet veers eastward through the night. Should be a fairly quick transition over to bowing structures with enough DCAPE to support good outflow pools. 0-3km shear vectors look to become quite favorably oriented to any meridional linear structures, suggesting that potential for mesovortices which in the least could support enhanced wind potential, and even a tornadic threat when located in a less stable environment. Potential is there, especially for areas near and north of I-90. However, this is not a slam dunk in terms of severe potential by any means, as the impact of the still +12 to +15C 700hPa temps over much of the area from near I-90 and south erode confidence. Could help to divert a greater portion of the severe threat to near/north of highway 14 and into southwest MN. Still, cannot discount that even areas southward toward the Missouri River could see some stronger to potential severe storms heading through the night as outflow pools are often underestimated. On Friday, boundary will not hang around too long across parts of northwest Iowa, before northerly flow freshens over the entire area. Some lingering convective threat as the wave shears through, interacting with whatever boundary from overnight convection resides across northwest IA and northeast NE. Secondary troughing falls through by early to mid afternoon, and could see a few showers/storms in the cyclonic flow even into the latter afternoon in southwest MN. Temps begin to find some upper 70s for highs in the north, with 80s elsewhere. .LONG TERM...(Friday Night through Thursday) Issued at 330 PM CDT Thu Aug 27 2020 The start of the weekend will be seasonable and quiet as deeper ridge builds back across the area. Highs upper 70s to lower 80s with no appreciable clouds or wind. By Saturday night, will begin to see warm advection develop back through central SD with elevated CAPE up to 250-500 J/kg which pushes toward the I-29 corridor but decreases by midday Sunday. Scattered convection seems likely, at least during the late night and early morning. The system Sunday night is very dynamic, and despite a cold frontal forcing for storms which typically does not perform well given the col structure to the pressure field, the degree of instability with limited moisture as sharp wavelength trough digs across the region should lead to a fairly solid threat for thunderstorms for most. While not mentioned, kinematics would suggest at least a nominal threat for a few severe storms even being somewhat off the diurnal cycle. Early to mid-week looks more quiet again. Some subtle chances for storms. A secondary trough around Monday night or early Tuesday really splits around the area, with the greater forcing diving southward through the western/central Plains. A few higher based showers/storms would be all that one might expect with this pattern. What is more certain is a least a couple days of seasonably cool temps ending August and starting September, with highs Monday perhaps not escaping the upper 60s for some spots north of I-90. A bit better mixed and slightly warmer, especially Wednesday, before another cooler shot in stronger northwest flow aloft by the end of the week. && .AVIATION...(For the 06Z TAFS through 06Z Friday night) Issued at 1154 PM CDT Thu Aug 27 2020 Main aviation concern continues to revolve around thunderstorms during the overnight hours. Organized cluster of storms continues to push toward KHON and has a history of producing winds in excess of 50 kts. It appears this cluster may stay just north of I-90, perhaps only clipping KFSD, if impacting it at all other than with showers and weaker thunderstorms. KSUX looks to stand the lowest chance of any thunderstorm activity. Lower ceilings and vsbys may accompany any thunderstorm activity. Showers may linger into mid morning Friday with the potential for a few additional showers by the afternoon. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...NONE. MN...NONE. IA...NONE. NE...NONE. && UPDATE...Kalin SHORT TERM...Chapman LONG TERM...Chapman AVIATION...Kalin
Hazardous Weather Outlook National Weather Service Sioux Falls SD 437 PM CDT Thu Aug 27 2020 IAZ001>003-MNZ071-072-080-081-089-090-097-098-SDZ038>040-050-052>067- 282145- Lyon-Osceola-Dickinson-Lincoln-Murray-Cottonwood-Nobles-Jackson- Pipestone-Rock-Beadle-Kingsbury-Brookings-Gregory-Jerauld-Sanborn- Miner-Lake-Moody-Brule-Aurora-Davison-Hanson-McCook-Minnehaha- Charles Mix-Douglas-Hutchinson-Turner- 437 PM CDT Thu Aug 27 2020 This Hazardous Weather Outlook is for northwest Iowa, southwest Minnesota, central South Dakota, east central South Dakota, south central South Dakota and southeast South Dakota. .DAY ONE...Tonight. Thunderstorms will develop across western and central South Dakota by early evening, then progress eastward across southeast South Dakota, southwest Minnesota and northwest Iowa overnight. It appears more probable that storms will organize into bowing segments, with a primary threat for damaging wind up to around 80 mph during the late evening and overnight hours. Some large hail up to half dollar size is also possible, with the greatest threat for hail west of Interstate 29. An isolated tornado is also possible. .DAYS TWO THROUGH SEVEN...Friday through Wednesday. Additional thunderstorms are possible on Sunday night. A few storms may be severe. .SPOTTER INFORMATION STATEMENT... Spotter activation may be needed late this evening and overnight. IAZ012>014-020>022-031-032-NEZ013-014-SDZ068>071-282145- Sioux-O'Brien-Clay-Plymouth-Cherokee-Buena Vista-Woodbury-Ida-Dixon- Dakota-Bon Homme-Yankton-Union- 437 PM CDT Thu Aug 27 2020 This Hazardous Weather Outlook is for northwest Iowa, west central Iowa, northeast Nebraska and southeast South Dakota. .DAY ONE...Tonight. By very late evening and early in the overnight, thunderstorms will move in from the west. A few of these storms could be severe with hail up to half dollar size and wind gusts to 70 mph. .DAYS TWO THROUGH SEVEN...Friday through Wednesday. Additional thunderstorms are possible on Sunday night. A few storms may be severe. .SPOTTER INFORMATION STATEMENT... Spotter activation may be needed late this evening and overnight.
The national radar mosaic for the moment the first event began.
16 thunderstorm wind, 2 tornado. Local standard time, as filed.
South Dakota RWIS station SD582, 3 miles north northwest of Wolsey, measured a peak wind gust of 61 mph.
A camper was flipped by strong thunderstorm winds. Property damage cost is a rough estimate.
A power pole was snapped off by thunderstorm winds. Property damage cost is a rough estimate.
A portion of a garage roof was torn off. A car and boat were rolled over near the house. Property damage costs are rough estimates.
A personal weather station measured a wind gust of 80 mph.
Automated weather station KHON near Huron, SD measured a peak wind gust of 84 mph.
Thunderstorm winds gusted several minutes to around 70 mph.
South Dakota RWIS station SD515, three miles east of Cavour, measured a peak wind gust of 75 mph.
A tornado formed just southwest of the intersection of 203rd Street and 428th Avenue. |During a brief touchdown and eastward travel, damage was limited to crops, marked by ground scour noted on high-resolution satellite data. The tornado lifted around a quarter mile east of 428th Avenue. Crop damage is an estimate from insured losses. Information provided by the United States Department of Agriculture.
A tornado spun up west of 432nd Avenue and north of 211th Street. As the tornado traveled generally eastward, ground scour was noted to crops. The greatest damage occurred mid-track where a barn collapsed, with the debris blown downstream into another building. A few other outbuildings were also damaged. Large branches in tree breaks were snapped and the upper half of a concrete silo collapsed. The tornado dissipated one quarter mile east of 432nd Avenue around 3 miles south-southwest of De Smet. Property damage costs are rough estimates.
Winds tipped over a trailer and ripped siding off a tin shed. Several trees had broken limbs. Property damage costs are rough estimates.
Strong winds collapsed part of a concrete silo and severely damaged the roof and walls of an old barn and another farm outbuilding. Several large trees were snapped by thunderstorm wind. Property damage costs are rough estimates.
Tree branches from 2 to 6 inches in diameter were removed by thunderstorm winds. Property damage costs are rough estimates.
Several trees were downed on a property, and one large tree crushed a swing set. Property damage costs are rough estimates.
Scattered tree damage was reported in Bruce. Property damage costs are rough estimates.
South Dakota RWIS station SD547, three miles southeast of Montrose, measured a peak wind gust of 58 mph.
South Dakota RWIS station SD577, 10 miles south southwest of Aurora, measured a peak wind gust of 58 mph. Crop damage is an estimate from insured losses. Information provided by the United States Department of Agriculture.
South Dakota State University mesonet station at Oak Lake measured a peak wind gust of 59 mph.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in South Dakota in 2020Every state in 2020This day in weather historyThe tornado archive for South Dakota, 2020Warnings in South Dakota, August 2020
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: 915269, 918900, 915263, 915264, 915270, 915271, 915272, 915273, 915260, 915262, 915265, 918901, 915266, 915267, 915268, 915274, 915275, 915276.