An isolated supercell developed across south central South Dakota late evening along an advancing cold frontal boundary. With appreciable DCAPE and deep-layer effective shear, upscale growth occurred rapidly with the development of widespread damaging winds, which spread out of eastern South Dakota into northwest Iowa.
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The episode as NWS FSD wrote it up.
An isolated supercell developed across south central South Dakota late evening along an advancing cold frontal boundary. With appreciable DCAPE and deep-layer effective shear, upscale growth occurred rapidly with the development of widespread damaging winds, which spread out of eastern South Dakota into northwest Iowa.
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 Sep 16, 9:28 PM CST, 2 hours 22 minutes earlier.
AREA FORECAST DISCUSSION National Weather Service Sioux Falls SD 1028 PM CDT Thu Sep 16 2021 .UPDATE... Issued at 931 PM CDT Thu Sep 16 2021 Quick update this evening on the ongoing convection risks. We're beginning to see elevated post frontal convection from across an area from Gregory to Huron and Watertown. These storms are forming in an area of moderate mid-lvl lapse rates, MUCAPE lifted from the 700 mb surface near 1500 J/KG, and somewhat modest effective shear around 30 knots. HRRR/RAP continue to pick up on this development reasonably well, with the HRRR being a bit slow as usual. We'll continue to see rapid development over the upcoming two hours along this axis given expansive ACCAS field, and have adjusted PoPs accordingly. The questions overnight is how far east activity will remain strong or severe. The area with the greatest potential for more organized wind, will be with a thunderstorm cluster moving into south central South Dakota where there is a higher degree of dry air below the lifting layer further east, and only a modestly stable boundary layer immediately post frontal. Further north towards highway 14, feel the main threat would be from marginally severe hail at times given more stable boundary layer. While lapse rates and MUCAPE values trend downward as you reach I-29 overnight, even weakening thunderstorms could still pose a marginal severe wind risk into western Minnesota and Iowa given better perpendicular alignment of 700 mb flow to the orientation of the convection and that modest DCAPE layer. && .SHORT TERM...(This Evening through Friday) Issued at 310 PM CDT Thu Sep 16 2021 Breezy pre-frontal winds have occasionally gusted between 35-45 mph so far today with a gradual decrease expected into the evening hours. This is ahead of an incoming cold front that currently runs from roughly Webster, SD to Valentine, NE. This boundary is moving into an environment that very well may stay capped to any surface parcels although occasional forecast soundings show at least minor hints that surface development is possible with north central Nebraska standing the best chance. Should surface based development occur along the front through the early evening hours (unlikely), between 2000-2500 J/kg of MLCAPE and around 40 kts of deep layer shear would support supercell structures and all modes of severe weather. What seems far more likely to occur is CI waits until closer to mid evening as a subtle wave ejects out of the Nebraska Panhandle with parcels originating on a boundary in the 800:700 mb layer. This activity will have similar values of MUCAPE initially (decreasing into the overnight) along with 30-40 kts of effective shear (increasing as you move further post- frontal). In terms of hazards with any elevated storms... initial updrafts may pose a threat of large hail up to 1.5 inches in diameter and may very briefly take on quasi-supercell characteristics. Upscale development should rather quickly occur with updraft interference reducing the hail threat. With DCAPE values between 1000-1400 J/kg, should transition to more of a damaging wind threat up to 70 mph with any organized clusters, particularly ones that can organize with more of a north to south orientation more favorably aligned perpendicular to the 700 mb flow. Corfidi vectors look to keep enough progression and 3 hr flash flood guidance between 2-3 inches should keep the excessive rainfall risk on the lower end. With this said, PWATs do peak around the 90th percentile of climatology so any sustained training could result in localized amounts in excess of 2 inches. This is further supported by the LPMM from the HREF. All of the above mentioned risks are captured by a Slight/Marginal from SPC and Marginal from WPC. Could still see some lingering showers across NW IA around daybreak Friday before the front clears our forecast area by mid day. Behind the front, cool surface high pressure will result in a rather fall- like day with temperatures in the mid 60s to mid 70s. .LONG TERM...(Friday Night through Thursday) Issued at 310 PM CDT Thu Sep 16 2021 Upper level ridging builds and moves east through the weekend keeping our region void of any precipitation and with temperatures working back to the warm side of normal. Deterministic GFS continues to run on the warm side and generally above its ensemble mean and thus will hedge away from this solution. Generally expect mid 70s to mid 80s Saturday with more widespread 80s to lower 90s for Sunday. Should also see a noticeable southerly wind through the weekend. Amplified troughing slides across the Northern Plains for the first half of the work week. Depending on eventual timing, could very well see some additional convection chances for the second half of Monday into Monday night with locations near/east of I-29 currently most favored. Cooler temperatures settle in by the middle of the week with 60s to lower 70s for Tuesday and Wednesday. && .AVIATION...(For the 06Z TAFS through 06Z Friday night) Issued at 1024 PM CDT Thu Sep 16 2021 Convection will continue to track east overnight, impacting most of the region. Some stronger winds are possible along and south of Interstate 90, with marginally severe hail at times. Localized MVFR ceilings may briefly accompany the elevated frontal passage through daybreak, but generally we may continue to see VFR conditions. Based on observations further northwest, also possible to see a period of 30 knots winds briefly Friday morning. Skies will take a bit of time to clear through the day on Friday, with winds remaining quite breezy. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...NONE. MN...NONE. IA...NONE. NE...NONE. && UPDATE...Dux SHORT TERM...Kalin LONG TERM...Kalin AVIATION...Dux
Hazardous Weather Outlook National Weather Service Sioux Falls SD 334 PM CDT Thu Sep 16 2021 IAZ001>003-012>014-020>022-031-032-MNZ071-072-080-081-089-090-097- 098-NEZ013-014-SDZ038>040-050-052>071-172045- 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- 334 PM CDT Thu Sep 16 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...This afternoon and tonight. Showers and thunderstorms are expected to develop this evening and become more widespread after 9 pm. The main threats from any stronger storms will be hail to half dollar and wind gusts to 70 mph. Locally heavy rainfall will also be possible with isolated amounts around 2 inches possible. .DAYS TWO THROUGH SEVEN...Friday through Wednesday. Thunderstorms may be possible again Monday. The risk for severe weather remains uncertain. .SPOTTER INFORMATION STATEMENT... Spotter activation may be needed.
PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SIOUX FALLS SD
1051 PM CDT THU SEP 16 2021
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
1027 PM HAIL BURKE 43.18N 99.30W
09/16/2021 M0.75 INCH GREGORY SD TRAINED SPOTTER
ACCOMPANIED BY STRONG WINDS, BUT NO DAMAGE.
&&PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SIOUX FALLS SD
1112 PM CDT THU SEP 16 2021
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
1048 PM HAIL BONESTEEL 43.08N 98.95W
09/16/2021 E0.70 INCH GREGORY SD LAW ENFORCEMENT
RELAYED REPORT FROM A DEPUTY REPORTING DIMES
IN BONESTEEL, SD.
&&
ATPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SIOUX FALLS SD
1222 AM CDT FRI SEP 17 2021
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
1158 PM TSTM WND DMG HUMBOLDT 43.64N 97.08W
09/16/2021 MINNEHAHA SD 911 CALL CENTER
RELAYED REPORT OF HIGH WINDS LEADING TO
TRANSFORMERS BLOWN, WITH POWER OUTAGES BEING
REPORTED ACROSS THE AREA.
&&
ATPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SIOUX FALLS SD
1226 AM CDT FRI SEP 17 2021
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
1204 AM TSTM WND GST 10 SSW AURORA 44.15N 96.76W
09/17/2021 M62 MPH MOODY SD MESONET
MESONET STATION 224 SD RWIS WARD.
&&
ATPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SIOUX FALLS SD
1240 AM CDT FRI SEP 17 2021
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
1215 AM TSTM WND GST 3 SE BALTIC 43.73N 96.68W
09/17/2021 M73 MPH MINNEHAHA SD MESONET
MESONET STATION BTCS2 BALTIC 3 SE (SD
MESONET).
&&
ATPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SIOUX FALLS SD
1248 AM CDT FRI SEP 17 2021
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
1228 AM TSTM WND GST 3 SSW BERESFORD 43.04N 96.80W
09/17/2021 M63 MPH UNION SD MESONET
MESONET STATION 16 SD RWIS BERESFORD.
&&
ATThe Sioux Falls radar (KFSD), 69 km away, was scanning. The volume nearest the first event began at 05:51 UTC.
11 thunderstorm wind. Local standard time, as filed.
Iowa RWIS station FW6302 in Hawarden measured thunderstorm wind gusts.
Automated weather station KSXK measured thunderstorm wind gusts.
A large tree branch was downed by strong thunderstorm wind gusts.
Automated weather station KSHL measured thunderstorm wind gusts.
Iowa RWIS site near Ashton measured thunderstorm wind gusts.
A roof was completely torn off the end of a strip mall and deposited in pieces on the roadway. Property damage costs are a rough estimate.
Several power lines were downed by thunderstorm wind gusts.
A large construction shed was lifted and thrown into a nearby corn field. Property damage costs are a rough estimate.
Large tree branches were downed by thunderstorm wind gusts.
The doors of a school bus garage building were blown in by thunderstorm winds, which resulted in failure of the most of the roof and an exterior wall. Property damage costs are a rough estimate.
Large tree limbs were downed by thunderstorm wind gusts. 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 Iowa in 2021Every state in 2021This day in weather historyWarnings in Iowa, September 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: 985556, 985557, 985550, 985558, 985559, 985551, 985552, 985553, 985554, 987193, 985555.