A strong mid-level wave pushed southeast toward southwest Minnesota overnight and the early morning hours on July 19. A large cluster of storms developed across eastern South Dakota after midnight, and with a high moisture content to the air mass, rainfall rates were quite high.
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The episode as NWS FSD wrote it up.
A strong mid-level wave pushed southeast toward southwest Minnesota overnight and the early morning hours on July 19. A large cluster of storms developed across eastern South Dakota after midnight, and with a high moisture content to the air mass, rainfall rates were quite high. One storm on the southern edge of the complex intensified, with strong 0-3km shear with low-level jet contributing to a brief meso-vortex development as the storm moved into the southern side of Brookings, SD.
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 Jul 18, 9:51 PM CST, 4 hours 9 minutes earlier.
Area Forecast Discussion National Weather Service Sioux Falls SD 1051 PM CDT Wed Jul 18 2018 .SHORT TERM...(This evening through Thursday) Issued at 445 PM CDT Wed Jul 18 2018 Focus for the forecast was generally on the first 24-36 hours today with the multi-faceted threat for severe storms and heavy rainfall. Convection fired early this afternoon near K9V9 near a broad warm frontal boundary as leading PV impulse pushing through central SD. Despite not being too strongly capped, was generally a solo cell until between 20-21z when additional destabilization with temps closer to 80 degrees expanded impact of lift near and north of the warm front. Surface boundary extends northwest back to low pressure which is a reflection of a secondary upper wave noted back across northwest SD. Atmosphere is not highly unstable with deeper moisture remaining locked up through the central Plains, but enough for up to 1500 J/kg MLCAPE. Deep-layer shear is primarily focused through the Missouri River corridor with 35-45 knots, lesser to areas northeast, but also fairly healthy 30-40 knots from 0-3km. Area of current TOR watch covers the stronger parameters late this afternoon and early this evening with good updraft acceleration from higher axis of 0- 3km CAPE concentrating along presence of west-northwest to east- southeast convergence axis, as well as low LCLs to suggest the better tornadic threat. Threat should continue into early evening with some tendency for evolution toward shorter bowing segments featuring wind and marginally large hail. However, as advertised more on than off in the high resolution solutions over the last 3-6 hours, a developing line of storms back toward the secondary PV /jet exit maximum across northwest SD is expected to evolve into a fairly distinct and larger scale bowing segment, which should propagate down the instability/shear axis through the Missouri River corridor later in the evening and early in the overnight with a likely stronger threat for damaging wind. 0-3km shear vectors would introduce potential for some smaller enhancements and perhaps mesovortex generation along the line during the evening. Better wind threat looks to diminish after 06z-07z around KSUX as the stronger area exits southeast. There will also be a threat for heavy rainfall over the next 18-24 hours as well. Precipitable water is around 1.5-1.7 inches in the pre-convective environment. The earlier storm across Brule county, despite a small stature, was able to produce a measured rainfall over 2" in an hour at K9V9, and much more estimated to the south where flash flooding occurred earlier in the afternoon. Slow motion to storms along with some backbuilding tendency will likely produce some locally heavy rainfall heading into the early evening hours. After this, the heavier precipitation threat may refocus toward areas east of the advancing mid-level wave sliding toward southwest MN later tonight and early Thursday morning, where a more broad and heavy rainfall could occur. Trend in solutions is to wrap up the mid-level circulation further south and slower as anticipated yesterday as it shifts from northeast South Dakota into southern MN through the day. For a brief time could see most precipitation lift north and east of the CWA, but southward wrap to feature should return precipitation as well as some stronger winds around low pressure in southwestern MN. Could be a very strong gradient in temps across the area on Thursday, especially if convection continues through much of the day across southwest MN. Should at least vary from mid 70s in southwest MN to upper 80s in south central SD. On the edge of clouds/precipitation and along the convergence axis near the eastern portions of southwest MN/northwest IA, could see a couple of random stronger storms tomorrow afternoon. The shear is fairly concerning from IA Great Lakes southward if can couple with some greater instability, perhaps enough for a low-topped tornadic threat toward Highway 71. .LONG TERM...(Thursday night through Wednesday) Issued at 445 PM CDT Wed Jul 18 2018 As the low continues to wrap up and pull away on Friday, should remain cool enough aloft, cyclonic enough at low-levels, and moist enough to bubble up isolated instability showers/storms on Friday afternoon near/east of I-29. After this, high pressure will present a terrific weekend for most with temps a bit below normal, a drop in humidity, and winds decreasing from a north to northeast direction through Saturday. The upper ridge will be knocked down Sunday by a weaker leading wave which could push a couple of storms into the James River valley within the elevated warm advection zone, with a better chance for thunderstorms across the area with a cold frontal zone later Sunday night into Monday. Another northwest flow system around Tuesday likely to produce a scattering of showers/storms, with temps increasingly below normal early next week. && .AVIATION...(For the 06Z TAFS through 06Z Thursday night) Issued at 1044 PM CDT Wed Jul 18 2018 Upper low over eastern South Dakota will slowly inch east. Waves of showers and thunderstorms are possible over the next 24 hours. Could see some lower ceilings develop east of the James River in the early morning hours on Thursday. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...None. MN...None. IA...None. NE...None. && SHORT TERM...Chapman LONG TERM...Chapman AVIATION...BT
Hazardous Weather Outlook National Weather Service Sioux Falls SD 338 AM CDT Wed Jul 18 2018 NEZ013-014-SDZ050-057>060-063>065-068>070-190845- Dixon-Dakota-Gregory-Brule-Aurora-Davison-Hanson-Charles Mix- Douglas-Hutchinson-Bon Homme-Yankton-Clay SD- 338 AM CDT Wed Jul 18 2018 This Hazardous Weather Outlook is for portions of northeast Nebraska, south central South Dakota, and southeast South Dakota. .DAY ONE...Today and Tonight Severe thunderstorms are possible this afternoon with hail up to quarter size and wind gusts up to 60 mph being the main threats. Additionally, an isolated tornado cannot be ruled out. Furthermore, heavy rain may become an issue this afternoon through tonight due to expected slow movement of showers and thunderstorms. Expect half an inch to an inch and a half of rainfall with locally higher amounts possible. .DAYS TWO THROUGH SEVEN...Thursday through Tuesday Thursday through Tuesday look to be mostly uneventful, with Friday and Sunday having a slight chance for thunderstorms. At this time, any thunderstorms are not expected to be severe. .SPOTTER INFORMATION STATEMENT... Spotter activation may be needed between 1 pm and 7 pm this afternoon. IAZ001-012-020-031-SDZ038-039-052>055-061-062-066-067-071-190845- Lyon IA-Sioux-Plymouth-Woodbury-Beadle-Kingsbury-Jerauld-Sanborn- Miner-Lake-McCook-Minnehaha-Turner-Lincoln SD-Union- 338 AM CDT Wed Jul 18 2018 This Hazardous Weather Outlook is for portions of northwest Iowa, west central Iowa, central South Dakota, east central South Dakota, and southeast South Dakota. .DAY ONE...Today and Tonight Severe thunderstorms are possible this afternoon with hail up to quarter size and wind gusts up to 60 mph being the main threats. Furthermore, heavy rain may become an issue this afternoon through overnight tonight due to expected slow movement of showers and thunderstorms. Expect half an inch to an inch and a half of rainfall with locally higher amounts possible. .DAYS TWO THROUGH SEVEN...Thursday through Tuesday Thursday through Tuesday look to be mostly uneventful, with Friday and Sunday having a slight chance for thunderstorms. At this time, any thunderstorms are not expected to be severe. .SPOTTER INFORMATION STATEMENT... Spotter activation may be needed between 1 pm and 7 pm this afternoon. IAZ002-003-013-014-021-022-032-MNZ071-072-080-081-089-090-097-098- SDZ040-056-190845- Osceola-Dickinson-O'Brien-Clay IA-Cherokee-Buena Vista-Ida- Lincoln MN-Lyon MN-Murray-Cottonwood-Nobles-Jackson-Pipestone- Rock-Brookings-Moody- 338 AM CDT Wed Jul 18 2018 This Hazardous Weather Outlook is for portions of northwest Iowa, west central Iowa, southwest Minnesota, and east central South Dakota. .DAY ONE...Today and Tonight Heavy rain may become an issue this afternoon through overnight tonight due to expected slow movement of showers and thunderstorms. Expect half an inch to an inch and a half of rainfall with locally higher amounts possible. .DAYS TWO THROUGH SEVEN...Thursday through Tuesday Thursday through Tuesday look to be mostly uneventful, with Friday and Sunday having a slight chance for thunderstorms. At this time, any thunderstorms are not expected to be severe. .SPOTTER INFORMATION STATEMENT... Spotter activation will not be needed today and tonight.
The Sioux Falls radar (KFSD), 81 km away, was scanning. The volume nearest the first event began at 07:58 UTC.
4 flash flood, 1 thunderstorm wind. Local standard time, as filed.
Third Street in Brookings is under water due to torrential rainfall.
Multiple mature trees blown over and bleachers flipped at the Southbrook Softball Complex. Dugouts damaged by blowing debris. Report from social media.
Multiple streets flooding in Brookings. Rainfall totalled 6.83 inches 2 miles northeast of Brookings, 7.50 inches on the southeast side of Brookings and 9.00 inches at Aurora.
Sixmile Creek at gaging site SMCS2 rapidly rose above flood stage at 0400CST and reached moderate flood stage at 0745CST. Rainfall of 6.83 inches was measured 2 miles northeast of Brookings. Numerous township roads were flooded north of Brookings.
Water over U.S. Highway 14. Report received through social media.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in South Dakota in 2018Every state in 2018This day in weather historyWarnings in South Dakota, July 2018
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 2026-09-30. Each event’s entry at NCEI: 773526, 773529, 773527, 773540, 773528.