A small complex of storms that moved out of northeast South Dakota late Saturday morning caused a concentrated area of straight-line wind damage. The Canby, Minnesota airport measured a wind gust of 72 mph.
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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 MPX wrote it up.
A small complex of storms that moved out of northeast South Dakota late Saturday morning caused a concentrated area of straight-line wind damage. The Canby, Minnesota airport measured a wind gust of 72 mph. These type of wind speeds continued across portions of Yellow Medicine and Lac Qui Parle counties where hundreds of trees and power lines were blown down. There was another wind gust of 68 mph at the Department of Transportation road sensor (Near Hanley Falls, MN) that went offline as the gust was recorded.
The worst area occurred in the city of Granite Falls where winds tore open two roofs and allowed water to pour through ventilation ducts into classrooms and hallways, causing hundreds of thousands of dollars in damage to the Yellow Medicine East School District building. The Granite Fun restaurant and golf course also sustained damage with 55 to 60 trees blown down. Due to the extensive damage, the city of Granite Falls declared a state of emergency.
Although wind damage occurred east of Granite Falls, mainly between Sacred Heart to Renville, this storm weakened as it moved eastward toward the western part of the Twin Cities.
A secondary area of storms developed across south central Minnesota during the mid-late afternoon near New Ulm, and produced another area of significant wind damage eastward toward Cannon Falls, and Red Wing. There was a swath of tree damage around Cannon Falls where one house lost its roof, with other having minor roof damage.
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 Twin Cities, MN wrote its last area forecast discussion before the first event at Aug 8, 11:28 AM CST, 14 minutes earlier.
Area Forecast Discussion National Weather Service Twin Cities/Chanhassen MN 1228 PM CDT Sat Aug 8 2020 .UPDATE... Issued at 1227 PM CDT Sat Aug 8 2020 Updated to include 18Z aviation discussion below. UPDATE && .SHORT TERM...(Today through Sunday Night) Issued at 300 AM CDT Sat Aug 8 2020 A potentially active weekend in terms of weather is on the horizon. The chance for strong storms exists right out of the gate this morning, as storms over northeast MN and northern WI shift into the west central WI portion of the forecast area. This activity could flare a bit after sunrise this morning as daytime heating ensues. The primary threat would be damaging winds. Given modest lapse rates around 7 C/km, large hail can't be ruled out, but is a slightly lesser threat. By mid to late morning we should see a break in any thunderstorm activity as we await the next round. Heat and humidity will build in the warm sector ahead of the front. Have increased temperatures into the mid to upper 80s across most of the area (slightly "cooler" in WI with low to mid 80s). Dewpoints will be pushing 75 degrees, which will mean heat indices will flirt with mid 90s. Given my forecast temps and dewpoints are near the higher end of numerical guidance, and considering the potential for clouds/convection to hinder heating a tad, will not issue a Heat Advisory, but highlight the threat in the HWO/graphical products. It will certainly be an uncomfortable afternoon for anyone active outside given the relatively light winds combined with the heat & humidity, as illustrated by WBGT (Wet Bulb Globe Temperature) values nearing 85 degrees this afternoon. The August-like humidity will also be revealed in progged MLCAPEs circa 4000 J/kg from southwest into central Minnesota this afternoon. H7-H5 lapse rates are also forecast to be around 8.5-9 C/km. Therefore, if we can get any convection to pop, strong to severe storms will be a decent possibility. The one big limiting factor will be warm H7 temps of +13C to +14C. Therefore, any afternoon activity would likely be isolated in nature, so a 20 POP seems sufficient for now. Tonight we expect to see a mid-level shortwave pass across the area, which will act to override any capping aloft and bring more widespread activity. Given the stalled boundary bisecting the area, precipitable water values around 2 inches, and forcing supplied by the shortwave, heavy rainfall is expected. Exactly where that sets up is still of low confidence, but as of now west central MN looks to have the greatest risk on the cool side of the boundary. Given antecedent soil conditions, thirsty crops, etc., think this area can withstand a fairly heavy bout of rainfall before we see much in the way of impacts, so will hold off on any flash flood headlines at this point. Showers and thunderstorms spread east across the area on Sunday attendant with the frontal passage. Steep lapse rates and low- level shear look sufficient to bring a threat for damaging winds. Sunday night differential divergence associated with the entrance region of the upper level jet streak over northwest Ontario supplies renewed forcing for showers and thunderstorms through Sunday night. .LONG TERM...(Monday through Friday) Issued at 300 AM CDT Sat Aug 8 2020 The long term period will start off cooler and drier before transitioning to a more typical August pattern. At the start of the period the cold front will have exited the CWA to the southeast and surface high pressure will move in behind it. High temperatures on Monday will be in the upper 70s with a few 80s possible. Drier air behind the cold front will drop the dewpoints back into the comfortable 50s where they will remain through Tuesday. Tuesday will be a few degrees warmer than Monday, but with relatively lower dewpoints should still feel reasonably comfortable. On Wednesday high pressure moves off to the east and warmer, more humid air starts to return to the area. Highs temperatures through the rest of the period will be seasonable in the low to mid 80s, and overnight lows mainly in the 60s. The weather should also become more active with shortwave energy bringing periodic chances for showers and thunderstorms through the rest of the long term. There is not much timing on the details of when and where any precipitation will occur, so will stick with the low PoPs from Wednesday on that were given by the NBM. && .AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon) Issued at 1227 PM CDT Sat Aug 8 2020 Anything but a high confidence forecast this go around with an unstable airmass and various weak forcing mechanisms in the area. Tried to time some convection this afternoon with the presence of upstream activity and some of the CAMs bringing activity through the central portion of the area. A better chance would seem to be later tonight when the shorwave trough over western South Dakota evident on water vapor imagery work into the area. Cloud cover remains an issue, with widespread MVFR ceilings slow to leave today. Satellite shows some mixing starting to occur, although debris clouds streaming in from the west could work against that for a time. However, continued with an improving trend in ceiling heights this afternoon. But, generally kept VFR and MVFR ceilings around at most sites through the period. KMSP...There is significant uncertainty in the likelihood and timing of any convection this afternoon and tonight, so it's likely that later forecasts will make some adjustments. Should see ceilings get into VFR fairly soon, but MVFR will return tonight, then should lift again sunday. /OUTLOOK FOR KMSP/ Sunday night...MVFR possible with a chance of showers and thunderstorms. South wind 5 to 15 kt becoming northwest. Monday...VFR. Northwest wind 5 to 10 kt. Monday night...VFR. Variable wind less than 5 kt. Tuesday...VFR. Southwest wind around 10 kt. Tuesday night...VFR. South wind around 5 kt. Wednesday...VFR. Southeast wind around 10 kt. && .MPX WATCHES/WARNINGS/ADVISORIES... MN...None. WI...None. && UPDATE... SHORT TERM...LS LONG TERM...CEO AVIATION...
Hazardous Weather Outlook National Weather Service Twin Cities/Chanhassen MN 956 AM CDT Sat Aug 8 2020 MNZ041>045-047>070-073>078-082>085-091>093-WIZ014>016-023>028- 091500- Douglas-Todd-Morrison-Mille Lacs-Kanabec-Stevens-Pope-Stearns- Benton-Sherburne-Isanti-Chisago-Lac Qui Parle-Swift-Chippewa- Kandiyohi-Meeker-Wright-Hennepin-Anoka-Ramsey-Washington- Yellow Medicine-Renville-McLeod-Sibley-Carver-Scott-Dakota- Redwood-Brown-Nicollet-Le Sueur-Rice-Goodhue-Watonwan-Blue Earth- Waseca-Steele-Martin-Faribault-Freeborn-Polk-Barron-Rusk- St. Croix-Pierce-Dunn-Pepin-Eau Claire- 956 AM CDT Sat Aug 8 2020 This Hazardous Weather Outlook is for portions of central and southern Minnesota...and west central Wisconsin. .DAY ONE...Today and Tonight. Thunderstorms are possible today and tonight. A few storms could become severe this afternoon and tonight, with the primary threats being large hail, damaging winds, and heavy rainfall. Heat and humidity will build today, with afternoon heat indices peaking in the low to mid 90s across the Minnesota portion of the outlook area. .DAYS TWO THROUGH SEVEN...Sunday through Friday. Additional thunderstorms are expected on Sunday and Sunday night. A few storms could become severe with the primary threats being large hail, damaging winds, and heavy rainfall. Periodic thunderstorms are possible Wednesday through Friday. Severe storms are not expected at this time. .SPOTTER INFORMATION STATEMENT... SKYWARN spotter activation may be needed this afternoon and evening.
Special Weather Statement National Weather Service Twin Cities/Chanhassen MN 1212 PM CDT Sat Aug 8 2020 MNZ054-056-064-081800- Chippewa-Yellow Medicine-Lac Qui Parle- 1212 PM CDT Sat Aug 8 2020 ...A STRONG THUNDERSTORM WILL AFFECT LAC QUI PARLE...NORTHWESTERN CHIPPEWA AND NORTHWESTERN YELLOW MEDICINE COUNTIES... At 1212 PM CDT, a strong thunderstorm was located 7 miles southeast of Marietta, or 21 miles northeast of Clear Lake, moving east at 40 mph. Winds in excess of 30 mph and pea size hail are possible with this storm. Locations impacted include... Montevideo, Madison, Dawson, Clarkfield, Watson, Boyd, Marietta, Cerro Gordo, Providence, Hazel Run, Montevideo Airport and Madison Airport. This includes the following highways... U.S. Highway 212 between mile markers 1 and 37. U.S. Highway 75 between mile markers 95 and 113. U.S. Highway 59 between mile markers 95 and 107. Torrential rainfall is also occurring with this storm, and may cause localized flooding. Do not drive your vehicle through flooded roadways. Frequent cloud to ground lightning is occurring with this storm. Lightning can strike 10 miles away from a thunderstorm. Seek a safe shelter inside a building or vehicle. This storm may intensify, so be certain to monitor local radio stations and available television stations for additional information and possible warnings from the National Weather Service. LAT...LON 4504 9645 4513 9570 4473 9571 4479 9645 TIME...MOT...LOC 1712Z 270DEG 35KT 4494 9632
Special Weather Statement
National Weather Service Twin Cities/Chanhassen MN
1238 PM CDT Sat Aug 8 2020
MNZ064-081815-
Yellow Medicine-
1238 PM CDT Sat Aug 8 2020
...AN AREA OF STRONG THUNDERSTORMS WILL AFFECT SOUTHWESTERN YELLOW
MEDICINE COUNTY...
At 1237 PM CDT, strong thunderstorms were clustered near Canby, or 16
miles east of Clear Lake, moving east at 40 mph.
Wind gusts up to 50 mph and dime size hail are possible with this
storm.
Locations impacted include...
Canby, Porter, Spring Creek, Burr and St. Leo.
This includes U.S. Highway 59 between mile markers 88 and 95.
Torrential rainfall is also occurring with this storm, and may cause
localized flooding. Do not drive your vehicle through flooded
roadways.
Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.
LAT...LON 4480 9645 4481 9621 4475 9575 4463 9582
4463 9645
TIME...MOT...LOC 1737Z 288DEG 34KT 4468 9635
ETAThe Sioux Falls radar (KFSD), 130 km away, was scanning. The volume nearest the first event began at 17:40 UTC.
12 thunderstorm wind, 1 hail, 1 lightning. Local standard time, as filed.
Several large trees were blown down or uprooted from Canby, and eastward toward St. Leo, and likely farther to the east along Highway 68. Due to the very rural area, and farmland of Yellow Medicine County, damage was isolated.
Several trees were blown down between Clarkfield to Hazel Run. Crop damage was also noted throughout the county.
There was damage in the city of Hanley Falls where a machine shed was blown down, with crop damage around the area. Upstream, and based on radar, isolated crop damage likely occurred from Hazel Run to Hanley Falls, and beyond as the core of the strongest winds moved across this area.
A measured wind report from the Department of Transportation wind sensor.
A measured wind report from a private weather station.
Widespread wind damage occurred around the city of Granite Falls. Numerous trees, power lines and structural damage occurred in the city with the Yellow Medicine East School District main building sustaining major damage. The winds tore off the heating and ventilation systems on the roofs, opening the duct work to the rain. Water poured through the ventilation system and directly into classrooms, hallways and on equipment. On the south side of the Minnesota River, the Granite Run golf course suffered significant damage to trees and their local restaurant.
Numerous trees and power lines were blown down on the north and northeast side of Granite Falls. There was a measured wind gust of 122 mph on a wind farm north of Granite Falls. However, this measurement was taken at 200 feet above the ground.
Numerous reports of downed trees and power lines from the eastern part of Granite Falls, eastward along Highway 212 to Sacred Heart. However, the damage became more sporadic eastward toward Sacred Heart.
A few large trees were blown down between Sacred Heart, and Renville, including a large tree that fell on a house in Renville.
There were multiple trees and power lines blown down from southwest of Cannon Falls, through Cannon Falls, and to the east of town. There were 3-4 houses that had roof damage, one of the houses had their entire roof blown off on Pine Street. One privately owned wind sensor between Cannon Falls and Red Wing had a report of 88 mph, but damage was too isolated for that type of wind severity.
A lightning bolt struck a park shelter where 40 people were attending a birthday party. Three individuals were injured, but with non-life-threatening injuries.
There was lots of wind damage in Red Wing where many trees and branches were blown down, including damage to roofs, and power outages throughout the city. There was a measured wind report from a private weather station of 61 mph west of Red Wing at 1635 LST.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Minnesota in 2020Every state in 2020This day in weather historyWarnings in Minnesota, 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: 900831, 900789, 900916, 900869, 900788, 900814, 900791, 900889, 901274, 900878, 900898, 900904, 902385, 900793.