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Overview

Widespread damage due to downbursts and tornadoes occurred Thursday, September 20 from south central Minnesota into west central Wisconsin. Some downbursts exceeded 100 mph. There were 27 tornadoes (one in Iowa, 24 in Minnesota, one that crossed from Minnesota into Wisconsin, and one in Wisconsin).  Two were rated EF2 with winds of 125 and 130 mph, 15 were rated EF1 with winds of 86-110 mph, and ten were rated EF0 with winds up to 85 mph. With 25 tornadoes in a matter of hours, this was Minnesota's third most prolific tornado day (48 on June 17, 2010 and 27 on June 16, 1992, though both of those days had a number of much stronger tornadoes).

This was definitely the most complicated array of damage in years for the area served by NWS Chanhassen, taking many weeks to finalize where the tornadoes were, where the damaging winds were, or both. It is entirely possible that there were other tornado paths that occurred where we were unable to be investigate before crops were harvested and damage cleaned up.  Damage was investigated through multiple on-site surveys, satellite imagery,  aerial photos by the Civil Air Patrol, and video/pictures taken by drones owned and operated by individuals, several counties, and broadcast media (WCCO-TV and KIMT-TV). Information regarding the Iowa tornado was provided by the NWS offices in Sioux Falls and Des Moines.

Below is a brief summary of the meteorology of the event.

A line of severe thunderstorms moved through southern Minnesota and into west central Wisconsin during the late afternoon and early evening of Thursday, September 20, 2018. Prior to the development of severe storms, numerous showers and thunderstorms affected much of central and southern Minnesota during the morning and early-mid afternoon. This activity developed well north of a warm front, which was located over northern Iowa at the time. The warm front quickly surged north during mid-late afternoon as a strengthening low pressure system moved northeast from Nebraska toward southern Minnesota. Scattered non-severe thunderstorms continued to develop north of the warm front as a broken line of strong to severe storms developed from the low pressure system southward ahead of the attendant cold front. These storms raced northeast at up to 70 mph, and moved across southern Minnesota and into west central Wisconsin from around 430 PM through 8 PM. Widespread severe weather occurred with these storms.

Here is a brief summary of all of the tornadoes in chronological order, and each one is detailed individually farther down on this page:

1.) Superior, IA tornado: EF1

2.) Granada, MN tornado: EF1

3.) Garden City, MN tornado: EF0

4.) near Skyline, MN tornado: EF0

5.) Lake Elysian, MN tornado: EF1

6.) Janesville, MN (east) tornado: EF0

7.) near Waldorf, MN tornado: EF1

8.) Waterville, MN tornado: EF1

9.) Waseca-Medford, MN tornado: EF1

10.) Morristown, MN tornado: EF2

11.) Roberds Lake, MN (southeast side) tornado: EF1

12.) Owatonna, MN tornado: EF1

13.) Faribault Airport, MN tornado: EF2

14.) Roberds Lake, MN (north side) tornado: EF1

15.) Medford Northeast, MN tornado: EF1

16.) Faribault Northeast, MN tornado: EF1

17.) Rice County Southeast, MN tornado: EF1

18.) Dundas, MN (south) tornado: EF1

19.) Dundas, MN tornado: EF0

20.) Stanton, MN tornado: EF1

21.) Wanamingo, MN tornado: EF0

22.) Cannon Falls, MN (south) tornado: EF1

23.) Cannon Falls, MN (north) tornado: EF0

24.) New Trier, MN tornado: EF0

25.) Zumbrota Northeast MN tornado: EF0

26.) Prairie Island, MN to Diamond Bluff, WI tornado: EF0

27.) Ellsworth, WI tornado: EF0

Radar 542 PM

The 5-minute radar loop shows the debris detected just northeast of Morristown MN as the storms raced northeast at nearly 70 mph. 

 

 

 

 

 

Radar 542 PM

Warnings and Preliminary Storm Reports from September 20, 2018

 

 

 

 

 

Radar 542 PM

Tornado paths from September 20, 2018

 

 

 

 

 

 

 

 

 

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