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 |
The 5-minute radar loop shows the debris detected just northeast of Morristown MN as the storms raced northeast at nearly 70 mph.
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Warnings and Preliminary Storm Reports from September 20, 2018
Tornado paths from September 20, 2018
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Tornadoes Tracks:
Tornado - Superior, Iowa
Track Map |
Tornado - Granada, Minnesota
Track Map |
Tornado - Garden City, Minnesota
Track Map |
Tornado - near Skyline, Minnesota
Track Map |
Tornado - Lake Elysian, Minnesota
Track Map |
Tornado - Janesville, Minnesota (east)
Track Map
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Tornado - near Waldorf, Minnesota
Track Map
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Tornado - Waterville, Minnesota
Track Map
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Tornado - Waseca-Medford, Minnesota
Track Map
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Tornado - Morristown, Minnesota
Track Map
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Tornado - Roberds Lake, Minnesota (southeast side)
Track Map
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Tornado - Owatonna, Minnesota
Track Map |
Tornado - Faribault Airport, Minnesota
Track Map |
Tornado - Roberds Lake, Minnesota (north side)
Track Map |
Tornado - Medford Northeast, Minnesota
Track Map |
Tornado - Faribault Northeast, Minnesota
Track Map |
Tornado - Rice County Southeast, Minnesota
Track Map
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Tornado - Dundas, Minnesota (south)
Track Map
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Tornado - Dundas, Minnesota
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Tornado - Stanton, Minnesota
Track Map
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Tornado - Wanamingo, Minnesota
Track Map
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Tornado - Cannon Falls, Minnesota (south)
Track Map
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Tornado - Cannon Falls, Minnesota (north)
Track Map
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Tornado - New Trier, Minnesota
Track Map
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Tornado - Zumbrota Northeast, Minnesota
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Tornado - Prairie Island, Minnesota to Diamond Bluff, Wisconsin
Track Map
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Tornado - Ellsworth, Wisconsin
Track Map
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The Enhanced Fujita (EF) Scale classifies tornadoes into the following categories:
EF0 Weak 65-85 mph |
EF1 Moderate 86-110 mph |
EF2 Significant 111-135 mph |
EF3 Severe 136-165 mph |
EF4 Extreme 166-200 mph |
EF5 Catastrophic 200+ mph |
Severe Wind:
Widespread wind damage occurred across the region, in addition to all the tornadoes. These storms were moving at 60 to 70 mph, and produced non-tornadic wind gusts of over 100 mph at some locations. This led to a very complex damage survey. Hundreds of areas of damage were analyzed to better understand this severe weather event, so that we can learn from it and hopefully improve the lead time and messaging for the next severe weather event. The images below show the tornado tracks, followed by the tornado tracks with tornado damage points, severe wind damage points, and finally the wind damage and reports on a single image. Thank you too everyone who helped contribute information to analyze this event.Wind damage points. Several locations had non-tornadic winds of 90 to 100 mph or more with this severe line of thunderstorms. | Tornado tracks. There were 27 tornadoes total. The different colors are related to the EF-scale shown in the lower right. | Tornado damage points (triangles) and tracks. The different colors are related to the EF-scale shown in the lower right. | All damage points and tornado tracks from the storm survey. |
Photos:
Here are a few images of tornadic damagae (top row) and non-tornadic wind damage (bottom row).Tornadic EF2 Damage in Morristown | Tornadic EF1 Damage SE of Northfield | Tornadic EF2 Damage at Faribault Airport | Tornadic EF1 Damage in Owatonna |
Non-tornadic wind damage in Camp Dells near Waterville MN. | Non-tornadic wind damage near Roberts Lake, west of Faribault MN. | Non-tornadic wind damage east of Northfield MN. | Non-tornadic wind damage near Prairie Island MN |
Radar & Satellite:
Radar Reflectivity Loop from 531 PM CDT to 749 PM CDT | |||
Radar Reflectivity at 551 PM CDT | Radar Reflectivity and Storm Relative Velocity at 632 PM CDT | Radar Reflectivity and Storm Relative Velocity at 642 PM CDT | Radar Reflectivity and Storm Relative Velocity at 658 PM CDT |
GOES-East Visible/IR Sandwich at 500 PM CDT | GOES-East Visible/IR Sandwich at 530 PM CDT | GOES-East Visible/IR Sandwich at 600 PM CDT | GOES-East Visible/IR Sandwich at 630 PM CDT |
Atmospheric Environment:
Basic Analyses
The surface analyses maps below show the progression of the surface low and its associated warm and cold fronts during the afternoon and evening hours. Note how the warm front lifts north into southern Minnesota and Wisconsin. This brought warmth and moisture northward, increasing instability across the area. Thunderstorms developed ahead of the surface low and cold front, and raced northeast across the area. The weather balloon sounding from 1 PM below shows the air mass in place in Chanhassen at the time, which had limited instability given its position well north of the warm front. However, it highlighted the strong wind shear (change in the speed and direction of winds) which would later help storms become better organized and take on rotation. The 300 mb analysis at 7 PM shows an anomalously strong upper level jet streak in the region, and highlights an area of divergence (purple lines) which helped to sustain storms by providing an exhaust mechanism for air at the storm top level.
Surface Weather Analysis - 1 PM CDT 9/20/2018 | Chanhassen, MN (KMPX) Sounding - 1 PM CDT 9/20/2018 | Surface Weather Analysis - 4 PM CDT 9/20/2018 |
Surface Weather Analysis - 7 PM CDT 9/20/2018 | 300 mb Analysis - 7 PM CDT 9/20/2018 | Surface Weather Analysis - 10 PM CDT 9/20/2018 |
Mesonalyses
These detailed atmospheric analyses at 6 PM provide an overview of the conditions that influenced the development and evolution of storms during the late afternoon and evening. Temperature and dewpoint analyses highlighted the location of warm and cold frontal boundaries, which could help to initiate/sustain/influence storms. The mean layer convective available potential energy (CAPE) analysis showed the presence of modest instability across the area, but the wind shear and helicity analyses helped illustrate that any lack in instability would be supplemented by strong shear due to significant changes in wind direction and speed over the lower to middle portion of the atmosphere. Lifted condensation level (LCL) height information indicated that the sub-cloud layer of the atmosphere would be sufficiently moist to favor the development of tornadoes should other supporting factors come together. Finally, the significant tornado parameter combines a variety of stability, wind shear, and temperature/moisture factors to indicate where the atmosphere could be conducive to the development of significant tornadoes should rotating thunderstorms develop. Overall, these mesoanalyses indicate that the atmosphere was favorable for the development of severe storms, which could produce both straight line wind damage from bowing segments of a squall line as well as tornadic circulations.
Temperature/Dewpoint/Wind - 6 PM CDT 9/20/2018 | Mean Layer CAPE - 6 PM CDT 9/20/2018 | Effective Shear - 6 PM CDT 9/20/2018 |
0-1 km Bulk Shear - 6 PM CDT 9/20/2018 | 0-1 km Storm Relative Helicity - 6 PM CDT 9/20/2018 | 0-3 km Bulk Shear/MLCAPE - 6 PM CDT 9/20/2018 |
Mean Layer LCL Height - 6 PM CDT 9/20/2018 | 850 mb/500 mb Crossover Winds - 6 PM CDT 9/20/2018 | Fixed Layer Significant Tornado Parameter - 6 PM CDT 9/20/2018 |
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