The line of severe thunderstorms that moved through Pine County, Minnesota, and northwest Wisconsin, produced widespread, non-tornadic wind damage. Such winds are also often referred to as "straight-line winds" or "downburst winds". The damage included some areas of very intense forest blowdown. The National Weather Service in Duluth conducted a storm damage assessment the following day in parts of Pine, Burnett, Washburn, and Douglas Counties. This assessment was aided by aerial flyovers of the damage by the Burnett County Sheriff's Department. Both the ground and air assessments concluded that the damage across the area was caused by widespread, non-tornadic, straight line wind gusts in excess of 60 mph, with smaller pockets of intense winds in excess of 100 mph.
Based on a combination of information from aerial and ground surveys, as well as spotter reports, we have created a map of the swaths of the most intense wind damage associated with these thunderstorms. Generally, these areas were a few miles wide and consisted of "straight-line" or diverging damage patterns.
Over the entire area pictured on the map there was at least sporadic tree damage, and wind gusts were likely into the 50 to 60 mph range at most spots. In the first level of shading, damage was observed to be more concentrated, and these were areas where many trees were damaged or blown over. Winds in those areas were likely closer to 70-80 mph. In the second level of shading, there was a combination of more intense tree damage and damage to a few structures. Winds were likely between 80 and 100 mph in those spots. And in the darkest level of shading, corridors of severe forest blowdown existed. Winds in those spots likely exceeded 100 mph.
The EF-2 tornado track near Solon Springs is also plotted in red.
Keep in mind that there will be some areas that aren't shaded that may have experienced much smaller areas of significant wind damage. However, in general, the shaded areas in the map represent the areas where the greatest concentration of damage existed.
Often times when meteorologists are assessing storm damage, aerial photographs can be extremely valuable in determining the extent and cause of the damage. This is especially true in parts of northeast Minnesota and northwest Wisconsin where large tracts of forest can limit the area of a ground survey. In this particular case, the aerial photographs revealed widespread wind damage. Non-tornadic damage will involve the vast majority of trees or objects being blown in a similar direction. Special thanks go to the Burnett County Sheriff's Department for conducting the flyover of the damage, and providing these images to National Weather Service meteorologists.
First, here are some annotated versions of the aerial photographs showing approximate damage vectors (the direction trees and objects were blown).
Here are some unedited versions of aerial photographs from around the Burnett County area. Photos were taken by Stacy Hopke of the Burnett County Sheriff's Department.
Ground Survey Photographs
National Weather Service meteorologists also conducted a ground survey. Here are some pictures from that survey.
Historical Forest Blowndown Events
Areas of northeast Minnesota and northwest Wisconsin are susceptible to forest blowdown events from time to time, some more severe than others. One can pretty easily compare the damage photos from Burnett County on July 1, 2011 to photographs from several other major forest blowdowns in the area. In all those events, broad swaths of trees were removed from the existing forest. However, in this particular case, the forest damage was not as widespread across the area as the examples provided below.
July 4, 1999: Minnesota Arrowhead and Boundary Waters
The intense wind damage that occurred in the Boundary Waters Canoe Area Wilderness will be remembered in northeast Minnesota for a long time. A large forest blowdown occurred very near the US-Canada border in the Minnesota Arrowhead in early July 1999. Prior to the storm reaching the Boundary Waters, it affected parts of the lakes region around and north of Brainerd, as well as the Iron Range. Damages in Cass, Itasca, and Aitkin counties exceed $3 million (in 1999 dollars). The Hibbing airport measured a wind gust to 81 mph and sustained some structural damage. In the Boundary Waters, over 600 square miles of forest and over 10 million trees were blown down. Dozens of people were injured by falling trees.
The line of storms eventually crossed through southern Ontario and Quebec, and re-entered the United States the following day in parts of Vermont, New Hampshire, and Maine. For more information, here is a writeup from the NWS Duluth (3.4MB PDF), as well as a web page from the Storm Prediction Center.
July 4, 1977: East Central Minnesota and Northwest Wisconsin
Another famous forest blowdown event occurred back in 1977. The storms moved from far eastern North Dakota through central Minnesota, northern Wisconsin, and into lower Michigan. The damage from this event was surveyed by Dr. T Theordore Fujita, a meteorology professor from the University of Chicago, and the person for whom the Fujita Scale is named. One of his damage maps is shown above (on the right). He surveyed significant damage from Sawyer County into Price County, Wisconsin, where wind speeds from the line of storms were likely in excess of 100 mph.
The image above and to the left is from the Storm Prediction Center web page on this event. Additional information can also be found on the NWS Green Bay web site.
Here's an old photograph of tree damage near Phillips from the NWS Green Bay web site:
Fujita, 1978: Manual of downburst identification for project NIMROD. Satellite and Mesometeorology Res. Pap. No. 156, University of Chicago, Dept. of Geophysical Sciences, pp. 104.