On July 1, 2011, several lines of severe thunderstorms swept across parts of Minnesota, Wisconsin, Lake Superior, Upper Michigan, Iowa, and South Dakota. The first line of storms was especially severe, and it produced a corridor of wind damage from northwest of Sioux Falls, South Dakota all the way into northwest Wisconsin and western Lake Superior. Many communities in east-central Minnesota and northwest Wisconsin were impacted by significant wind damage, caused by hurricane-force wind gusts.
The line of storms had a stunning visual appearance as they raced through the area at forward speeds of over 60 mph. Many residents observed a shelf cloud, something that is a common occurrence on the leading edge of a strong line of thunderstorms. The shelf cloud usually marks the leading edge of the strong thunderstorm outflow winds. The pictures below are from the Spooner, Wisconsin area of Washburn County, and were taken by Nancy Olson and Tucker McCumber.
Here are some other pictures taken by Ron Wilhelm from near Grantsburg, WI in Burnett County. There is a similar shelf cloud evident in the photographs, on the leading edge of the approaching line of thunderstorms:
Shelf clouds generally slope down and away from the rain core of the line of thunderstorms, and they tend to indicate thunderstorms dominated by strong outflow winds. Often times, there will be a ragged appearance to the bottom of the shelf cloud, something that is visible in the pictures from Grantsburg. These low-hanging, ragged clouds are referred to as "scud clouds", and by themselves these clouds are quite harmless. Scud clouds are the result of abundant moisture in the atmosphere and sufficient rising motion in the column of air between the ground and the predominate cloud base. The invisible water vapor quickly condenses into a visible cloud fragment which is subsequently raised up to the shelf cloud base. This effect can create clouds that hang very low to the ground, but do not rotate or cause damage. The main threat from an intense line of storms like the one that is pictured above is damaging wind gusts.
For more information on these types of clouds, you can visit a web page put together by the National Weather Service in Sullivan, Wisconsin.
July 1st was a very hot and humid day across much of Minnesota and Wisconsin. High temperatures were in the 90s in many locations. The Duluth airport recorded a high of 91 degrees - the first 90 degree high temperature there in approximately 5 years. The high at Minneapolis was 99 degrees.
The heat was accompanied by an abundance of low level moisture. Surface dew points were well into the 70s, and values as high as 79 were recorded in parts of north central Wisconsin. The combination of heat and humidity lead to heat index values as high as 110 in some locations, and the development of an extremely unstable air mass.
By mid afternoon, a weakly defined front extended from western Ontario, along the north shore of Lake Superior, into central Minnesota, and into eastern South Dakota. This would eventually become the focus for widespread thunderstorm development. Rapid thunderstorm development initially occurred very close to this front to the northwest of Sioux Falls, South Dakota around 2 PM as a strong mid-level low pushed into the area. These storms quickly became severe and then tracked northeast up the front.
Within an hour, the storms had rapidly formed into a compact line and began producing extremely strong and damaging winds. While a relatively broad corridor received wind damage and gusts probably in excess of 60 mph, there were also smaller swaths of very intense wind damage along much of the path through Minnesota and northwest Wisconsin. These areas of significant damage were generally related to more substantial downbursts.
Some scattered supercells that formed ahead of the main line of storms along the front also produced some very large hail in central Minnesota and northwest Wisconsin. There were a few reports of hail between the size of baseballs and softballs.
In total, several hundred wind damage and wind gusts reports were received along a roughly 500 mile long area in the region. Many of these reports included significant damage, with large stands of trees blown down, and some damage to structures.
Other NWS Offices' Web Pages About This Event