The National Weather Service found evidence of a tornado within the widespread straight-line wind damage left behind after the Monday night storm. The damage path is approximately 28 miles long through Polk and Red Lake counties and was discovered by the nature of the damage; debris was lifted higher and thrown further in a manner usually associated with tornadoes. No one actually saw the tornado because it was entirely wrapped in heavy rain and was surrounded by a large area of very strong non-tornado wind. A few people along did report hearing the “freight train roar” often associated with tornadoes, butthis sound can sometimes be heard in strong straight line wind storms as well. Rain-wrapped tornadoes are not common in the northern Plains but are more common in the South where more humid environments often produce more widespread rain around tornadoes. Rain-wrapped tornadoes are often difficult to detect except by Doppler radar, and the National Weather Service did have a tornado Warning in effect at the time. This illustrates the need for people to take all Tornado Warnings seriously. Fortunately, there were no injuries Monday night.
The tornado was a part of a huge thunderstorm complex which produced wind damage along a more than 500 mile path through North Dakota and Minnesota into northern Wisconsin and northern Michigan. It began Monday afternoon and lasted into early Tuesday. The storm complex, called a derecho, happens from time to time when hot and humid air in the lower atmosphere is topped by an upper air disturbance of higher winds and cooler temperatures. The cool air aloft makes the air very unstable. As the developing thunderstorms form into a line, some mechanism forces some of the stronger upper-level wind down to the ground. This condition can continue for hours, causing near-continuous wind damage. In the Monday night storm, the mechanism responsible for forcing the wind was a small area of low pressure which formed over northeast North Dakota and then continued to move east with the storm.
By far, most of the wind damage Monday night was due to straight-line wind. There were many reports of wind speeds estimated to be in the 60 to 80 mph range. The tornado produced wind in this range as well, along with a few locations of 110-120 mph wind, all in Polk County near and southwest of Crookston. The 110+ mph wind allow for the tornado to get an EF-2 rating.
Meteorologist John Wheeler