National Weather Service United States Department of Commerce
May 21, 2010
Tornadoes & Flash Flooding

An area of upper level low pressure moved across the Great Lakes, with a trough extending into the Ohio Valley. The airmass had become unstable, and shear in the low levels of the atmosphere became favorable for the formation of tornadoes. In addition, thunderstorms tended to rebuild over the same areas, and rainfall was very heavy. This led to flash flooding across the region as well.