The National Weather Service from Des Moines took pictures of the tornado damage in Parkersburg. They use these pictures along with other information in their storm survey to determine which Enhanced Fujita (EF) category a tornado would fall in.
This one was rated an EF-5. The last one, if I'm not mistaken, was Greensburg, Kansas. You all remember the incredible images that came out of Greensburg.
Here are the Parkersburg pictures.
More information on Parkersburg from Wikipedia.
Even more information from City-Data.