Why was there lots of damage in Parkersburg Ia after the tornado?

Answer 1

See the related question for what causes tornadoes in general. The event started when a low pressure system started pulling warm, moist air off the Gulf of Mexico several days before the Parkersburg tornado. As the system shifted northward it brought that air to Iowa. Then the system drew in a dry air mass from the north.this forced the moist air mass upwards, triggering the development of severe thunderstorms. Very strong wind shear set these storms rotating. One of these was exceptionally powerful, producing the EF5 tornado that struck Parkersburg.

Answer 2

The tornado that hit Parkersburg, Iowa was ab EF5, the strongest category of tornado on the Enhanced Fujita scale. Winds near the center of the tornado exceeded 200 mph at time, enough to wipe a well-built house clean off its foundation. A large portion of its damage path experience winds of at least 140 mph, which can partially demolish most houses.

When it hit Parkersburg the tornado was half a mile wide, meaning that damage occurred in a swath that wide.