Severe Wisconsin Weather and Weird Clouds

Wisconsin Severe Weather

My dad took this picture of me standing in a field near our house in southern Wisconsin on July 28, 2009. They were the strangest clouds we'd ever seen. They looked like big round pillows in a grid pattern across the whole sky. What kinds of clouds are these?

Here is a weather report from that day, to give you an idea of the conditions under which these clouds formed:

Severe weather occurred in Southern Wisconsin on Monday evening. There were reports of heavy rains, hail, strong winds and possible tornadoes. The State Emergency Operations Center was activated to Level 3 earlier this evening with partial staff, but has now returned to Level 4 activation with normal monitoring.
Numerous counties reported downed power lines, trees, and road closures, but little structural damage. There has been no request for state assistance. (Region V, Wisconsin EM)

http://www.fema.gov/emergency/reports/2009/nat072809.shtm- article
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