A series of storms that has pummeled the Upper Midwest is part of a very unusual weather pattern.

Jonathan Martin is chairman of the Atmospheric and Oceanic Sciences Department at the University of Wisconsin-Madison. He says it only comes around once every 100 or 200 years.

Energized by warm air and fueled by moisture from the Gulf of Mexico, the storms start in Nebraska and Iowa in the afternoon, roll through southern Minnesota and then into Wisconsin and Illinois.

They've been running on schedule for six days.

The storms have caused more than $48 million in damage in Wisconsin so far.