The May 2013 Moore tornado, which topped the intensity scale for such storms, contributed to an outbreak that caused $1.1 billion in insured losses.
Swiss Re took the track of that tornado and laid it over Cook County, Illinois, which includes the Chicago area. Applying similar ratios of damage and loss to those experienced in Moore, the company arrived at the $20 billion property damage figure (which it acknowledged was not precise).
"Instead, this estimate of loss demonstrates the potential magnitude and far exceeds historical loss experience, even when the historical loss experience is adjusted for inflation, economic growth and population increases," the firm said in its report.
"Densely populated and developed Cook County, Illinois has the greatest potential exposure to a violent and long track tornado" compared to areas like Dallas and St. Louis, Swiss Re added, with more than 112,000 people living in the damage zone of a Moore-like tornado.
"In short, an informed reexamination of history is instructive, for it dispels the myth that tornadoes cannot impact major cities," the firm concluded.