Dear Tom,
Having been a construction engineer on the first phase of the Deep Tunnel Project and in light of recent heavy rains, I have a question: What are the definitions of 50- and 100-year rain events?
Ron Chojecki
Dear Ron,
Hydrologist Bill Morris at the Chicago National Weather Service forecast office says that a 100-year rainstorm is a storm that has an annual probability of occurrence of 1 in 100 at a specific location.
Note that when such a storm takes place in a given year, its likelihood of happening again in the next year is still 1 in 100. It is therefore possible that several 100-year events could occur in just a few years, or that hundreds of years might pass between the occurrence of such rains but, averaged over time, a 100-year rain will occur once every 100 years. Comparable definitions apply to 10-year storms, 50-year storms, etc.
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Tom Skilling is chief meteorologist at WGN-TV. His forecasts can be seen Monday through Friday on WGN News at noon and 9 p.m.
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