Dear Tom,
When the forecast calls for five inches of rain and the news talks about flooding, my children ask, “How can five inches of rain flood anything?” Its depth is only up past my ankles, but you see four feet of water around houses.
Michelle Connor
Dear Michelle,
If the Earth were smooth like a cue ball, a five-inch rain would measure five inches. Add texture to the planet, and water flows from high ground to low, flooding the lowlands if there’s enough water. The flood threshold differs across the country. In Chicago, three inches of rain cause problems, while five inches cause widespread flooding. Fortunately, NOAA has developed a sophisticated stream gauge, rainfall and snowmelt network to report these flooding events. Computer models now predict flood crests and duration in advance. Accurate forecasts included the great Dakota snowmelt in 1997 and the record Mississippi River flood in 1993.
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Tom Skilling is chief meteorologist at WGN-TV. His forecasts can be seen Monday through Friday on WGN-TV News at noon and 9 p.m.
Write to: ASK TOM WHY, 2501 Bradley Pl., Chicago, IL 60618 or asktomwhy@wgntv.com (Mail volume precludes personal response.)
WGN-TV meteorologist Dennis Haller plus weather producer Bill Snyder contribute to this page.
