Dear Tom,
For hours during the morning of Aug. 18, several counties in southwest Wisconsin and northwest Illinois had thunderstorm warnings, but no watch was in effect. Since a watch means severe storms are possible, why is a watch not automatically issued when a severe storm occurs?
Tim Rushmeyer
Dear Tim,
The answer to this apparent contradiction relates to the varying criteria for issuing watches and warnings. The National Weather Service issues a severe thunderstorm watch when it expects widespread severe weather (3/4 inch hail and/or 58 m.p.h. winds) or when it believes a localized but more intense event (hail two inches in diameter and/or 75 m.p.h. winds) might occur.
On the date that you mentioned, warnings were issued without a watch because the severe thunderstorm activity was isolated in nature.
———-
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 meteorologists Steve Kahn, Richard Koeneman and Paul Dailey plus weather producer Bill Snyder contribute to this page.
