Dear Tom,
What is the difference between scattered showers, isolated showers and a chance for showers?
David Tanny
Dear David,
The National Weather Service (NWS) has set guidelines for their forecasters for convective weather. For areal coverage, the forecast usually spans one or more counties in 10 percent increments within a 12 hour period. “Isolated” forecasts 10 and 20% coverage, “scattered” 30 through 50%, “numerous” 60 and 70% , and no qualifier 80 through 100%, for example–t-storms.
Precipitation probability encompasses all types of precipitation, i.e. rain, snow, showers, etc. A “slight chance” implies 10 and 20%, i.e.. rain or snow falls 10 or 20% of the time. With a “chance,” the odds go up to 30 through 50%, while “likely” brings a 60 to 70%, and unqualified an 80 through 100% chance of the event occurring.
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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 meteorologists Steve Kahn, Richard Koeneman and Paul Dailey plus weather producer Bill Snyder contribute to this page.
