Heat is headed for Chicago. Sunday’s highs rise to within striking distance of 90 degrees away from local (limited coverage) lake breezes and into the 90s Monday, Tuesday and possibly Wednesday as well. The deluge-generating thunderstorm clusters, which have been rotating through Iowa and downstate Illinois on the periphery of an expanding dome of hot air, are likely to ease in coming days as the depth of the air mass increases here. The expansion produces a rain-inhibiting “cap”–a layer of warm air which slashes the rate at which temperatures decline with height. Caps choke off the upward motion of hot humid air which fuels thunderstorms. At the same time, the atmosphere is to grow decidedly moister with dewpoints surging into the upper 60s or lower 70s early next week–Gulf Coast levels. The moisture available from a column of air over Chicago will grow to as much as 1.80″ by midweek. It’s moisture which will be available for thunderstorm development Wednesday afternoon and night. The increase in dewpoints and humidities predicted early next week could drive heat indexes into the 94-102 degree range, which would be the highest of 2007.
Sources: National Weather Service-Chicago, GLERL, Frank Wachowski, NOAA-NCEP
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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.
WGN-TV meteorologists Steve Kahn, Richard Koeneman and Paul Dailey plus weather producer Bill Snyder contribute to this page.
