Prompting comments of "Ugh" from lovers of cool temperatures and "It's about time" from those of an opposite persuasion, a much warmer and more humid weather regime has begun to establish itself across Chicago and the Midwest. The city's high temperature climbs into the lower 80s on Thursday and then, as the warmth gathers strength, into the upper 80s on Friday. Afternoon readings push to 90 degrees or higher from Saturday onward through the 7-day forecast cycle. And, as is often the case with multi-day periods of high temperatures, Gulf air dripping with moisture will accompany this hot spell. That means warm, muggy nights and, in addition, a day-to-day chance for  mainly late-day thunderstorms. And what a difference a year makes. Through the 19th, temperatures this June have averaged 8.0 degrees cooler than one year ago

A heat wave is a period of abnormally and uncomfortably hot and usually humid weather. In 1900, climatologist A. T. Burrows defined it as a spell of three or more days on each of which the high temperature reaches or exceeds 90 degrees. By that commonly accepted standard, Chicago experiences an average of three heat waves per year, and this year's first heat wave is now in sight.

But first, Chicago's temperatures will have to recover from the sharp, 17-degree cool-down that took Monday's high of 88 degrees down to 71 on Tuesday.

A major weather pattern change is now under way because the primary west-to-east jet stream is shifting into Canada. A jet stream typically forms between hot and cold air masses and its northward shift signals an expansion of heat across the United States. For Chicago, 90s arrive Saturday and persist into next week.