The mercury touched 92° at O’Hare on Tuesday for the first official 90° reading in Chicago this year. Midway’s 93° was the third consecutive day of 90° or better, while the high for the state was Pontiac’s 97°. A lake breeze kept the lakefront a little cooler, with only 84° reported at Northerly Island. Consecutive model runs get hotter and hotter each day and now forecast no significant relief from the heat and humidity for the next seven days and beyond. If the forecast verifies, it would develop into an unprecedented heat wave so early in the season.
Amidst this heat and humidity is and unstable atmosphere. Thunderstorms in this environment have developed somewhere in the Chicago area the past four afternoons, and this aspect of the forecast should also remain through the period. The best moisture and instability gather Thursday and Friday, and hence best chance for needed rain then.
Midway airport topped out at 91° Monday, with O’Hare reaching 89°, both the highest readings of the year thus far. The developing heat now spreads far and wide, from the mid Atlantic to the Rockies. Temperatures in the 90’s in Chicago during the first 10 days of June are not that unusual, occurring about half the time (53 percent), but a string of three or more days are unusual this early in the summer.
Some relief will be moving east into the high Plains over the next few days, but warm, humid conditions are expected over northern Illinois right through the weekend. Changes in the upper pattern now project the weak jet stream, the delineator between warm and cool air, will settle well north off Chicago, with an associated cold front sinking only as far south as the Wisconsin line late in the week. This heat and humidity in an unstable environment adds up to a chance for t- storms and much needed rain almost every day, with the best chance on Thursday.