UPDATED FORECAST AS OF 4:24 P.M.
- TONIGHT: Coolest night in just under 4 months. Extensive cloudiness, a few possible sprinkles this evening and early tonight. Clouds scatter and diminish late. Windy, cooler. Lows range from upper 40s at the coolest inland locations to 59 Chicago lakeshore.
- SATURDAY: Mostly sunny, breezy and cool. High 68 -- 10 degrees below normal. (Gale warnings on the open lake: Waves 2-3 ft. near shore, 5 to 9 feet well offshore; rip current advisories Illinois and Indiana beaches).
- SATURDAY NIGHT: Clear, cool. Low 55.
- SUNDAY: Partly sunny, becoming breezy from the southwest and warmer in the afternoon. Slight chance of an isolated thunderstorm or two at night affecting 20 percent of the Chicago area. High 78.
- MONDAY (Labor Day): Partly sunny, windy, much warmer. Chance of gusty nighttime thunderstorms. Southwesterly wind gusts to 35 mph possible in the afternoon. High near 90.
- TUESDAY: Partly sunny, windy, not as warm. High 83.
Happy Friday! The coolest weather in three months has settled in after the biggest official rainfall here in over a month.
We're looking at a high of about 72 today as cooler air pushes into the area from the northwest -- we haven't seen anything like that since early in the summer. Saturday promises to be even cooler, with a high of 68.
If back-to-back highs Friday and Saturday fail to reach 70, it would mark the first time since May 17-18 that Chicago's high temperatures have failed to break out of the 60s.
Both days also promise to be windy -- perhaps forcing a return to jackets after a hot, humid, and wet summer.
We're looking for some of today's cottony cumulus clouds to build into brief shower producers because of faster than normal vertical temperature declines. The atmosphere is said to be "unstable" when this happens and daytime heating enhances the rate temperatures drop with height.
The temperatures at 5,000 feet about 2:30 p.m. were 43 degrees. With ground level highs to peak at 72, that sets up a near 30-degree drop through the first mile of atmosphere. That's a very unstable atmosphere. The typical drop in temperature would be closer to 16 degrees. Such a rapid decline encourages air to rise and there's enough moisture in the air that we expect brief, scattered instability showers to pop up this afternoon and evening.
Temperatures at 5,000 feet: 43 degrees. With ground level highs to peak at 72, that sets up a near 30-degree drop through the 1st mile of atmosphere. That's a very unstable atmosphere. The typical drop in temperature would be closer to 16 degrees. Such a rapid decline encourages air to rise. That's why we expect brief, scattered instability showers to pop up this afternoon and evening.
Last night's storms yielded 1.72 inches at O'Hare, but more than 2 inches at a number of sites across the area. Temperatures will rebound expeditiously Sunday and Monday, a rebound which might fire an isolated thunderstorm late Sunday or Sun night.
Last evening's storms produced 7,500 cloud-to-ground lightning strokes within a 225-mile radius of Chicago. The downpours broke a long dry spell. Parts of this area hadn't seen rain in 17 days.
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