Temperatures Tuesday won't be quite as hot as they were Monday, but they'll remain close to 80 for some into the afternoon. A cold front passing through the Stateline Tuesday morning will shift winds from the southwest to the northwest. There isn't much cold air initially behind the front which is why afternoon highs will remain above average.
Most of Tuesday will see dry, and likely, sunny conditions. But clouds will increase through the evening as a surge of moisture returns late Tuesday night. This moisture will arrive with increasing winds from the low-level jet (stronger winds a few thousand feet above the surface). The front from Tuesday morning will have settled to our south, sitting roughly across central Illinois. Showers and thunderstorms will develop after sunset (between 8pm and 11pm) across southern Iowa, moving towards northern and central Illinois around Midnight - and beyond.
The Stateline will sit on the northern edge of the thunderstorm activity, and north of the front, which will help keep the highest risk for widespread thunderstorms to the south. But a few of those thunderstorms could reach some of the region, mainly in the form of elevated thunderstorms. With just enough instability the strongest of the storms could produce some locally large hail (quarter of an inch, or greater) along with gusty winds late Tuesday night. Because of that risk the Storm Prediction Center has placed areas along and west of I-39 in northern Illinois under a 'marginal risk' for isolated severe storms. At this point it doesn't look to be anything too widespread, but something we'll need to keep an eye on heading into Tuesday night.
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