textproduct: Chicago

KEY MESSAGES

- A pleasant pattern will continue through the holiday weekend.

- An early fall storm system will move through the Great Lakes region toward the middle to end of next week accompanied by a shot of well below average temperatures.

DISCUSSION

Issued at 301 AM CDT Sun Aug 31 2025

A tranquil and pleasant weather pattern will persist through through Tuesday as high pressure remains dominant across the Great Lakes region. More sunshine is anticipated today through Tuesday, which should foster a bit warmer temperatures than what the area experienced on Saturday. Accordingly, upper 70s are forecast this afternoon, with readings up around, or just above, the 80 degree mark on Monday and Tuesday, particularly inland from the Lake Michigan shore. A persistent onshore easterly wind will result in a bit cooler temperatures (lower to mid 70s) along the Lake Michigan lakeshore.

Beyond Tuesday, attention continues to focus on the significant weather pattern shift expected across North America in response to the anomalously strong mid/upper-level ridging building across northwestern Canada. The associated buckling of the Polar jet over this region will send impulses originating from the Beaufort Sea south-southeastward across south central Canada, and ultimately carving out a large upper trough/low over the Upper Great Lakes region later in the week. There does continue to be some ensemble spread in regards to the overall strength and southward extend of the Upper Low into the Upper Great Lakes late in the week. This spread becomes particularly noteworthy in regards to the potential for a secondary impulse and reinforcing shot of cold air to dig into our region along the western periphery of the upper low on Friday. Nevertheless, the main message of a period of precipitation with a strong cold frontal passage late Wednesday into Wednesday evening, followed by a turn towards much cooler weather by Thursday remains unchanged.

Robust cold air advection (especially for early September) on breezy northwest winds will usher in a significant and notable cool down for Thursday through the end of the week. High temperatures still look likely to struggle to climb out of the low to mid 60s on Thursday and Friday as well. For climate comparison, the record coldest high temperature for Thursday the 4th of September is 60 and 61 in Chicago and Rockford, respectively. Similarly, overnight low temperatures will be well below average, and may make a run for the lower 40s (if not colder) outside of Chicago and away from Lake Michigan.

As noted above, ensemble spread and forecast confidence lowers Friday into the early part of next weekend. Of particular forecast interest, is the potential for a secondary impulse and reinforcing shot of cold air to dive southward into our area on Friday along the western periphery of the main upper low. If this potential continues to look increasingly likely, the threat will be higher for another period of rain showers on Friday than the current 20% probabilities offered up by the latest NBM. Temperatures could also end up slower to moderate next weekend.

KJB

AVIATION /12Z TAFS THROUGH 18Z MONDAY/

Issued at 553 AM CDT Sun Aug 31 2025

No significant aviation weather concerns are anticipated through the period.

Surface high pressure will remain dominant today over the Great Lakes region. Accordingly, this will continue to foster light (10 kt or less) east-northeasterly surface winds through the period. Otherwise, VFR conditions are anticipated with decreased cloud cover today and tonight.

KJB

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...None.