September 1-4, 2026: Abnormally Hot and Humid Start to September

Overview

Following an August where no 90+°F temperature observations were recorded in either Chicago or Rockford, September 2026 started off hot and humid with four straight days of 90+°F high temperatures observed in both cities and most other locations across northern Illinois and northwestern Indiana.

September 1-4, 2026 was the hottest four-day start to September on record for both Chicago and Rockford. The average temperature during this 4-day stretch was 85.4°F in Chicago and 82.5°F in Rockford, which was 1.4°F warmer than the next warmest September 1st through 4th period in both cities (1960).

Peak afternoon heat indices between 100°F and 110°F were observed across most of northern Illinois and northwestern Indiana on all four days, though outflow from thunderstorms did provide a bit of relief to some locations in far northern Illinois on both the 3rd and the 4th.

The abnormal warmth came as a result of a dome of high pressure and heat becoming established across the central and southeastern United States and persisting for several days. This kind of weather pattern is more typical for mid-summer, but can occasionally occur during other times of the year as well. Since the summertime warmth and moisture present across the eastern half of the country had yet to meaningfully subside and since our area was able to largely remain south of the extensive thunderstorm activity along the northern periphery of this heat dome, this weather pattern resulted in July-like temperatures and heat indices being observed here in early September.

Heat Indices

Peak heat indices observed at local area airports on September 1st

Peak heat indices observed at local area airports on September 2nd

Peak heat indices observed at local area airports on September 3rd

Peak heat indices observed at local area airports on September 4th

Additional Information

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