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Overview
Heat Indices
Additional Info

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.

While no daily record high temperatures were set for Chicago and Rockford during this hot start to the month, multiple record warm low temperatures were set or tied in both cities as a result of the humid air mass limiting how much nocturnal cooling occurred after sunset.
- September 2nd:
  - Chicago -- tied daily warm low temperature record of 78°F (previously set in 1913)
- September 3rd:
  - Chicago -- broke daily warm low temperature record with a low temperature of 77°F (previously 75°F set in 1898, 1953, and 1973)
  - Rockford -- tied daily warm low temperature record of 73°F (previously set in 1939)
- September 4th:
  - Chicago -- broke daily warm low temperature record with a low temperature of 76°F (previously 74°F set in 1960 and 1971)
  - Rockford -- broke daily warm low temperature record with a low temperature of 73°F (previously 72°F set in 2018)

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