Excessive Heat — Union, Illinois

2025-07-22 to 2025-07-30 · Union, Illinois

Event narrative

Extreme heat continued from the third week of July.

Wider weather episode

An extended heat wave across the Quad State region occurred from mid to late July. On the synoptic scale, at 500mb, a 594dam ridge was present across the Southeast U.S., with the ridge building northward to include the Ohio Valley and Great Lakes by July 22nd-23rd. Convection coverage became more isolated the 22nd-25th as the ridge strengthened over the Ohio Valley. The ridge relaxed slightly on the 26th-27th, before the worst of the heat arrived on the 28th-29th. A cold front moving through on the 31st allowed for a much cooler and drier airmass to move across the region.

High temperatures in the low to mid 90s combined with very high humidity (dew points in the mid 70s to near 80) produced heat index values routinely in the 102 to 112 range from July 22nd through the 30th in Southern Illinois, following a more off and on heat the third week of July for northern portions of Southern Illinois. Slight decreases in heat indices occurred on the 26th-27th as more clouds were around, followed by the most oppressive heat with widespread heat indices of 110-120 on the 28th. The heat wave continued through the 30th, then a cold front moving south through the region brought relief to close out the month. Heat Advisories or Extreme Heat Warnings were in effect for portions of the Quad State for 15 consecutive days.

For the entirety of the mid-late month heatwave, Carbondale, IL experienced 21 consecutive days with a 75 degree dew point, their record longest. Carbondale observed 9 consecutive days reaching a heat index of at least 100 degrees from July 22-30, which is tied for their 3rd longest streak on record (records going back to 1996). The highest heat index in Southern Illinois for late July was 121 at the Crab Orchard RAWS site in Williamson County on the 29th.

Source: NOAA Storm Events Database, event_id 1290516. Narrative written by the NWS forecast office that issued the report.