ST. LOUIS, GLEN CARBON/MARYVILLE, IL. – The National Weather Service in St. Louis has issued an Extreme Heat Watch for Madison and St. Clair counties and nearby areas from Sunday afternoon, June 28 through Thursday evening, July 2, 2026, warning that heat index values of 105 to 110 are possible as a prolonged stretch of heat and humidity begins.
The watch was issued at 1:59 a.m. CDT Saturday, June 27, 2026, and covers St. Clair, Madison and Monroe counties in Illinois, along with Jefferson, St. Charles, St. Louis County and the city of St. Louis in Missouri, according to the National Weather Service. The agency said the watch will begin Sunday afternoon, June 28, 2026, and continue through Thursday evening, July 2, 2026.
“Dangerously hot conditions with heat index values of 105 to 110 possible,” the weather service said.
Earlier Saturday, June 27, 2026, scattered thunderstorms moved through the region, and police responded to an area of Route 159 on the edge of the Glen Carbon/Maryville line after water covered one of the lanes. Motorists were encouraged not to drive through roadways with high water. Drivers encountering that area were using the other lane while police used flashing sirens to alert travelers.
According to the provided summary, scattered rain and storms were expected across the southern third of the St. Louis viewing area Saturday, with temperatures in the mid-80s. Conditions are expected to turn hotter Sunday, with highs in the mid-90s and heat index values of 105 to 110.
The weather service said heat-related illnesses increase significantly during extreme heat and high humidity events. It urged residents to “drink plenty of fluids, stay in an air-conditioned room, stay out of the sun, and check up on relatives and neighbors.”
The agency also advised people to avoid prolonged time in the heat.
“If you have to be outside, take precautions. Drink plenty of water and take breaks in air conditioning or shade,” the post said.
The summary also said an Extreme Heat Watch was issued for St. Louis and nearby counties, while a Heat Advisory covers the rest of the region from Sunday afternoon through Thursday evening.
