Triple-digit temperatures, more commonly seen in the middle of summer and not in the spring, made an unwelcome visit to Texas and other parts of the southeastern United States this week, placing millions of Americans under extreme heat warnings.

“Definitely more like August this week than May,” Cameron Self, a meteorologist with the National Weather Service’s office in the Houston and Galveston area, said Tuesday. A very strong ridge of high pressure over the south-central United States that is centered over the Gulf of Mexico will be responsible for the extreme heat.

A First Alert Weather Day is in effect Wednesday due to the extreme heat.

Temperatures across the counties under the extreme heat warning are expected to rise to at least 105 degrees, which can cause heat-related illnesses.