St. Louis, Chicago and Des Moines were among the cities that saw heat indexes hit triple digits on Saturday, with the East Coast bracing for similar conditions by Monday.

Cities across the Midwest and eastern U.S. braced Sunday for another day of dangerously hot temperatures as a rare June heat wave continued to grip a portion of the country. The temperature had already reached 80 degrees Fahrenheit (26.

Major cities across the United States are preparing for extreme temperatures caused by a heat dome that hit the U.S. Plains on Friday and is expected to expand to much of the rest of the country over the coming days.