A punishing heat dome that meteorologists are calling the strongest on the planet right now has settled over the central and southern United States this weekend, triggering Extreme Heat Warnings from the Mid-South into the Midwest and putting more than 100 million Americans under some form of heat alert.

The Washington Post reported Friday that the dome — a zone of high pressure that traps hot air and suppresses cloud cover — is centered over the South this time, driving extreme humidity levels that make the heat feel even more oppressive than the raw temperatures suggest.

Triple-Digit Heat Index Readings

In St. Louis, the National Weather Service has an Extreme Heat Warning in effect through 9 p.m. CDT Saturday, with heat index values expected to climb between 105 and 110 degrees as afternoon highs push toward the century mark. Forecasters in Nashville and Charleston, South Carolina, issued similar warnings, describing Saturday as a "First Alert Weather Day" for dangerous heat, with a chance of strong afternoon and evening thunderstorms as a weak cold front tries to nose into the muggy air mass.

This marks the latest in a string of major heat domes to grip the country this summer. An earlier, separate dome baked the West earlier in August, pushing temperatures as high as 118 degrees in Phoenix and 115 in Las Vegas before beginning to ease in recent days. The South-centered dome now taking over is forecast to be more humid, which raises heat-index values and makes it harder for the body to cool itself through sweating.

Relief on the Way — For Now

Forecasters say Saturday should mark the peak, with a weak cold front expected to nudge temperatures down a few degrees by Sunday across much of the affected region, even if the humidity lingers a while longer. Local meteorologists cautioned residents to check on elderly neighbors, limit outdoor exertion during peak afternoon hours and never leave children or pets in parked cars.

Public health officials in several of the affected states urged residents to use cooling centers where available and to watch for symptoms of heat exhaustion and heat stroke, including confusion, rapid pulse and a lack of sweating despite high heat — a marker of the more dangerous heat stroke that requires immediate emergency care.