Until midweek, as many as 107.5m people will be affected by a heatwave engulfing a large number of states from Kansas to West Virginia, north into Wisconsin, and south into Mississippi, the National Weather Service (NWS) said.
Daily temperatures records had been tied in Chicago and broken in both Nashville and Toledo by the afternoon, NWS tweeted. Several areas were expecting "well above normal to record-breaking temperatures", added the NWS, issuing excessive heat warnings and heat advisories for much of the country.
The agency encouraged people to hydrate and watch for signs of heat-related illness. Officials in Milwaukee were investigating whether two deaths, of an 89-year-old and a 39-year-old, were due to the heat, reports the Associated Press.
"Plan ahead to avoid heat-related illness and check on relatives and neighbours. The heat should break toward the end of the week," the weather service said.
St Louis, Memphis, Minneapolis, and Tulsa were also under excessive heat warnings, with temperatures forecast to reach about 100 degrees Fahrenheit (38C) in parts of the south, along with high humidity that could make the temperature feel almost 109F (43C).
The excessive heat is caused by an intense high-pressure zone often referred to as a heat dome. The dome, which was centred over the Southwest on Saturday has shifted east.
An early-season heat wave will continue this week, expanding from the Plains into the Midwest and Southeast. Much above normal record tying and record breaking high and low temperatures are forecast. The warm lows will provide little relief from the heat overnight. pic.twitter.com/RB8KkdntUs
— NWS Weather Prediction Center (@NWSWPC) June 12, 2022
Somewhat cooler conditions were expected the rest of the week in Chicago following a cold front that will bring storms tonight, NWS said.
High temperatures had powered through much of the southwest over the weekend, as cities like Phoenix, Las Vegas and parts of Texas and California saw sweltering conditions last for days. Some residents of Odessa, Texas also faced high temperatures on Tuesday and Wednesday without running water after a water main break.
Heatwaves are expected to get more common and more intense over the coming decades as the planet warms due to the climate crisis. Heatwaves that ocurred once every 50 years would happen almost 14 times more often and be 2.7C hotter if the world warms 2C above the 19th-century average, according to the most recent report from the Integovernmental Panel on Climate Change, a global authority on the climate crisis.
Currently, the world is heading towards 2.7C of warming by 2100, according to the Climate Action Tracker.