A record-breaking heatwave continued to spread across the central US on Wednesday, placing about 130 million people under heat alerts in 22 states, and prompting warnings from weather experts of potentially deadly conditions. Temperatures above 100F (37.7C) stretched south to states on the Gulf coast, the National Weather Service (NWS) warned in a morning advisory, with “brutal humidity levels” pushing the heat index as high as 120F (48.8C) in some areas. School officials in numerous states, including Colorado, Iowa, Missouri and Oklahoma, closed classrooms or were sending students home early in response to the heatwave, which has seen record temperatures this week in cities from Texas to Louisiana. “It is imperative to take the heat seriously and avoid extended time outdoors, as temperatures and heat indices will reach levels that would pose a health risk and be potentially deadly to anyone without effective cooling and/or adequate hydration,” the NWS said in its statement. “Remember heat is the number one weather-related cause of death in the US.” Meanwhile, the deluge from now tropical depression Harold continued to cause problems in Texas, New Mexico and Arizona, where flooding prompted evacuations Tuesday night from homes, hotels and a school near the south rim of the Grand Canyon. “Severe thunderstorms with damaging winds, hail and heavy rain are expected in parts of the Great Lakes and Ohio Valley, especially tonight,” the NWS statement added. This week’s heatwave, which will last until the weekend, forecasters say, extends an unprecedented summer of record-breaking heat and other extreme weather events. In July, rangers at Death Valley, California, the hottest place on Earth, recorded a temperature of 128F (53.3C), close to its previous high. “With global warming, such temperatures are becoming more and more likely to occur,” Randy Ceverny of the World Meteorological Organization told the Guardian in an email. Chicago was among several heavily populated cities expected to set or equal records on Wednesday and Thursday, with a forecast high of 99F and a heat index of up to 115F. The last time the city saw a temperature above 100F was July 2012. In Louisiana, New Orleans is forecast to reach at least 100F for the fourth straight day, with central and northern parts of the state climbing to 110F. The NWS said the extreme conditions were caused by “a huge dome of high pressure” building across the middle of the country, and little relief was expected for the next two to three days. “[The] searing August heatwave is forecast to continue through at least the end of this week,” it said, adding that “a cold front dropping south begins to usher in cooler temperatures over the upper midwest on Friday.” In the Atlantic, the National Hurricane Center was monitoring three active weather systems, including tropical storm Franklin, which was crossing the Dominican Republic on Wednesday, but posed no threat to the mainland US. Joe Biden said he was sending federal personnel and resources to Puerto Rico and the US Virgin Islands to help with the aftermath. “Across the country, people are experiencing the devastating impacts of extreme weather worsened by climate change,” the president said in a statement. “As peak hurricane season approaches, my administration continues taking action to bolster the country’s preparedness, and support response and recovery efforts.”