Excessive heat is baking US Southwest and expected to get worse

A prolonged heatwave blanketed a swath of the US stretching from California to South Florida on Wednesday, with forecasters expecting temperatures that could shatter records in parts of the Southwest in the coming days.

The National Weather Service issued excessive heat advisories, watches and warnings for areas where about 100 million Americans live. The sweltering conditions are expected to worsen over the weekend and continue into next week.

While stifling temperatures gripped many parts of the country, Vermont and other Northeastern states barely had time to recover from historic flooding in recent days when the National Weather Service forecast more heavy rainfall across parts of New England, where rivers and streams are already running high.

Floodwaters turned the Vermont state capital of Montpelier into a swirling, brown waterway, damaged roads and may have compromised the city's water supply. Officials told the city's 8,000 residents to boil their water before using it until further notice.

Extreme weather also threatened the Chicago area, where at least eight tornadoes touched down in four counties in northeastern Illinois. The twisters and thunderstorms forced O'Hare and Midway airports to temporarily halt all air traffic, as tornado sirens echoed through the third biggest US city.

The growing frequency and intensity of severe weather across the US is symptomatic of global, human-driven climate change, experts in the field say.

The Southwest was bracing for potentially deadly heat with much of the area under an excessive heat warning.

Phoenix posted its 13th straight day with a temperature of at least 43 Celsius, with forecasts predicting the city next week will break its record of 18 straight days over 110, set in June 1974.

Moreover, the overnight lows are holding above 32C, offering little relief.

"The heat is going to be expanding across California, Nevada, Arizona and all the way into West Texas into this weekend and the core of the strongest heat seems to be setting up for Friday through Monday. So we'll be looking at records," said Tom Frieders, Warning Coordination Meteorologist with the National Weather Service in Phoenix.

At the family-owned Six Points Hardware store in Phoenix, fans and air conditioner units have been flying off the shelves, said store manager Drew Materniak.

The heat means "business is good," he said, noting the biggest seller has been large cooling fans, sold mostly to businesses like auto shops that cannot cool the air.

"Just stay inside man, just stay inside," was Materniak's advice for dealing with the heat. Las Vegas peaked at 42C on Wednesday and was forecast to match its all-time high mark of 47C on Sunday, the weather service said.

A ridge of stagnant air parked in the atmosphere was causing the excessive temperatures, said Ashton Robinson Cook, a forecaster with the weather service's Weather Prediction Center. The mass blocks cooler air and storm systems from rolling through the area, so it is "just full sun and heat," he said.

In Texas, where temperatures reached the upper 90s and topped 37C on Wednesday, the heat index will make it feel like 46C in some places through the weekend. Warm ocean water is causing the moist, humid air over much of the state that drives the heat index higher, Cook said.

In a cooling center in the West Texas city of Lubbock, where the temperature rose to 36C by midday, Courtney Martin, 41 and homeless for the past two weeks, sat quietly inside a public library doing needlepoint, keeping cool and staying hydrated with free bottles of water.

"I'm in here to beat the heat," said Martin, who recently moved to Lubbock from Michigan and is not accustomed to the high temperatures. "I don't know what I'd do without the libraries as cooling centres."