(Bloomberg) -- It’s going to get so hot in California that people swimming in mountain streams could freeze to death.Much of California, parts of Nevada and Arizona, will be getting their first extreme heat of the year and by Tuesday temperatures will reach 100 degrees Fahrenheit (38 Celsius) in many places and stay there through Thursday. Car interiors will become death traps for children and pets left alone, outside work will be difficult, and the elderly will be at risk, the National Weather Service said.So much mountain snow will melt that streams will turn into icy torrents. Anyone venturing in to cool off risks being swept away and suffering “rapid onset of hypothermia,” the agency warned.“Daytime highs will be in the triple digits for most Valley locations, especially Tuesday through Thursday,” the weather service said. “Overnight temperatures will remain warm and offer little relief.”An excessive heat watch in California’s Central Valley reaches all the way to the coast where temperatures in Oakland could approach 90 and push to 103 in Napa. The heat risk also spreads to southern Nevada and into western Arizona, where temperatures could reach 108 by Wednesday and linger until Friday.Energy demand is set to spike across the region as people switch on air conditioners.For more articles like this, please visit us at bloomberg.comSubscribe now to stay ahead with the most trusted business news source.©2020 Bloomberg L.P.