 Least a couple more in the forecast. A rep for the National Weather Service calls this the most extreme heat wave in the history of record-keeping in Las Vegas since 1937. More than 142 million people across the U.S. are under heat alerts, many out west, where the duration of the heat has been a major concern. A motorcyclist in California's Death Valley dying from heat exposure earlier this week, where temperatures soared to 128 degrees. Kristen Goodwin, Fox News.