Millions of people in a large swath of the western United States are bearing an unusually early and enduring heatwave with temperatures in some places breaking or matching historic records. Phoenix tied a record for the second day in a row with 115 degrees. The city is expected to heat up to 117 degrees each of the next two days, according to the National Weather Service. Palm Springs hit a high of 120 degrees on Tuesday. The heat is expected to last until at least Sunday. High atmospheric pressure is believed to be contributing to the scorching weather. In California, the operator of the state’s power grid asked residents to voluntarily conserve power for a few hours Thursday evening to relieve stress from the grid. CEO Elliot Mainzer said the grid was stable and there was no expectation of rotating power outages, but that could change as temperatures spike in the coming …