A heatwave is expected to bring dangerously hot weather to parts of the US, with millions of people in the west, and possibly other regions, advised to stay inside. The extreme heat may break records, potentially making this July 4th the hottest ever in some areas, and has already led to the cancellation of some events. The heatwave's reach is not entirely clear, with some reports focusing on the western states, while others mention the Midwest, Northeast, and eastern cities like DC and Philadelphia.