Much of the U.S. is facing either extreme heat or excessive rainfall and potential flooding on Tuesday. Here's where the greatest threats are expected.

Heat alerts were issued for millions across parts of the western U.S. Sunday as an unusually prolonged heat dome reached its peak.

Heat alerts were issued for millions across parts of the western U.S. Sunday due to an unusually prolonged heat dome, which is starting to move east.

Much of the U.S. is facing either extreme heat or excessive rainfall and potential flooding on Tuesday. Here's where the greatest threats are expected.