Texas beachgoers worry 'common' phenomenon may 'ruin' July 4th vacations It's the most gooey during the heat of the afternoon.

Hundreds of thousands of people have been advised to stay out of the sun this week due to "dangerously hot conditions" forecast in two states.

Heat-related advisories were in effect for millions across 13 states on Monday, with the National Weather Service (NWS) warnings of "dangerously hot conditions" in some parts of the country.

An NWS update released on Saturday showed extreme heat warnings are in place for large sections of Arizona along with parts of Southern California.