The sunny highs of July are numbered, with a dramatic drop in temperature next week when a second polar blast hits large parts of the U.S. Temperatures are forecast to be up to 20 degrees below average for this time of year, with some places experiencing a chill more usual in the fall.

A jet stream from polar regions will bring the unseasonable weather in on Sunday, with a band of thunderstorms leading it in Ohio and along the East Coast. MORE