It will break the grip of the humid heat wave that has smothered the northeast quarter of the nation this week like a musty wool blanket.
The relief will arrive in the upper Midwest late Friday before cooling off the Northeast Saturday night, the National Weather Service said.
People should avoid celebrating outdoors, as thunderstorms are predicted to electrify the ground with lightning in many areas. Michigan could see large hail, the weather service said. High gusts may blow loose objects around.