(CNN) -- It'll be firecracker hot in the Southwest on the Fourth of July, but the Southeast and Ohio River Valley will be so drenched that floods are possible.
Meanwhile, the Northeast will get a reprieve from storms that left knee-deep flooding in some areas.
"The most widespread showers and thunderstorms on Thursday will be from the central Gulf Coast northward into the Ohio River Valley," the National Weather Service said. "There is a slight risk of excessive rainfall/flash flooding across much of the Southeast through Wednesday."
As the wet weather system moves up toward the Midwest, the relatively cool temperatures along the East Coast will give way to more summer-like conditions in the Mid-Atlantic and Northeast.
Still sweating in the West
But the East Coast warm-up will have nothing on the oppressive heat wave scorching the West Coast.
While the Pacific Northwest will get some relief, "extreme heat will continue across the Western states, especially the Southwest," the weather service said.
Cities in California, Nevada and Arizona have already suffered through heat topping 120 degrees in the past few days.
The heat was so torrid in Idaho that Boise residents could bake cookies without an oven.
CNN affiliate KTVB placed a pan of chocolate chip cookie dough on the dashboard of a car. Within a few hours, the cookies were fully baked -- and even overcooked.
Forecasters in Las Vegas noted that a similar heat wave in 2005 killed 17 people. They urged neighbors and relatives to check on those most susceptible to heat-related illness -- children, the elderly and the chronically ill.
Relief and trouble in Arizona
The extreme heat continues to wreak havoc for firefighters trying to stop Arizona's Yarnell Hill wildfire, which has scorched more than 8,400 acres, about 13 square miles of land.
By Wednesday afternoon, the fire was 8% contained. Officials estimated the blaze might not be contained until July 12.
The fire killed 19 members of an elite firefighting squad on Sunday when fierce, erratic winds whipped flames in different directions. Parched land from Arizona's drought has added fuel to the fire.
The possibility of thunderstorms this week could drizzle much needed rain over the fire. But storms could also bring two serious dangers: wind gusts of up to 20 mph and lightning, the suspected cause of the inferno.
CNN's Tina Burnside and Michael Martinez contributed to this report.