( 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 . As temperatures sizzle , fireworks even more risky . 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 .
NEW : Wednesday will be either hot or wet for 19 regions of country . NEW : Extreme heat will continue in The Pacific Northwest , but rain will drench Southeast . Southwest will get a break from blazing temperatures . Nevada 's massive wildfire could get rain , but also wind gusts and lightning .