Over 20 million people in the mid-Atlantic faced the threat of severe storms Thursday, while California braced for "record breaking heat" this weekend that is expected to be even more intense than the heat wave that contributed to massive, destructive wildfires in mid-August.
Another 4 million people were under flash flood watches in parts of the South and the Midwest.
"From a hazards perspective, it is the Desert Southwest and throughout California's Great Valley that stand out most with record breaking high temps likely this weekend and into early next week," the National Oceanic and Atmospheric Administration said Wednesday afternoon.
About 44 million people were to be under excessive heat watches and warnings that go into effect Friday across the Southwest and the West Coast, with high temperatures possibly reaching 104 to 117 degrees in cities like Phoenix and Las Vegas and up to 95 degrees in San Diego.
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Record heat and dry weather could envelop much of the West, the National Weather Service said. More than 100 daily-record high temperatures will likely be set, including several all-time record highs. The high temperatures will begin Friday and extend through Monday.
In Los Angeles, daytime highs away from the coast are expected to be 100 to 115 degrees. "It is not recommended to spend any extended period outside during the heat of the day," the weather service said.
The heat will also intensify the threat of wildfires in what is already one of the most active fire seasons in California's history. More than 7,000 blazes have torn through about 1.4 million acres, fueled in part by dry conditions.
Climate change will make heat waves like that longer and more intense, and they will last later into the summer.
There is a risk of severe storms Thursday in Philadelphia, Baltimore, Washington, D.C., and other parts of the mid-Atlantic. Tornadoes are possible, with the threat greatest in the corridor between Washington and Baltimore.
The flash flood watches cover parts of Kentucky, Indiana, Ohio and West Virginia. Cities in the flood zone include Louisville, Kentucky; Cincinnati; and Charleston, West Virginia.
Heavy rain in the Ohio Valley led to flash flood concerns around Louisville, and more rain is forecast Thursday night.
In the Caribbean, Nana made landfall as a hurricane overnight on the coast of Belize before it was downgraded to a tropical storm. It will continue to affect parts of Belize, Mexico, Honduras and Guatemala on Thursday with heavy rain and gusty winds. Maximum rainfall of 8 to 12 inches in isolated areas is expected to cause dangerous flash flooding.