A long and possibly record-setting heat wave is expected to hit California this week, bringing temperatures into the 90s and lower 100s on Wednesday with little relief expected overnight, according to the National Weather Service.
"There's a large dome of strong high pressure building over the Great Basin and it will shift westward over California tomorrow and Thursday and result in temperatures rising up to well above normal," said David Spector, a meteorologist for the National Weather Service.
A map of the U.S. on the National Weather Service website shows that most of Northern California is under an excessive heat watch, while Southern California is under an excessive heat warning, as temperatures could exceed 110 degrees in the next few days.
Counties under excessive heat warning include Los Angeles, where temperatures up to 115 degrees can be expected, and San Diego, where temperatures may hit 109 degrees. The heat warning is in effect from Wednesday until Monday, the National Weather Service said.
Parts of southern Nevada and western Arizona are also under an excessive heat warning from Tuesday until Sunday and will experience temperatures 8 to 12 degrees above normal.
In Northern California, where an excessive heat watch is in effect from Saturday to Tuesday, Sacramento, the San Francisco Bay Area and other areas could experience record temperatures of up to 115 degrees.
In the Pacific Northwest, parts of eastern Washington, eastern Oregon and southwestern Idaho are under heat advisories, meaning temperatures could reach about 105 degrees. The scorching conditions could intensify the Rum Creek Fire in Oregon, which has burned over 11,000 acres.
In Maricopa County, Arizona, an air quality alert was issued because "weather conditions combined with existing ozone levels are expected to result in local maximum 8-hour ozone concentrations that pose a health risk," the weather service said.
About 52 million people are under heat alerts in the Southwest, Southern California and the Pacific Northwest. Scores of records could be broken across the region Tuesday.
The National Weather Service warned that the extreme heat "will significantly increase the potential for heat related illnesses, particularly for those working or participating in outdoor activities."
"It's going to be dangerous outside for a long a period of time, especially for the elderly and pets," Spector said.
Heat kills more people annually in the U.S. than other weather disasters, including floods, hurricanes and tornadoes. It's recommended to drink plenty of fluids and stay indoors.
The hot and dry conditions also bring an elevated risk for fire in California, which has already suffered a number of wildfires this year — including the deadly McKinney Fire, the state’s largest blaze of the year.
The dangerously hot conditions can also worsen the state's historic drought.
"In terms of the drought, it's only going to get worse," Spector said. "The dry humidity will make even drier conditions."
