LOS ANGELES — As Southern California's worst heat wave of the year intensifies and expands across the region Thursday, officials warn the next few days could bring record-breaking high — and low — temperatures.

Widespread heat advisories have been issued across California and the U.S. Southwest, even encompassing much of the often-spared coastline. While daytime temperatures in the 90s and triple digits could set records that will exacerbate health and fire concerns, climate scientists warn it's the overnight highs that might be the biggest concern.

"The entire L.A. Basin, even downtown L.A., [is] not getting below the low 70s," Daniel Swain, a UCLA climate scientist, said in a recent briefing. Parts of the San Gabriel Valley will remain above 80 degrees at night, and much of the Palos Verdes peninsula won't fall below 70, according to the National Weather Service.

Phoenix is not forecast to drop below 90 degrees over the next few days.

"Those overnight minimum temperatures will set records," Swain said. "We don't talk about them as much, but they're hugely consequential from a human health and, in particular, wildfire perspective."

An extreme heat warning is in place for much of Los Angeles, Ventura, Santa Barbara and San Luis Obispo, warning of "dangerously hot conditions" through at least Saturday. Temperatures Thursday are expected to hover in the 90s inland from the coast, and somewhere between 100 and 109 degrees in the valleys, lower mountains, farther inland to the Antelope Valley. In the Mojave desert and Coachella Valley, temperatures will climb above 110 degrees.

"This dangerous level of heat will pose a threat to anyone without effective cooling and adequate hydration," the weather service's Weather Prediction Center warned. "Highs into the 90s more broadly across the West will still pose a threat to anyone more sensitive to heat or engaging in strenuous outdoor activities."

In a Thursday morning forecast, the weather service's Oxnard office warned: "Only the beach areas will not have dangerous heat."

Woodland Hills is forecast to hit 107 on Thursday and downtown Los Angeles may hit 97. It's forecast to hit 101 degrees in Pasadena.

Compounding those safety concerns, officials have warned the heat wave could foster an environment ripe for fast-moving wildfires. A red flag warning is in effect for the Los Angeles and Ventura County mountains and foothills through Saturday night.

"If fire ignition occurs, conditions are favorable for extreme fire behavior and rapid fire growth, which could threaten life and property," the red flag warnings said.

The dry, hot weather coupled with an unstable atmosphere — that could bring some weekend thunderstorms — is a recipe for dramatic fire growth, officials warn. It's particularly a concern in Southern California, where back-to-back wet years helped increase vegetation growth before this year's dismal rainy season.

"We have the legacy of this wet-to-dry whiplash event," Swain said. "There's literally more biomass, more potential fuel, for the fires to burn."

The high-pressure system driving this heat wave will not only persist over the region for a prolonged period, but is also expected to be "stronger than normal," Kristan Lund, another weather service meteorologist in Oxnard, said earlier this week.

The hot weather could be accompanied by some monsoonal thunderstorms in the mountains and deserts by the weekend, which could further elevate fire concerns given the chance for lightning. The storms, if they materialize, may also create localized flooding, destructive winds and debris flows, especially in recent burn scars.

That pattern will keep the area unstable — and likely hot — into next week, when long-range forecasts show mercury could begin falling, said Sam Zuber, a weather service meteorologist in San Diego.

We're "probably going to see a slight cooldown into early next week — but it's still going to be above normal," Zuber said.

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