California is facing a prolonged late-summer heat wave this week, with widespread triple-digit temperatures starting in the south and spreading northward, the National Weather Service said Monday.

Excessive heat watches will go into effect Wednesday morning and remain in effect through Sunday evening in a large swath of Southern California, including much of the normally temperate coastline, forecasters said.

Temperatures were predicted to top 100 degrees (37.7 Celsius) in many valley and mountain locations.

“This heat may be record breaking and will likely produce a very high heat illness risk,” the Los Angeles-area weather office wrote.

The main highway from Los Angeles to Phoenix was damaged by a flash flood that washed out part of the road through the Southern California desert in the latest bout of punishing monsoonal thunderstorms that have hit the region this summer.

The newest round of flooding started Wednesday evening, damaging a roadway that was part of a detour past a repair project along eastbound Interstate 10 near the small community of Desert Center, about 165 miles east of Los Angeles.

Traffic in both directions was halted initially, but westbound lanes for motorists heading from Arizona to California reopened later.

The McKinney Fire in Northern California was still out of control on Wednesday despite cloudy, cooler conditions on Tuesday night.

The fire has burned nearly 90 square miles since it started on Friday in an unpopulated area in Klamath National Forest.

CalFire listed the blaze as 0% contained as of Wednesday afternoon, unchanged from its level of containment on Monday.

Multiple evacuation orders and warnings have remained in effect since Monday.

Wildfires in California and Montana exploded in size overnight amid windy, hot conditions and were quickly encroaching on neighborhoods, forcing evacuation orders for over 100 homes Saturday, while an Idaho blaze was spreading.

In California’s Klamath National Forest, the fast-moving McKinney fire, which started Friday, went from charring just over 1 square mile to scorching as much as 62 square miles by Saturday in a largely rural area near the Oregon state line, according to fire officials. The fire burned down at least a dozen residences and wildlife was seen fleeing the area to avoid the flames.

A destructive wildfire near Yosemite National Park burned out of control through tinder-dry forest on Sunday and had grown into one of California’s biggest blazes of the year, forcing thousands of residents to flee remote mountain communities.

Some 2,000 firefighters battled the Oak Fire, along with aircraft and bulldozers, facing tough conditions that includes steep terrain, sweltering temperatures and low humidity, according to the California Department of Forestry and Fire Protection, or Cal Fire.

“It’s hot out there again today,” Cal Fire spokesperson Natasha Fouts said Sunday. “And the fuel moisture levels are critically low.”

Preston Brown knows the risk of wildfire that comes with living in the rural, chaparral-lined hills of San Diego County. He’s lived there for 21 years and evacuated twice.

That’s why he fiercely opposed a plan to build more than 1,100 homes in a fire-prone area he said would be difficult to evacuate safely. Brown sits on the local planning commission, and he said the additional people would clog the road out.

“It’s a very rough area,” Brown said. “We have fires all the time now.”

A wildfire threatening the largest grove of giant sequoias in Yosemite National Park more than doubled in size in a day, and firefighters were working in difficult terrain Sunday to protect the iconic trees and a small mountain town as the U.S. weathers another very active year for fires.

Campers and residents near the blaze were evacuated but the rest of the sprawling park in California remained open, though heavy smoke obscured scenic vistas and created unhealthy air quality.

“Today it’s actually the smokiest that we’ve seen,” Nancy Phillipe, a Yosemite fire information spokesperson, said Sunday. “Up until this morning, the park has not been in that unhealthy category, but that is where we are now.”

A wildfire that erupted in Northern California forced evacuations as it threatened about 500 homes and other buildings, authorities said.

The Rices Fire erupted at around 2 p.m. near the Yuba River in Nevada County and had spread to more than 900 acres, according to CalFire.

The rural area is in the Sierra Nevada, northeast of Sacramento and about halfway between the state Capitol and the Nevada border.

A Southern California regional passenger rail service announced that it is testing technology that will use the West Coast’s earthquake early warning system to automatically slow or stop trains before shaking begins.

The five-county Metrolink system said the technology is an advancement of a previous version deployed in September 2021 that sends automated messages instructing train crews to slow or stop but does not have automated braking.

The combination of tropical moisture and an offshore storm will result in an uptick in thunderstorms across portions of California on Wednesday and Thursday, weather experts are warning.

With most of the storms will contain little or no rain and could lead to a surge in wildfires, AccuWeather meteorologists warn.