‘Heat dome’ brings California’s first heat wave of the season, with triple-digit temperatures

A significant, early-season heat wave is expected to broil much of inland California this week, with highs set to top 100 from the Sacramento Valley to the Antelope Valley — dangerously high temperatures that experts warn could bring health risks given that nighttime cooling will be limited in many areas.

Much of the western U.S. is bracing for the effects from a high-pressure ridge, or heat dome, which will begin warming up the region Tuesday, likely driving temperatures to near-record or record-breaking levels, with several excessive heat watches and warnings already issued.

In Southern California, the most extreme temperatures are expected Tuesday through Thursday in the inland mountains and deserts, with Wednesday supposed to be the hottest.

“It’s going to start heating up really tomorrow,� Joe Sirard, a National Weather Service meteorologist, said Monday morning. “Those temperatures in the lower mountains and deserts will be anywhere from 15 to 17 degrees above normal for this time of year.�

Highs in the Antelope Valley are expected to reach 103 to 108 degrees Wednesday and Thursday, while the Los Angeles County valleys will likely peak around 90 degrees. Statewide, the coast will generally avoid the worst of this heat wave, with L.A. beaches remaining in the 70s and 80s this week.

Lancaster and Palmdale are forecast to tie or break daily high temperature records, Sirard said, with historical highs topping out at 104 degrees in both cities. An excessive heat watch has been issued for Wednesday and Thursday across the Antelope Valley and its surrounding foothills, as well as a warning of “dangerously hot conditions� for the Apple and Lucerne valleys, where temperatures may go as high as 106 degrees.

National Weather Service officials are focused on the serious and potentially deadly health effects from extreme heat, especially for vulnerable populations, such as elderly people or pregnant women. The weather service reminds residents in these hot spots to stay hydrated, avoid the sun and the heat of the day and use air conditioning.

The heat is also expected to bring elevated fire conditions across the state.

This past weekend, the Golden State likely saw its busiest fire activity so far this year, with a 14,000-acre blaze in San Joaquin County that temporarily forced evacuations. Several other small fires popped up across the state, including two vegetation fires in Santa Barbara County and one in Riverside County.

The most extreme heat this week is expected near California’s border with Nevada, where an excessive heat warning will be in effect from Wednesday through Friday, with dangerously hot temperatures. Forecasters warned of highs hitting 115 degrees around Lake Mead and Lake Havasu City, 114 degrees in Las Vegas and up to 120 degrees in Death Valley.

“As the heat builds day by day there will be little relief during the overnights, especially within the Las Vegas valley and Death Valley National Park,� the warning said.

The excessive heat warning will also be in effect across the Mojave Desert on Wednesday and Thursday, where “dangerously hot conditions� are expected to bring highs between 96 and 106 degrees. In the San Joaquin Valley and lower Sierra Nevada foothills, highs are forecast to top out between 103 and 108 degrees.

In Northern California, the heat wave is expected to hit even sooner, with heat warnings going into effect Tuesday morning, including across the Sacramento and San Joaquin valleys and the Sierra foothills, where warnings of excessive heat have been issued. The mercury there is forecast to reach 95 to 108 degrees, and “limited overnight relief, with low [nighttime] temperatures in the 60s to mid 70s,� the warning said.

Across inland North and East Bay and the Sonoma County mountains, highs are expected to reach nearly 100 degrees Tuesday, Wednesday and Thursday, with nighttime temperatures dropping only into the 60s and 70s, according to the heat advisory issued for that region.

“There is a moderate to high risk for those who are heat sensitive, especially those without effective cooling or adequate hydration,� the National Weather Service Bay Area warned.

The widespread warming will also increase snowmelt, raising concerns about cold, fast-moving waterways across the state, creating what the weather service warned could be “potentially dangerous conditions for those seeking relief in rivers and lakes.�

The upper-level high pressure system driving this heat wave is expected to slowly weaken and move east by the end of the week, Sirard said.

“By Friday, temperatures do start to cool down a little bit,� Sirard said. “We are expecting a gradual cool-down through the weekend,� with more seasonally expected temperatures by Sunday.