ORANGE COUNTY — Southern California today enters the worst part of a dangerous heat wave expected to linger through most of the week, bringing triple-digit temperatures to many areas while increasing health risks and the risk of wildfires.

An excessive heat warning issued by the National Weather Service will be in force until 10 p.m. Wednesday in the San Gabriel, Santa Clarita, San Fernando and Antelope valleys, the San Gabriel Mountains, the Santa Monica Mountain Recreational area and both coastal and inland Orange counties.

The heat wave is expected to peak today, and National Weather forecasters said temperatures would be 10 to 20 degrees above average.

The NWS forecast sunny skies in Orange County today along with highs of 77 in Laguna Beach; 78 in San Clemente; 79 in Newport Beach; 93 in Anaheim and Irvine; 96 in Fullerton and Mission Viejo; and 98 in Yorba Linda.

The weather service attributed the heat wave to a “strong upper-level high-pressure system over the region combined with weakening onshore flow.”

The NWS said it is certain that even coastal areas “will also warm significantly during this heat wave, with highs mostly in the 80s near the beaches and between 90 and 100 across inland portions of the coastal plain, including Downtown Los Angeles.”

The heat wave will also bring warm overnight temperatures, the statement said, adding there is a potential for “significant heat impacts across much of southwest California” for residents without air conditioning.

Additionally, “the hot and dry conditions combined with very dry fuels” — the vegetation — “will bring an extended period of elevated fire danger to southwest California.”

Forecasters urged residents to protect themselves and those close to them by finding access to air conditioning, reducing time spent outdoors, drinking plenty of water, checking on friends and neighbors — especially the elderly — and by never leaving children, seniors or pets in parked cars, even with windows cracked open. The interior of a vehicle parked in hot weather can rapidly reach lethal levels.