SAN DIEGO - Scorching temperatures again will plague portions of San Diego and Riverside counties today, but Southern California's heat wave is expected to dissipate beginning this weekend, forecasters said.

An Excessive Heat Warning issued by the National Weather Service will go into effect at noon today, lasting until 6 tonight in the San Diego County mountains and in Riverside County's far inland valleys and foothills, and mountain locales under 5,000 feet.

The warning will be extended two hours in the San Diego County deserts and the Coachella Valley in Riverside County.

Temperatures in San Diego County will reach up to 107 in the mountains and 117 in the deserts, according to meteorologists.

The mercury could reach 115 degrees in the Coachella Valley, and foothills and mountain regions below 5,000 feet in Riverside County could swelter in 110-degree heat, forecasters said.

The weather service advised people to avoid heat-related health problems by limiting their time in the sun, staying well-hydrated and checking up on potentially vulnerable relatives and neighbors.

The extreme heat has been baking the region all week long, but NWS forecaster Greg Martin said a marine layer that has been looming along the coast began to slowly move eastward on Independence Day, which will cause temperatures to drop gradually.

"That zone of cooling has been creeping further inland each day," Martin said.

A steady cooling trend will begin tomorrow and lead into next week, bringing the high temperatures down to their expected seasonal levels, according to Martin.