San Mateo area residents face a weekend of unusual heat after the National Weather Service issued a Heat Advisory covering the San Francisco Bay Shoreline and much of the Bay Area.

The advisory takes effect at 11 a.m. Saturday, Oct. 3, and runs through 10 p.m. Monday, Oct. 5. Inland temperatures could reach the mid-90s to near 103 degrees, according to the NWS. Overnight lows in the upper 60s to mid-70s at higher elevations will limit the body's ability to cool down between hot days.

San Mateo sits along the waterfront, where temperatures tend to run cooler than inland valleys. The Mercury News reported on Sept. 28 that San Mateo was expected to reach the mid-to-low 70s during the initial warm-up earlier in the week.

NWS meteorologist Matt Mehle told the Mercury News on Sept. 28 that a high-pressure ridge was driving the heat. "We're going to keep the above-normal temperatures through the weekend," Mehle said. "Temperatures are going to be 5-10 degrees above normal all the way into next week."

ABC7 meteorologist Drew Tuma said highs will run 10 to 20 degrees above average, with some locations potentially reaching record highs. The warm spell will persist for about two weeks, Tuma said.

The NWS warned that this level of heat affects anyone without effective cooling or adequate hydration, with likely impacts on health systems and heat-sensitive infrastructure. Heat is the leading weather-related cause of death in the United States, exceeding fatalities from thunderstorms, flooding and tornadoes, according to the NWS.

That risk is compounded locally. About half of households in the San Francisco metro area lack air conditioning, one of the highest rates among major U.S. metro areas, according to Brookings Institution data and an ABC7 Data Team analysis of Census figures cited by Travelers Today.

Multi-day heat events with warm overnight temperatures are more dangerous than single-day spikes because warm nights can limit recovery from daytime heat, according to NWS guidance.

The NWS advises residents to drink plenty of water or sports drinks, shift outdoor activities to cooler parts of the day, take frequent breaks in air-conditioned spaces or shade, and never leave people or pets in unattended vehicles.

The advisory covers the East Bay, South Bay, Santa Cruz Mountains, Sonoma Coastal Range, North Bay interior mountains, and parts of Monterey and San Benito counties.