Why California's coming heat wave could be more dangerous than you think

The Bay Area is heading into its first heat event of the season and it's arriving abruptly.
A heat advisory is in effect for California's interior Friday and Saturday, including parts of the East Bay, North Bay and South Bay. It comes as a strong area of high pressure builds over the West, triggering a sharp temperature spike that could push some Central Valley cities toward record highs for May.
In the Bay Area, it's not just the peak temperatures that make this heat event notable, it's how quickly they arrive. In places like Livermore, highs are expected to jump from the low to mid-80s on Thursday to triple digits by Friday: that's an almost 20-degree spike in just 24 hours. This kind of surge would rank among the most abrupt single-day warmups the region has seen in the last quarter century.

The South Bay will see a similar surge. San Jose is forecast to rise from the low 80s on Thursday to the upper 90s by Friday, a 13- to 15-degree spike in one day. That would rank among the sharpest single-day warmups in the past 25 years. The jump will be a bit less dramatic in the North Bay, where hotter spots like Santa Rosa and Napa are expected to climb about 10 degrees, from around 90 on Thursday to near 100 by Friday.
What makes the heat even more striking is how unremarkable things have been until now. Most of the Bay Area's inland cities have cruised through May with average highs, warm, but far from hot. San Jose hasn't cracked 90 degrees yet this year; Livermore's and Santa Rosa only did it once. For much of the region, this week marks the first true taste of summer heat.
With temperatures ramping up quickly, it's the kind of shift that can catch people off guard. Our bodies need time to adjust to extreme heat, and most of us simply aren't acclimated to triple-digit temperatures yet. That's part of what makes early-season heat especially risky. The National Weather Service's Heat Risk Index notes that first heat events of the year tend to carry higher health risks across all demographics, even for otherwise healthy individuals.
Wednesday breakdown
San Francisco: A deep and persistent marine layer will blanket much of the city in the morning, potentially producing pockets of drizzle. Partial afternoon sun may break through, but it'll be a chilly day overall. Highs will reach the upper 50s in the Outer Sunset and Richmond and the low 60s downtown, with a steady west wind at 10 to 20 mph. Clouds return overnight with lows around 50 degrees.
North Bay: The marine layer will penetrate deeper into the North Bay valleys in the morning, eventually retreating to the immediate Marin coastline. The rest of the day will be mostly sunny but cool. Highs will be in the upper 60s in Vallejo, the low 70s in San Rafael and Petaluma and the upper 70s to around 80 degrees in Santa Rosa and Vacaville. Winds from the west will funnel through the valleys at 10 to 25 mph. Partly cloudy skies overnight with lows in the 50s.
East Bay: Marine layer clouds will linger west of the Berkeley Hills through much of the morning, with drizzle possible in the hills. It'll be sunny inland, with sunshine gradually pushing toward the bay shore by afternoon. Highs will reach the upper 60s in Richmond and Oakland, and the low to mid-70s in Hayward, Concord and Livermore. A stiff westerly breeze will kick up along the bay shore, with gusts up to 25 mph. Overnight, clouds return to the water, while inland areas stay mostly clear with lows in the low 50s.
Pacific Coast and Peninsula: Patchy drizzle and low clouds will dominate the morning hours along the coast, especially in Pacifica and Half Moon Bay, where highs will stay stuck in the upper 50s. A bit of sun may peek through in the afternoon, but don't expect much clearing along Highway 1. Daly City and Redwood City should see partial afternoon sunshine, with highs ranging from the low 60s to around 70 degrees. West winds will gust up to 25 mph in the afternoon. Overnight, clouds and patchy drizzle return to the coast, with lows in the low 50s.
South Bay and Santa Cruz: Morning clouds will give way to sunshine across the South Bay, with highs in the upper 60s in Mountain View, low 70s in San Jose, and upper 70s in Gilroy. Santa Cruz starts off gray with patchy drizzle, but skies clear by midday, with highs in the low to mid-70s. The Santa Cruz Mountains will stay cooler, topping out in the 60s to low 70s. Winds will pick up out of the northwest at 10 to 20 mph by afternoon. Overnight, patchy drizzle may return to the coast, with lows in the upper 40s to low 50s.

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