California Heat Wave Will Be ‘Exceptionally Dangerous’ for Days, Forecasters Warn

Updated 2:06 p.m. Tuesday

An extreme early summer heat wave hitting the Bay Area this week will bring little chance for relief given its length and high nighttime temperatures, posing danger even for generally cooler places like San Francisco and spurring dire warnings from forecasters.

Shortly before noon Tuesday, the National Weather Service extended its excessive heat warning to last an entire week, in effect until 11 p.m. next Tuesday, July 9, for wide swaths of inland Northern California. The risk of heat-related illness will be high, the weather service warned, noting that heat is the top weather-related killer in the U.S.

“An exceptionally dangerous situation is expected to unfold over the next week as we enter a potentially historic and deadly heat event. Several days of temperatures well above normal will lead to compounding effects among people and infrastructure, with the possibility of numerous heat related fatalities,” the weather service said in its warning. “It cannot be stressed enough that while one day at these temperatures may be manageable for some, an event of this scale, magnitude, and longevity will likely rival anything we’ve seen in the last 18 years.”

Some inland parts of the Bay Area are forecast to reach triple-digit temperatures by early Tuesday afternoon, and highs in the Sacramento Valley and surrounding foothills are expected to reach 110 to 115 this week.

In San Francisco, where fog and cool ocean air generally curb the dangers of high temperatures, a heat advisory will be in effect for much of Tuesday and Wednesday.

The Mission, Tenderloin and South of Market neighborhoods are among those at the greatest risk of reaching temperatures ranging from the low 80s to the mid-90s this week, according to the city’s Department of Emergency Management. Those “urban heat island neighborhoods” are also home to many unhoused people, older adults and others who are at higher risk of heat illness.

According to the Department of Homelessness and Supportive Housing, the city will implement a hot weather protocol — including increased wellness checks, water bottle distributions and transportation vouchers — through Friday to help those feeling adverse effects of the heat.

Two additional temporary shelters will also have walk-up services available through Friday near the South of Market and Tenderloin neighborhoods, and the Department of Emergency Management said people should visit public places with air conditioning, including the city’s libraries and Southeast Community Center.

Being able to cool down will be important, but because the high-pressure ridge spurring the heat is coming from over the Pacific Ocean instead of inland, even coastal areas like San Francisco won’t feel the usual overnight reprieve from the marine layer, UCLA climate scientist Daniel Swain said during his “Weather West” office hours on YouTube.

The combination of high daytime and nighttime heat can pose health risks without an opportunity to cool down.

“This is a problem for human health and physiology because the human body requires at least a certain number of cool hours to maintain itself at a reasonably cool temperature in those overnight hours,” Swain said. “Ideally, the natural world drops to that temperature overnight, or at least you have access to some active cooling like air conditioning.”

The duration of such an extended heat event is a major risk especially for older people and people with underlying health conditions, such as heart disease or kidney disease, said Dr. Gina Solomon, the chief of UC San Francisco’s Occupational, Environmental and Climate Medicine Division.

“People’s bodies really are stressed by extreme heat, and the longer the heat goes on, the worse it is. That’s especially true with heat waves like this one, where the temperatures stay high day after day, they stay high even at night, and our bodies don’t get a respite,” she told KQED.

“We get gradually more dehydrated, our cardiovascular systems are stressed, people’s kidneys are stressed. For people with underlying conditions, that’s a real problem and a health risk.”

Solomon noted that sometimes, older people can lose their thirst reflex, so remembering to drink water is very important. She also recommended that all people find cool places to spend time and avoid liquids that can be dehydrating, such as caffeinated drinks and alcohol.

Heat conditions are also expected to last well into next week, according to Nicole Sarment, a meteorologist in the National Weather Service’s Bay Area office. Because the Climate Prediction Center is forecasting above-average temperatures at least through the next two weeks, people should take extra precautions to stay cool, she said.

“We’re still in a potentially deadly heat event that is going to last for the better part of two weeks,” she told KQED. “People need to be in air conditioning; they need to wear light, loose clothing, not be outside if they don’t have to and stay hydrated.”