National Weather Service officials warned of "hazardous heat" that's expected to impact the San Francisco Bay Area early this week, as temperatures are forecast to climb as much as 15 to 20 degrees above seasonal averages across the inland valleys.
A heat advisory was issued for interior portions of the East Bay, the South Bay and the Santa Cruz Mountains at 11 a.m. on Monday morning and will remain in effect until 8 p.m., Dial Hoang, a meteorologist for the weather service's Bay Area office, told SFGATE. The advisory will go into effect again Tuesday during the same time frame for the East Bay, where sweltering conditions are anticipated to persist.
Hoang was hesitant to describe the event as a "heat wave," which is typically characterized by high temperatures that are sustained for three days or longer. "The bulk of the impacts are today and tomorrow," he said over the phone. "It's a little less intense than some of the heat events we've already seen this summer, but because there's a moderate to high risk of heat-related illness for sensitive groups, we still urge people to stay hydrated, avoid strenuous activities in the afternoon if possible, and never leave children or pets unattended in vehicles."
An upper-level ridge developing in the Pacific Northwest and extending toward California is driving the heat as it mingles with a low-pressure system forming off the coast of San Luis Obispo. The interaction between the two is "pretty weak," Hoang explained, but is nonetheless contributing to offshore flow in the upper atmosphere, which is leading to steadily increasing temperatures across the region.
The heat is expected to peak Monday afternoon, with the hottest temperatures reaching the mid-to-upper 90s throughout inland portions of the Bay Area and in eastern Marin County.
"Our current forecast has some isolated spots in the East Bay and North Bay that might get up to triple digits," Hoang said. "But at this point, we don't expect most of the region to see triple-digit heat."
Some of the forecast highs for Monday are as follows:
Relief is anticipated Monday night, with temperatures dipping into the mid-60s to lower 70s. Forecasters are also monitoring the marine layer, which may quell some of Tuesday's heat for cities along the coastline. In downtown San Francisco, temperatures will likely drop around 5 to 6 degrees, Hoang said.
Typical temperatures for the month of September are expected to return by Wednesday, with temperatures between the upper 70s and mid-80s in the inland valleys and in the mid-60s near the bay shoreline.