The Bay Area is in for a finicky week of weather as temperatures plummet to the mid-to-upper 50s, joined by possible drizzle, before changing course and soaring to the mid-90s by the middle of the week, leading to a potential heat advisory.
A low-pressure system moving through the Central Coast and approaching the southern tip of the Bay Area is driving the initial cooldown, Dylan Flynn, a meteorologist for the National Weather Service’s Bay Area office, told SFGATE on Sunday. Accompanying it are high clouds that block the sun, causing temperatures to drop five to 10 degrees below normal, he explained. For people residing along the coastline, “September is one of the warmer months out of the year,” Flynn said. “When it’s cooler, it tends to feel pretty nice.”
By Monday, some drizzle is expected, but it’s difficult to pinpoint where exactly it could end up, Flynn said. Right now, meteorologists are eyeing parts of the Central Coast and the South Bay, like Half Moon Bay, Monterey, Big Sur and the Santa Cruz Mountains.
“We’re looking at drops less than half of a millimeter here,” Flynn said. “When that happens, we don’t get much accumulation. Maybe a couple hundredths of an inch here and there. You might have to use your windshield wipers once or twice.”
Following the low-pressure system is a ridge of high pressure that’s forecast to gradually build temperatures to five to 10 degrees above normal through midweek, with Wednesday and Thursday likely becoming the hottest days. The heat could climb into the mid-90s in interior portions of the Bay Area, with temperatures likely reaching around the mid-80s in San Jose and the mid-70s in San Francisco and along the coastline.
Flynn said that even though similar temperatures are expected between Wednesday and Thursday, the weather service is looking at issuing an advisory for heat risk only for Thursday. Because forecasters are tracking the cumulative effect of high temperatures over multiple days, that day will likely see a stronger impact.
“It’s not uncommon to have a heat advisory in September, but it’s usually expected with strong offshore winds that bring the heat down along the coast,” Flynn said. “We don’t have that here. So, if anything, that’s when it would be, and it would be targeted to the interior.”
Meteorologists expect to make the final call by Tuesday. Regardless, “there is an end in sight,” Flynn said. A low-pressure system coming from the Pacific Northwest is supposed to bring temperatures back to normal by Friday.
“So it’s a little bit of a roller coaster this week,” he continued. “But nothing looks to be too extreme.”
