A significant but slow-moving storm arrived in the Bay Area around midday Monday, bringing with it showers and a chance of thunderstorms, promising to impact the evening commute and linger through Tuesday morning before the skies dry up again through the rest of the week, meteorologists said.
The “notable early season storm” began brewing off the Pacific Coast last week and was forecast to bring rainfall to even the Bay Area’s driest spots. The storm comes just days after federal scientists announced that a La Niña seasonal effect has started, meaning that the waters off the coast of South America are cooler than normal and may bring drier winter weather to California.
The storm was forecast to arrive in the North Bay then move into the heart of the Bay Area by early afternoon, said NWS meteorologist Roger Gass. The storm was expected to continue to bring “scattered rain showers and isolated thunderstorms” through the evening and into early Tuesday morning.
“Within any heavier rain shower, there can be downpours at times, so just be mindful of water on the roadways,” Gass said. “We’re not seeing much in the way of rainfall right now, but the evening commute especially will be treacherous.”
The rest of the week was projected to dry out and warm up, with temperatures reaching into the high 60s and low 70s, said NWS meteorologist Dalton Behringer. On Wednesday, highs for the San Jose, Oakland and San Francisco are all projected to sit in the mid- to high 60s. Temperatures across the Bay Area are expected to creep into the 70s and even the low 80s by Saturday.
“It’s going back to normal after this little anomaly we’re having,” Behringer said.
Isolated showers started in the Bay Area around 5:30 a.m. Monday, according to a social media post from the National Weather Service. By early afternoon, scattered showers had started along the San Francisco Peninsula and in Sonoma County, Behringer said.
“We really aren’t (expecting) this to be a major rainmaker,” Gass added. “Being the first of the season, we would only expect nuisance flooding and very, very isolated flash flooding, but we’re not expecting any kind of flash flooding or anything like that.”
By Tuesday afternoon, the rainfall was expected to dry out across much of the Bay Area, Gass said. Some showers were projected to linger in the East Bay through most of the day, Behringer said.
“It’s really going to be in the morning hours and really by the commute, it looks like it should be mostly clear,” Behringer said. “We’ll have some lingering showers in the East Bay and portions of the North Bay, but most of the widespread, heavier rainfall will be well out of the area by then.”
There was a 20% to 25% chance of thunderstorms across the Bay Area that could bring brief, heavy rainfall, Gass said.
“When thunder roars, go indoors,” he advised.
The rain was brought to the Bay Area by an upper-level low system coming from the Pacific Northwest coast that passed first through the Bay Area then headed down to the Central Coast, Gass added. An upper-level low is a type of low pressure system that sits high in the atmosphere and brings unsettled and stormy weather, he explained.
“It is kind of out of the ordinary, but we’ve had early season storms this early in the past,” Gass said, adding as an example that San Francisco gets about 0.94 inches of rain in October. “This amount of rainfall, will get us right about normal … for the month.”
Rainfall totals across the Bay Area from Monday to Tuesday were expected to range from half an inch in lower elevations to 2 inches in higher elevations such as the Santa Cruz Mountains, Gass said.
San Jose was projected to receive between 0.75 and 0.9 inches of rain, Behringer said. Oakland, Walnut Creek and San Francisco were all expected to receive 0.8 inches, Gass said.
The North Bay “looks to be the least winners of the rain maker,” and was forecast to receive between a quarter-inch and an inch of rainfall, Gass added.
Behringer added that beachgoers should be cautious of rough seas brought on by the storm through Tuesday afternoon.
Gass advised drivers to drive slowly and leave extra time to arrive at their destinations.
“Turn around, don’t drown,” he said. “Never drive through flooded waterways.”
