Following a dry fall, the Bay Area is bracing for significant rain this week, increasing the risk of flooding in portions of the North Bay and putting much of Northern California above its seasonal normal rainfall for this time of year.
According to National Weather Service meteorologist Nicole Sarment, the first atmospheric river of the season will arrive Tuesday night, delivering five days of pretty steady rain to the Bay Area.
The North Bay will be hammered hardest and first, with Napa and Sonoma projected to receive 4-6 inches of rain inland and 7 inches on the coast between Wednesday and Thursday night. In locations north of Healdsburg, up to 10 inches may fall.
According to Sarment, San Francisco might receive up to an inch and a half of rain during the storm’s initial wave, which consists of two low-pressure systems.
She stated that after the first sweeps through Wednesday, the second is predicted to start Friday and last through the weekend, spreading further south and inland.
Significant rainfall is expected in the North Bay and San Francisco this weekend. Both locations are expected to receive roughly the same amount of rainfall as earlier in the week, if not somewhat less. Except for the Santa Cruz Mountains, the storm is predicted to weaken as it moves south, with total rainfall in the South Bay expected to be less than an inch.
Despite early warnings that the flooding risk was low, a flood watch has been issued for Napa, Sonoma, and Marin counties from Wednesday until Friday morning. Dial Hoang, a National Weather Service meteorologist, stated that the risk will increase in the storm’s last days as soil that has been parched over the summer becomes wetter.
“As we go through Wednesday and Thursday, the soils will begin to saturate and small creeks and streams will begin to fill up,” Hoang told me. “So we see the flooding concerns increase through Friday and the weekend, even after the intensity of the rain begins to drop off.”
The storm might push Northern California above its seasonal rainfall average for mid-November, which seemed implausible after a drier-than-usual October.
“It would put us above normal,” Sarment added.
San Francisco typically receives approximately 2.4 inches of rain by this point in the year but has only received about 0.6 inches since the water year began on October 1. Following the storm, the city is expected to receive approximately 3.6 inches of rain.
The North Bay might potentially exceed its seasonal norms, which are approximately 3.5 inches in Sonoma and 1.7 inches in Napa.
“Our winter and rainy season is early here in Sonoma County,” stated Jeff DuVall, the county’s director of emergency management.
While urban and local spring flooding is possible, officials do not expect big rivers such as the Russian River to reach the monitor stage, at which preparations for potential flooding are recommended.
San Francisco is more vulnerable to flooding when rain falls at a rate greater than an inch per hour, thus it is unlikely that flooding will occur this week unless drains become clogged or there is an unanticipated traffic obstacle.
“I’m not trying to undercut what we’re expecting, but [that’s] the good news,” Sarment told KQED on Tuesday.
Nonetheless, people should make every effort to prepare for rainy and windy weather. Sarment recommended homeowners clean their gutters, identify any trees with loose branches or roots on their property, and secure any outdoor furniture that may be blown away.
Once the storm starts, keep an eye out for downed power lines that may be active, and avoid driving through flooded areas.
“Honestly, it would just be a good weekend to stay home and just stay out of the way,” she told me. “I’m sure there’ll be trees down, power lines down, and all that kind of stuff.”
