Flood warnings blanketed the Bay Area on Saturday, with heavy rains forcing Highway 101 in southern San Francisco to be closed for nearly eight hours. The highway for a while was submerged.
Downtown San Francisco recorded 5.46 inches of rain, marking the second wettest day in records from 1849. December is the city’s wettest month, averaging 4.76 inches of rain. But the New Year’s Eve deluge raised the December 2022 total to more than double: 11.7 inches. Nine weather stations in the Bay Area recorded at least 25 percent of their 2022 precipitation in a single day, according to the National Weather Service.
Nearby Oakland recorded its wettest day since records began in 1970, with 4.75 inches of rain. Redwood City saw 4.88 inches, the third-largest daily total in the past 116 years.
The same storm caused rockslidesalso on highway 1 south of Big Surand fallen trees whose roots were loose in the saturated soil. On Buena Vista Avenue in East San Francisco, a tree fell on parked carsthus stopping the progress of a nearby bus.
Flooding was reported Saturday at numerous locations in northern and central California. In San Ramon, east of San Francisco, the fire department tweeted that it has responded to more than 100 flood and storm-related incidents. Flood warnings were also issued around Sacramento, where up to 3 inches of rain fell.
More rain is coming to California, which is welcome given the state’s prolonged severe drought. However, the extra precipitation on saturated soil also entails the risk of more flooding.
A fast-moving system coming Monday night into Tuesday is expected to drop a quick half-inch, primarily in the central and northern parts of the state. It will be followed by a more robust and slower-moving storm system that could dump at least 2 to 3 inches of rain on Wednesday and Thursday.
The storm is due to a parade of atmospheric rivers, or conveyor belt-like corridors of tropical moisture originating as far away as Hawaii. Each reaches California through the same basic processes: Each filament of moisture is pulled eastward between two counter-rotating weather systems — a counterclockwise-rotating low to the north, with a clockwise high-pressure in the tropics to the south. The two systems work together like interlocking gears, pulling ribbons of smooth tropical air eastward.
Atmospheric rivers carry their heaviest moisture about a mile above the ground, which is why higher elevations, such as the Coastal Range and the Sierra Nevada, usually end up with the jackpot rainfall totals. Saturday’s setup was a bit different – what started out as an atmospheric river turned into a more classic soaking wet event as a center of low pressure passed overhead. That brought downpours and isolated thunderstorms, leading to higher rainfall.
During a three-hour period in San Francisco between about 10 a.m. and 1 p.m., the airport recorded 2.63 inches of rain.
Remember… just because the rains stop, the stream/creek levels take longer to clear because of all the runoff from nearby higher ground. Therefore, the effects of flooding could last overnight after the rain moves south. #cawx https://t.co/DvkLZegIhl
— NWS Bay Area (@NWSBayArea) December 31, 2022
High precipitation rates were also a staple of the snowfall that accompanied the system in the Sierra Nevada. Snow fell predominantly above 7,000 feet, with exceptional accumulations above 9,000 feet.
The UC Berkeley Central Sierra Snow Lab, which studies Sierra snowfall and water resources, observed snowfall rates as high as 7.5 inches per hour Saturday afternoon. An on-site researcher noted that the snow was light and fluffy, with frigid temperatures increasing the snow’s “fluff factor.”
Connections related to climate change
Although these types of storms occur periodically, heavy precipitation is more likely due to the effects of human-induced climate change, which is warming the atmosphere.
As the atmosphere warms, the air can hold more water. When moisture is not available, it translates into a parched landscape and drought. But when storm systems introduce moisture, as they did on Saturday, the atmosphere can store and drain more water, leading to higher precipitation totals.
The frequency of upper-level heavy rains is increasing significantly, even though annual precipitation totals may not show the same trend. Days with 2 inches or more of rain at San Francisco International Airport are more than 60 percent more common today than they were in the 1950s.
