San Francisco sees historic deluge as atmospheric river slams California
Jan. 1, 2023 Updated Sun., Jan. 1, 2023 at 7 p.m.
A powerful atmospheric river drenched northern and central California on New Year’s Eve, unloading copious amounts of lowland rain and mountain snow. San Francisco posted its second wettest day in over 170 years of records because of this fire hose of tropical moisture.
The storm system triggered widespread flooding that inundated roads, forced high-water rescues and disrupted travel. It was the latest in a conga line of storm systems to affect the Golden State; at least two more are on the way in 2023’s opening week.
Flood advisories blanketed the Bay Area on Saturday, with heavy rains forcing the closure of Highway 101 in south San Francisco for nearly eight hours. For a time, the highway was completely submerged.
Downtown San Francisco registered 5.46 inches of rain, marking its second-wettest day in records dating to 1849. December is the city’s wettest month, with an average of 4.76 inches of rain. But the New Year’s Eve deluge boosted December 2022’s total to more than twice that: 11.7 inches.
Nearby Oakland nabbed it wettest day since records began in 1970, with 4.75 inches of rain. Redwood City saw 4.88 inches, its third-greatest daily total in the past 116 years.
The same storm caused rockslides, including on Highway 1 south of Big Sur, and toppled trees whose roots were loosened in the saturated soils. On Buena Vista Avenue in East San Francisco, a tree came crashing down onto parked cars, halting the progress of a nearby bus.
Flooding was reported in numerous locations in northern and central California on Saturday. In San Ramon, east of San Francisco, the fire department tweeted that it 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 the Golden State, which is welcome considering its long-standing severe drought. However, the additional precipitation on saturated ground also poses the risk of more flooding.
A fast-moving system coming Monday night into Tuesday will drop a quick half-inch or so in mainly in the central and northern parts of the state. It will be followed by a more robust and slower-moving storm system that has the potential to dump at least 2 to 3 inches of rain Wednesday and Thursday.
The storminess is due to a parade of atmospheric rivers, or conveyor belt-like corridors of tropical moisture with origins as far away as Hawaii. Each reaches California through the same basic processes: Each filament of moisture is drawn eastward in between two oppositely spinning weather systems – a counterclockwise-rotating low to the north, with clockwise-spinning high pressure in the tropics to the south. The two systems work together like meshing gears, tugging ribbons of soupy tropical air toward the east.
Atmospheric rivers carry their heaviest moisture about a mile above the ground, which is why higher elevations, like the Coastal Range and the Sierra Nevada, ordinarily end up with the jackpot precipitation totals.
Saturday’s setup was a bit different – what began as an atmospheric river transitioned into more of a classic soaking rain event as a center of low pressure passed overhead. That brewed downpours and isolated thunderstorm activity, which in turn led to higher rainfall rates.
During a three-hour window in San Francisco between roughly 10 a.m. and 1 p.m., the airport measured 2.63 inches of rain.
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 in elevation, with exceptional accumulations above 9,000 feet.
The U.C. Berkeley Central Sierra Snow Lab, which research 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 boosting the “fluff factor” of the snow.
While storms like this do periodically occur, heavy precipitation rates are made more likely by the effects of human-induced climate change, which is warming the atmosphere.
As the atmosphere heats up, the air can hold more water. When moisture is unavailable, that translates to a desiccated landscape and drought. But when storm systems introduce moisture, as was the case Saturday, the atmosphere can store and unload more water, leading to higher precipitation totals.
The frequency of top-tier heavy rain events is increasing markedly, even if overall precipitation totals over a year may not exhibit the same trend. Days with 2 inches or more of rain at San Francisco International Airport are more than 60 percent more common now than the 1950s.
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