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San Francisco is preparing for a barrage of heavy rain, high winds, and flooding as it faces the twin threat of an atmospheric river and a bomb cyclone. The weather front is expected to hit on Wednesday evening and continue into Thursday, with further storms anticipated in the coming days.
The National Weather Service (NWS) issued a flood watch for San Francisco and many of the surrounding areas of northern California, predicting between 2-4 inches of rain in the valleys, and up to 10 inches in coastal mountain regions.
Forecasting that "a strong atmospheric river will bring heavy rain and damaging winds to the region," the NWS has warned of "multiple" mud or landslides, swelling rivers, and blocked roads.
The weather front will be the third storm to hit California in recent weeks; one which broke over the state on New Year's Eve and caused severe flooding. At least two people died as a result of the storm.
London Breed, San Francisco's Mayor, wrote on Twitter yesterday that the city was preparing for the next onslaught by activating emergency operations to clear storm drains and distribute sandbags. "We're going to do everything we can to prepare, but we can't control the weather," she said in a briefing with city officials the same day.
Meanwhile, officials in the nearby city of San Jose declared a local emergency on Tuesday night and ordered the evacuation of areas near waterways.
Meteorologists are tracking a bomb cyclone—so named due to a rapid drop in pressure that brings strong winds and heavy rain—over the northern Pacific, which is expected to combine with an atmospheric river flowing into California.
Atmospheric rivers are narrow channels in the atmosphere that are able to carry a large amount of moisture with them. According to the National Oceanic and Atmospheric Administration, a strong river is able to transport as much as 15 times the amount of water flowing through the mouth of the Mississippi River.
"What you have [with atmospheric rivers] is cold air coming down from the Arctic and warm air coming up from the subtropics. They meet together and it's in that place that the cold air chills the warm air that causes there to be a lot of moisture in the atmosphere and a lot of rain," Mark Maslin, a professor of earth system sciences at University College London and a climate author, told Newsweek.
"What California has at the moment is it has lots of moist air being brought across the Pacific by the Jetstream. It's then being hit by a massive storm coming down from the Arctic, which has huge amounts of cold air, and as it hits the California warm air, it's causing massive amounts of rain," he said.
"And so you have these two sources of water: one coming from the Arctic cold air, which is being warmed up, and therefore the warm air is then precipitating rainfall, and you have lots of rain coming from the Pacific."
The NWS said in a discussion of the Bay Area forecast that "the meteorological setup is actually quite different than the NYE storm. But at the end of the day, we expect similar types of impacts though not necessarily in the same locations that saw them on Saturday."
After a brief reprieve on Friday, meteorologists are predicting further storms to come into next week, which may exacerbate the flooding.
The NWS noted that the oncoming weather front "will not be a 'one and done' storm. Details are still hard to pinpoint but the resounding message is several more storms look to be lined up for this weekend, into next week, and towards MLK weekend. The cumulative effect of these storms could be quite debilitating."
Even when the rainfall does abate, albeit only for brief spells, there will still be a risk of flooding due to the large amounts of water that have already fallen, saturating the ground and waterways.
"This is where we have to start to watch for issues on the rises of creeks and streams and street flooding concerns as well," Tamara Berg, a meteorologist for Sacramento's KCRA3 TV station, said on Tuesday.
In San Francisco, the highest threat of flooding will come between 2 p.m. and 9 p.m. Wednesday, according to Drew Tuma, a meteorologist at local news network ABC7. In a tweet on Tuesday, he wrote: "Given our wet soil conditions produced by recent heavy rains, flooding is likely."
AccuWeather meteorologists also attributed the New Year's Eve storm—which Mayor Breed said dropped 5.5 inches of rain on the city, not seen since 1849—to an atmospheric river.
During that storm, a 72-year-old resident of Santa Cruz was found under a fallen tree at Lighthouse Field State Beach, the California Department of Parks and Recreation said in a statement on December 31. The following day, another person was found dead in a submerged vehicle by a fire department helicopter, CBS Sacramento reported.
"California does get rainfall in the winter—actually it gets quite heavy rainfall," Maslin said. "What's happening here—and its partly weather, partly climate change—is rain is falling more heavily because there is more moisture in the atmosphere because the atmosphere is warmer.
"What's been happening, we've seen, for the last 50 or 60 years is rainfall events are becoming shorter and more intense as that weight of water in the atmosphere, when it gets to a critical point, literally drops."
Update 01/04/23, 8:10 a.m. ET: This article was updated with comments from Professor Mark Maslin.
About the writer
Aleks Phillips is a Newsweek U.S. News Reporter based in London. His focus is on U.S. politics and the environment. ... Read more