San Francisco is readying for heavy rain, high winds, and flooding as it confronts the threats of an atmospheric river and a bomb cyclone.
The weather front is expected to hit Wednesday night and continue into Thursday, with further storms expected in the coming days, Newsweek reported.
The National Weather Service 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, Newsweek reported.
A strong atmospheric river will bring heavy rain and damaging winds to the region, multiple mudslides or landslides, swelling rivers, and blocked roads.
The weather front will be the third storm to hit California in recent weeks. One broke on New Year's Eve and caused severe flooding. At least two people died, Newsweek reported.
San Francisco Mayor London Breed tweeted that the city was preparing for the next onslaught by activating emergency operations to clear storm drains and distribute sandbags.
"In preparation for the storm, we're working with our City partners to clear storm drains, monitor low-lying areas & provide sandbags for San Francisco residents and businesses whose properties are prone to flooding during heavy rains. Learn more how you can get rain ready," Breed wrote.
Due to San Francisco's large homeless population, Breed tweeted that "the city has made shelter beds available for walk-up at some of our shelters to respond to the incoming storm."
Officials in San Jose, California, declared a local emergency on Tuesday night and ordered the evacuation of areas near waterways.
Meteorologists are tracking a bomb cyclone — 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, Newsweek reported.
Atmospheric rivers are narrow channels in the atmosphere that can carry a large amount of moisture.
"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, said to 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.
"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 New Year's Eve storm dropped 5.5 inches of rain on San Francisco.
"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."
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