Atmospheric river arrives in Northern California as bomb cyclone intensifies damaging storm
Bay Area and Northern California residents woke up Wednesday to the arrival of a storm fueled by a strong atmospheric river and intensified by a bomb cyclone off the Pacific Northwest coast. The storm is expected to bring significant impacts including heavy rainfall, damaging winds and flooding.
Most of the impacts are expected in the North Bay and the northeast portion of the state. The National Weather Service has issued a flood watch for Marin, Napa and Sonoma counties through 4 a.m. Saturday morning with 5 to 12 inches of rain expected. A wind advisory is also in effect for the North Bay, San Francisco Bayshore and Pacific coast through early Thursday for gusts reaching up to 45 to 50 mph.
In addition, a high surf advisory was in effect from Wednesday morning through Friday morning as the Weather Service said breaking waves of 14 to 22 feet were possible.
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A "major event" for the North Bay
The rain totals will be highest in the North Bay and low-lying spots will likely see localized flooding.
"Here's what we know, this will be a major event for the North Bay," the Weather Service said in its San Francisco area forecast discussion Wednesday. "There is now a 90% chance that Santa Rosa will record more than 5" and a 10% chance of exceeding 12" by the end of the weekend. The normal for the entire month of November is 3.25"."
North of the Bay Area, the storm is expected to be especially treacherous while the storm effects will lessen farther south. The Weather Service's Eureka office has issued a series of storm and flood watches, warnings and advisories across Northern California up to the Oregon border. Meanwhile, a winter storm warning has been issued for counties along the northern Sierra Nevada.
Power outages across region
As of 8 a.m., several thousand PG&E customers were without power according to the utility's power outage map. Most of the outages were concentrated in the North Bay in Sonoma and Marin counties. Farther north in the Eureka area and Humboldt County where the storm was the most intense, thousands more were also without power.
Pacific Gas and Electric said its crews have been preparing for outages caused by the intense winds expected across the region.
"With the high winds, we can expect tree branches to break off and contact power lines or tree lines to fall into power lines and cause power outages," said PG&E spokesman Paul Moreno. "We've been making sure that we have staffing available and resources such as materials and poles and wires available to meet this challenge."
What is an atmospheric river? And what is a bomb cyclone?
An atmospheric river — a long band of heavy moisture from the tropics — is flowing into Northern California at the same time a polar air mass is colliding with a tropical air mass in the Pacific. The two air masses caused the atmospheric pressure to drop quickly, creating a bomb cyclogenesis, or a bomb cyclone, intensifying the storm and increasing its winds.
Atmospheric rivers can produce heavy rain and snow, and strong ARs can transport water vapor roughly equivalent to 7.5 to 15 times the average flow of the Mississippi River. When the atmospheric river and bomb cyclone coincide, a major weather event is expected.
When will the storm end?
The Weather Service said there may be a slight lull in the storm Thursday ahead of another intensified cyclone on Friday, pushing another round of heavier rain across the region. The rain will be accompanied by a cold air mass and increased instability, shifting from steady rainfall to showers and potential thunderstorms late Friday through Saturday.
Will there be flooding in Northern California?
While the heaviest rain will be over by the weekend, the chance for additional light rain will continue through Wednesday. The Weather Service said that since it is still early in the water year, the largest rivers still have a lot of capacity and are not expected to flood. However, flooding of smaller creeks and streams, as well as urban and roadway flooding, are all expected and can bring significant impacts.
