Residents of the Bay Area and Northern California woke up Wednesday to the arrival of a storm, fueled by a strong atmospheric river and intensified by a bomb cyclone off the coast of the Pacific Northwest. The storm is expected to bring significant impacts, including heavy rainfall, damaging winds and flooding.
The most impacts are expected in the North Bay and northeastern part of the state. The National Weather Service has issued a flood warning for Marin, Napa and Sonoma counties until 4 a.m. Saturday, with 2 to 12 inches of rain expected. The North Bay, the San Francisco Bayshore and the Pacific Coast are also under a wind advisory until early Thursday for gusts of up to 70 to 80 km per hour.
In addition, a high surf advisory was in effect Wednesday morning through Friday morning as the weather service said breaking waves of 15 to 20 feet were possible.
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A “major event” for the North Bay
Rain totals will be highest in the North Bay and localized flooding is likely in low-lying areas.
“Here’s what we know: This will be a significant event for the North Bay,” the Weather Service said Wednesday in its forecast discussion for the San Francisco area. “There is now a 90% chance that Santa Rosa will record more than 5″ by the end of the weekend and a 10% chance of exceeding 12″ by the end of the weekend. Normal for the entire month of November is 3.25”.
North of the Bay Area, the storm is expected to be particularly treacherous, while storm effects will diminish further south. The Weather Service’s Eureka office has issued a series of storm and flood warnings, warnings and advisories throughout Northern California to the Oregon border. Meanwhile, a winter storm warning has been issued for counties along the northern Sierra Nevada.
Power outages throughout the region
Several thousand PG&E customers were without power as of 8 a.m., according to the utility’s outage map. Most of the outages were concentrated in the North Bay in Sonoma and Marin counties. Further north, in the Eureka area and Humboldt County, where the storm was most intense, thousands of others were also without power.
Pacific Gas and Electric said its crews are preparing for outages caused by the intense winds expected in the region.
“With the high winds, we can expect to see tree limbs snapping and coming into contact with power lines, or tree lines falling into power lines and causing power outages,” said PG&E spokesman Paul Moreno. “We have ensured we have the personnel and resources such as materials, poles and wires to meet this challenge.”
What is an atmospheric river? And what is a bomb cyclone?
An atmospheric river – a long stretch of heavy moisture from the tropics – flows into Northern California, while a polar air mass collides with a tropical air mass in the Pacific Ocean. The two air masses caused atmospheric pressure to drop rapidly, creating a bomb cyclogenesis or bomb cyclone, which strengthened the storm and made the winds stronger.
Atmospheric rivers can produce heavy rain and snow, and strong ARs can transport water vapor approximately equal to 7.5 to 15 times the average flow of the Mississippi River. When the atmospheric river and the bomb cyclone coincidea major weather event is expected.
When will the storm end?
The weather service said there may be a slight lull in the storm on Thursday, ahead of another intensified cyclone on Friday, which will push another round of heavier rain through the region. The rain will be accompanied by a cold air mass and increased instability, transitioning from steady rainfall to showers and possible thunderstorms late Friday through Saturday.
Will there be flooding in Northern California?
Although the heaviest rain will end this weekend, the chance of additional light rain remains through Wednesday. The Weather Service said that since it is still early in the water year, the largest rivers are still at high capacity and are not expected to overflow. However, flooding of smaller creeks and streams, as well as flooding of towns and roads, are all expected and could have significant impacts.
