A series of storms fueled by a strong atmospheric river are targeting the Bay Area and Northern California, amplified by a so-called “bomb cyclone” that promises heavy rain and potential flooding.
The National Weather Service said the storms will lash the North Bay with rainfall on Wednesday, move south and last through the weekend. More rain is expected across the region on Friday and the weekend.
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A Flood Watch was issued from 4 a.m. Wednesday to 4 a.m. Thursday for the Coastal North Bay, Marin Coastal Range, inland North Bay mountains and valleys, and the Sonoma Coastal Range.
Atmospheric river + bomb cyclone
The arrival of one atmospheric river – a long, narrow band of heavy moisture from the tropics – coincides with conditions in the Pacific Ocean where a polar air mass collides with a tropical air mass, causing atmospheric pressure to drop rapidly. Meteorologists call such low-pressure systems a bomb cyclogenesis, bombogenesis or a bomb cyclonecausing the storm to intensify and the wind to blow harder.
Bay Area residents were urged Monday to prepare for the elements during a two-day dry spell. The Weather Service said Monday in its daily forecast discussion that the atmospheric river would begin Wednesday and bring 1 to 3 inches of rain to the North Bay through Sunday, and up to 11 inches in far coastal areas of the North Bay.
North of the Bay Area, rain is expected to be even heavier and conditions will be treacherous. 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 was issued for counties along the northern Sierra Nevada.
Less rain further south
The Bay Area’s rain totals from the first storm were expected to decrease further south. The Weather Service said about 4 inches of rain was expected in areas between San Pablo Bay and Santa Rosa, 1 to 2 inches from the Bay to Marin and Sonoma counties on the Bay, 0.5 to 1 inch from Santa Rosa Cruz area to the Bay, and generally 0.25 inches or less south of Santa Cruz with little to no rainfall expected south of Monterey/Salinas.
However, from Friday morning through Sunday afternoon, the weather service said the North Bay could see an additional 2 to 3 inches of rain in the North Bay; 1 to 2.5 inches for northern parts of the Bay Area, including San Francisco, Santa Cruz Mountains, Big Sur Coast; and elsewhere 0.5 to 1 inch.
Heavy gusts of wind are expected
The Weather Service also issued a wind advisory for Marin, Napa and Sonoma counties, the Peninsula coast and the Bay coastline from 10 p.m. Tuesday through 6 a.m. Thursday. Strong southerly winds of 20 to 30 mph are expected, with gusts up to 45 mph, resulting in difficult driving conditions. In combination with the heavy rainfall, tree damage and associated consequences are expected.
With the arrival of the first major storm of the season, the Weather Service said soils should be able to handle at least the first rainfall, but some areas in the North Bay are likely to become saturated very quickly. The arrival of the second storm system on Friday could lead to more flooding, even though less rain is expected.
The Weather Service urged Bay Area residents to stay tuned to changing weather forecasts for the latest information as a potentially serious situation develops during the week.
