Strong atmospheric river and bomb cyclone to hit Bay Area with heavy rain, possible flooding

A series of storms fueled by a strong atmospheric river is taking aim at the Bay Area and Northern California, intensified by a so-called "bomb cyclone" that promises heavy rain and potential flooding.

The National Weather Service said the storms will begin battering the North Bay by Wednesday with rainfall, moving south and lasting into the weekend. More widespread rainfall across the region is expected by Friday and into the weekend.

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A Flood Watch was issued for the Coastal North Bay, Marin Coastal Range, North Bay interior mountains and valleys, and the Sonoma Coastal Range from 4 a.m. Wednesday through 4 a.m. Thursday.

Atmospheric river + bomb cyclone

The arrival of an atmospheric river - a long, narrow band of heavy moisture from the tropics - coincides with conditions in the Pacific where a polar air mass collides with a tropical air mass, causing the atmospheric pressure to drop quickly. Meteorologists call such low-pressure systems a bomb cyclogenesis, bombogenesis, or a bomb cyclone, which intensifies the storm and increases its winds.

Bay Area residents were urged Monday to prepare for the weather onslaught during a two-day dry spell. The Weather Service said in its daily forecast discussion Monday that the atmospheric river would begin Wednesday, bringing 3 to 7 inches of rain to the North Bay through Sunday, and up to 11 inches in the far North Bay coastal ranges.  

North of the Bay Area, the rainfall is expected to be even heavier and conditions will be treacherous. The Weather Service's Eureka office issued a series of storm and flood watches, warnings, and advisories across Northern California up to the Oregon border.

Less rainfall further south

Bay Area rain totals from the initial storm were expected to taper off further south. The Weather Service said rain totals of about four inches were expected for areas between the San Pablo Bay and Santa Rosa, 1 to 2 inches from the Bay to bayside Marin and Sonoma counties, 0.5 to 1 inch from Santa Cruz area to the Bay, and generally 0.25 inch or less south of Santa Cruz with little to no rainfall expected south of Monterey/Salinas. 

Beginning Friday morning through Sunday afternoon, however, the Weather Service said the North Bay could see an additional 2-3 inches of rain in the North Bay; 1 to 2.5 inches for the northern portions of the Bay Area including San Francisco, Santa Cruz Mountains, Big Sur Coast; and 0.5 to 1 inch elsewhere.

Freezing temps tonight, keep an eye on forecast

The Weather Service also upgraded a Freeze Watch to a Freeze Warning for interior portions of the North Bay and Central Coast, Sacramento Valley, and Central Valley from midnight Tuesday to 8 a.m. Tuesday. A Frost Advisory was issued for the East Bay hills and valleys, Santa Cruz coast, and Watsonville areas. Meanwhile, a Winter Storm Warning was issued for counties along the northern Sierra Nevada.   

With the arrival of the first major storm of the season, the Weather Service said soils should be able to handle at least the initial rainfall, but some North Bay locales will likely become saturated very quickly. However, the second storm system coming on Friday could bring more flooding fears even if less rain is expected. The Weather Service urged Bay Area residents to stay tuned to changing forecasts for the latest information as a potentially serious situation develops over the week. 

In:
• Storm
• Atmospheric River
• Bomb Cyclone