Atmospheric river could bring heavy rain to the North Bay, Northern California

A "strong" atmospheric river appears poised to soak Northern California, including parts of the North Bay, later this week, according to the National Weather Service.

The storm system, expected during the mid- to late-week time frame, could bring periods of heavy rain and gusty winds to the North Bay and areas north, the weather service said.

The weather service said finer details on timing and rainfall amounts are still uncertain at this time.

The weather service has issued a flood watch for Sonoma, Napa and Marin counties between 4 a.m. Wednesday and 4 a.m. Thursday.

"Localized flooding looks likely and we could see some regional flooding as well as [atmospheric river]-boosted rain rates increase runoff as the ground becomes increasingly saturated," NBC Bay Area Meteorologist Rob Mayeda said in a statement. "Areas south of San Francisco may initially see much less rain vs. the North Bay, but by Friday into Saturday we should see higher rain rates regionwide as the cold front moves through the region."

Atmospheric rivers are long and relatively narrow bands of water vapor that form over an ocean and flow through the sky, transporting much of the moisture from the tropics to northern latitudes.

They occur globally but are especially significant on the West Coast of the United States, where they create 30% to 50% of annual precipitation and are vital to water supplies but also can cause storms that produce flooding and mudslides, according to the National Oceanic and Atmospheric Administration.

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