Big Bay Area storm may unleash flooding

An atmospheric river is expected to hit the Bay Area on Tuesday night, bringing heavy rain, strong wind gusts and an increased risk of floods and slides.

The weather phenomenon, a narrow corridor carrying water vapor from the tropics near Hawaii onto California shores, is expected to dump up to 2 inches of rain in San Francisco and Oakland. Some of the North Bay and South Bay mountains could see up to 4 inches of rain, said Spencer Tangen, a National Weather Service meteorologist.

“All the rain we’re going to get on top of that is just going to lead to worsening flood problems,” Tangen said. “There are higher potentials for landslides and debris flows than we saw before all this rain.”

The heaviest rain is expected to arrive Wednesday morning during commute hours, forecasters said. The National Weather Service issued a flash flood watch for the Bay Area from 10 p.m. Tuesday to 10 a.m. Thursday.

Accompanying the storm is a round of strong winds that could uproot trees and knock down power lines, especially given recent repeated rainstorms that have saturated the ground. Winds along the coast on Wednesday afternoon could reach 30 to 35 mph, with gusts up to 50 mph. Higher elevations may see wind gusts up to 70 mph, Tangen said.

Last week’s snow in the East Bay hills, the Santa Cruz mountains and other spots will likely melt with the arrival of the atmospheric river, Tangen said, and that could contribute to rising river levels.

“This warmer weather is going to melt that snow and that’ll add a little bit to the runoff,” Tangen said.

Warmer temperatures are also expected to prevail in the Bay Area with the tropical moisture, with highs bouncing up to the middle to upper 50s midweek.