'Over an inch an hour': Flash flood watch in 3 Bay Area counties

The National Weather Service issued a flash flood watch for parts of the San Francisco Bay Area ahead of an atmospheric river forecast to drench the region in soaking rains Sunday into Monday.

The watch impacts portions of Sonoma, Napa, Marin and San Mateo counties as well as Monterey and Santa Cruz counties to the south. 

In the North Bay, the weather service called out the following cities as areas of concern for flash flooding: Rohnert Park, Santa Rosa, Lagunitas-Forest Knolls, Angwin, San Rafael, Woodacre, South Santa Rosa, Petaluma, Napa, and Novato

Weather service forecaster Brayden Murdock said that at the peak of the storm event these areas could see "over an inch [of rain] an hour."

The biggest concern with this event is flooding and debris flows in 2020 wildfire burn scars such as the CZU Lightning Complex burn scar in the Santa Cruz Mountains. Burn scars in the North Bay are also a concern. Mudslides and washouts could occur in steep terrain.

Urban and small stream flooding is likely as the heavy rain band passes through Sunday afternoon and night, the weather service said.

The flash flood watch is in effect for parts of Sonoma, Napa and Marin counties in the North Bay 5 a.m. to 11 p.m. Sunday, the Santa Cruz Mountains 1 p.m. to 11 p.m. Sunday, and portions of Monterey County 5 p.m. Sunday to 4 a.m. Monday.

Clogged storm drains could cause some flooding on urban streets and highways.

The weather service advised people to use Saturday to complete routine storm maintenance around their properties such as clearing rain gutters.

"The combination of strong winds and heavy rain will be a nasty combination," the weather service said. "Please plan accordingly. Avoid unnecessary travel during the peak of the storm and never cross flooded roadways."

The storm will first arrive in the North Bay Sunday morning, delivering the heaviest rain to the region around noon. The central Bay Area is forecast to receive the brunt of the storm Sunday evening, from about 5 p.m. to 10 p.m.

The North Bay Mountains could receive 6 to 9 inches of rain with this storm, the Santa Cruz and Santa Lucia Mountains 3 to 6 inches and valley areas including San Francisco and Oakland 1 to 3 inches, the weather service said.