Bay Area Storm: Atmospheric river set to arrive Thursday afternoon; heavy rainfall expected

A strong atmospheric river storm system rapidly approached the Bay Area Thursday morning, sending agencies into a frenzy as a three-day long flood watch issued by the National Weather Service was set to go into effect at 1 p.m.

Thursday’s storm, expected to peak in the afternoon through Friday morning, was forecast to bring the Bay Area’s wettest conditions since early January, when a series of nine atmospheric river systems ravaged parts of the region with historic amounts of rainfall and wind.

Quick science lesson ahead of the atmospheric river!

What is an AR? In short: A relatively narrow band of water vapor originating from the tropics that redistributes water to higher latitudes on Earth. They are responsible for most of the moisture transport out of the tropics. pic.twitter.com/3ELzVYINNC

— NWS Bay Area (@NWSBayArea) March 9, 2023

NWS forecasts predicted one to three inches of rain Thursday for inland regions, with the lower totals occurring in San Jose and the rest of the South Bay. Totals could come near three inches in San Francisco and Oakland, while much of Contra Costa County was predicted to see between two and three inches of rain.

Updated excessive rainfall outlook for the upcoming #AtmosphericRiver event. Considerable river flooding is a concern from the Santa Cruz mountains down through northern Monterey County. Preparations should be completed by the end of the day today. #CAwx pic.twitter.com/V5hHVxfFxd

— NWS Bay Area (@NWSBayArea) March 8, 2023

As it was in the January storms, flooding was of strong concern to agencies on Thursday. The official NWS flood watch was expected to be in effect from 1 p.m. Thursday to 10 a.m. Sunday for the entire Bay Area, plus Monterey and San Benito counties.

According to the NWS, excessive runoff was expected throughout the weekend due to the already pre-saturated soils from past rain storms. Those soils won’t be able to absorb the new rainfall at a rapid pace, causing runoff and flooding. While some urban flooding was expected in areas with poor drainage, the NWS said the largest probability of floods exists in the Santa Cruz and Santa Lucia Mountains, where rain totals could reach six to eight inches Thursday.

A flood watch has been issued for our area starting Thursday, March 9.

Visit https://t.co/VVVSP1ASTR for tips and resources. https://t.co/53NUpuNDTD

— valleywater (@valleywater) March 7, 2023

In a storm briefing posted Wednesday, the Santa Cruz County Office of Response, Recovery and Resilience said flooding in low-lying areas of Paradise Park and Felton Grove is “probable,” and the San Lorenzo and Parajo rivers were expected to approach flood stages. In the same briefing, the agency said downed trees, slope failures, road impacts and power outages are “likely” as wind gusts could approach 60 miles per hour Thursday.

This is a developing story. Check back for updates.