Update: Napa County flood watch extended to Saturday amid 'bomb cyclone' storm (2024)

Updated at 7:58 p.m. Wednesday—Heavy rain began pelting Napa County early Wednesday morning as the Bay Area received its first major atmospheric river of the season, prompting a North Bay flood watch that will continue at least into the weekend.

Cities and Napa County opened sandbag stations to help residents protect their properties against small-stream flooding. Meanwhile, Pacific Gas & Electric Co. reported its first storm-related power outage in the county, affecting more than 300 customers north of the city of Napa.

Wednesday’s rainfall was on the leading edge of a major regional storm battering California and the Pacific Northwest, causing widespread power outages, shuttering schools and downing trees. Two people died in northwest Washington state after being struck by falling trees in separate incidents.

The Weather Prediction Center issued excessive rainfall risks through Friday and hurricane-force wind warnings were in effect as the strongest atmospheric river — a large plume of moisture — that California has seen this season overwhelmed the region. The storm system is considered a “bomb cyclone,” which occurs when a cyclone intensifies rapidly.

In Napa, Sonoma and Marin counties, the National Weather Service issued a flood watch starting at 4 a.m. Wednesday. Forecasters later extended the watch by a day, to 4 a.m. Saturday.

The weather service also issued a wind advisory in North Bay counties from 10 p.m. Tuesday to 6 p.m. Thursday, with south winds of 20-30 mph expected to gust up to 45 mph. Tree limbs may be blown down, raising the risk of power outages.

By the end of the week, many parts of the region are expected to receive about a month's worth of rain, with the North Bay forecast to receive up to 10 inches or more in some Sonoma County coastal mountain ranges, weather service meteorologist Dial Hoang said.

“At least for these first couple of days, the North Bay will really see the largest impacts from this system,” Hoang said.

On top of the heavy rains and high winds, there are chances of thunderstorms in the Bay Area late Friday into Saturday and the wet weather is expected to continue through at least Tuesday of next week, according to the weather service.

As of 7:55 p.m. Wednesday, rainfall had totaled 1.89 inches at the Napa city corporation yard on Jackson Street, according to the Napa County OneRain website. Mountainous and rural parts of the county already had received far greater amounts, as much as 5.67 inches at the Petrified Forest west of Calistoga, 5.44 inches at the Napa River on Dunaweal Lane, and 5.24 inches in Angwin.

Flooding is possible due to an expected atmospheric river bringing heavy rainfall, especially in urban and poor-drainage areas, the weather service said in its flood watch announcement.

Hoang said soil conditions in the Bay Area are somewhat dry because of the hot summer and this being the first big winter storm to hit the region so that might mitigate flooding concerns somewhat.

“They might be able to soak up some of it initially, but it will saturate really quickly” and lead to flooding in some urban areas and along small streams and creeks, particularly in the North Bay, Hoang said.

Saturation of Napa County soils could occur as soon as Friday if current rain forecasts hold, according to Jeremy Sarrow, operations manager for the county’s Flood Control and Water Conservation District.

There was no indication that the Napa River would approach flood stage at Oak Knoll Avenue north of the city, according to the California Nevada River Forecast Center website. As of Wednesday afternoon, the river was expected to rise to 12.18 feet at 6 p.m. Friday before receding – well below the monitoring stage of 22 feet and the flood stage of 25 feet.

The storms are not likely to prompt the closing of floodgates at downtown Napa’s Oxbow Commons flood bypass, but the Napa County Flood Control and Water Conservation District is keeping an eye on smaller waterways, especially Napa Creek in downtown Napa, which has two sets of bypass culverts to increase its flow, according to Sarrow.

“If the flows get high, and there’s a lot of debris, we stage an excavator there to ensure that water can flow freely through those culverts,” he said Wednesday afternoon of Napa Creek.

“I don’t think any channels will reach capacity or come out of their banks based on what’s being forecast,” Sarrow added. “But obviously that could change over the next 24 hours.”

The Napa County Sheriff's Office and city authorities said sandbag locations are stocked and ready to go ahead of this week’s predicted storms.

Sandbags can prevent or reduce flood water damage. Sandbags are available at the following locations:

- Napa city corporation yard, 770 Jackson St., 707-257-9520

- American Canyon City Hall, 4381 Broadway (Highway 29), parking lot near the tree and picnic table; self-service, bring your own shovel

- Yountville yard, 7294 Silverado Trail, 707-944-0196

- Napa County Fire/Dry Creek Lakoya Volunteer Fire Station 16, 5900 Dry Creek Road, 707-944-8887

- Circle Oaks Community Water District, end of Circle Oaks Drive

- Napa County Fire/Gordon Valley Volunteer Fire Station 22, 1345 Wooden Valley Cross Road, 707-428-2050

- St. Helena City Hall parking lot, 1088 College Ave. (at the Napa Valley College Upper Valley Campus)

- Crane Park, 360 S. Crane Ave., St. Helena (parking lot of the Little League baseball fields)

- St. Helena California Department of Forestry station, 3535 N. St Helena Highway (Highway 29/128) in Calistoga, 707-967-1467

- Angwin Fire Station 18, 275 College Ave., in Angwin, 707-965-6551

For a full list of sandbag locations throughout the county, visit https://bit.ly/4eDpmgn.

Sheriff's deputies say people should stay off the roads if possible.

Wednesday’s rain and wind contributed to a PG&E outage shortly after 10 a.m. that cut off service to 336 customers in the Lake Hennessey area, according to the utility’s online outage map. The blackout affected areas along Highway 128, Chiles Pope Valley Road and Lower Chiles Valley Road.

Service was restored later Wednesday.

Rainfall on Wednesday and Thursday is expected to total 3 to 7 inches in valley areas, increasing to 6 to 10 inches at higher elevations.

A second band of rain is predicted to cross the North Bay from Friday morning to Sunday afternoon, carrying another 2 to 3 inches, according to the weather service.

As a result of the stormy forecast, Fort Ross and Montgomery elementary schools, both located along the Sonoma Coast, were closed on Wednesday, county education officials said.

Cities on the North Coast could be pelted with over 10 inches of rain, while San Francisco and Oakland might get 1 to 4 inches, according to the National Oceanic and Atmospheric Administration’s weather prediction website.

This story includes reporting by Napa Valley Register city editor Howard Yune; Bay City News Service reporters Dan McMenamin, Tony Hicks and Katy St. Clair; and The Associated Press.