Next Sonoma County atmospheric river to bring 60 mph gusts, possible thunderstorms

Possible thunderstorms coupled with strong winds on Tuesday and early Wednesday could trigger multiple weather hazards, including mudslides, in the North Bay, according to the National Weather Service.

Residents are encouraged to secure outside items, monitor the weather and have a plan in place in case the storm, which is one of the strongest to hit the region this winter, causes evacuations or power failures, said Warren Blier, a meteorologist with the weather service’s Monterey office.

Widespread rains were expected to begin about midnight Tuesday after a day of scattered lighter showers.

Steadily increasing winds will peak about 3 a.m. at 20 to 35 mph with gusts around 45 mph in the Sonoma County valleys, said National Weather Service meteorologist Patrick Ayd.

As winds pick up, so too will rain showers, which are expected to drop as much as 6 inches in the Mayacamas Mountains and 1.5 to 3 inches from Monday night to Tuesday night in Sonoma County valleys.

Thunderstorms will be embedded in the swath of rain, which is expected to slow down about midday, turning into scattered showers that could last until Wednesday morning.

The weather service issued a high wind warning for the Bay Area and Central Coast, which will take effect from 11 p.m. Monday to 5 a.m. Wednesday in anticipation of 20 to 35 mph winds with gusts around 45 mph in the Sonoma County valleys, Blier said.

A flood watch will be in effect from Monday evening through Tuesday night in multiple areas, including the North Bay interior mountains and the Sonoma Coast.

Accumulation rates from Tuesday morning to afternoon could reach about 1 inch per hour during the heavier parts of the storm, with increased accumulation occurring in higher elevations, Ayd said. Some areas could see up to 2 inches per hour.

The ground is already saturated due to previous storms in early January and over the past few days, meaning these increased rain rates could trigger landslides.

“The zones are very saturated,” Ayd said, adding that “mudslides are definitely a threat.”

In addition to rapid slides, Santa Rosa officials also have their eyes on a slow-moving mass on a hillside near Cooper Drive, which slid into about six properties and prompted the red-tagging of two of them Sunday.

The two were deemed uninhabitable because of damage already caused and impending risk, Santa Rosa Fire Marshal Lowenthal said.

The fire department has been monitoring the area since early January, during the onslaught of rain storms. However, the land movement “escalated” the last few days, Lowenthal said.

While the slippage is slow-moving, the area stirring is so big that it could potentially speed up and cause some damage.

“It’s a pretty large mass that obviously had a lot of rain behind it,” Lowenthal said. “That’s what the concern is, is what the rain could potentially do to it.”

Sandbags and large plastic tarps were placed over the area to prevent additional rain from seeping into the soil, causing the area to move even more.

The fire department is also in contact with neighbors in the area who have damaged properties or fences but do not face as high of a risk as the red-tagged properties.

Because the ground is already so wet, it “won’t take much” for the winds to knock down multiple trees, Lowenthal said. So, with the stronger gusts predicted, he believes there will be many trees felled during the incoming storm, some of which he expects will hit power lines.

Some could fall into freeways, streets, cars and homes, like they have in previous storms, Lowenthal said.

“That becomes a pretty significant safety concern,” he said.

Other concerns during the storm, especially peak hours, include flooding - specifically rapidly rising creeks and streams and flooded roadways - and winds carrying items not secured, Blier said.

During the storm, Sonoma County residents should avoid unnecessary travel and refrain from driving through roads that are closed due to flooding, Lowenthal said.

“If you don’t need to be out during the peak storm, don’t,” he said.

You can reach Staff Writer Madison Smalstig at madison.smalstig@pressdemocrat.com. On Twitter @madi.smals.