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

Officials are discouraging unnecessary travel due to the potential for weather hazards such as rocket mudslides and downed trees.|

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.

The Santa Rosa Fire Department responded to a request on Cooper Drive in Santa Rosa to assess soil movement on a section of hillside property affecting homes. It was determined that with the ongoing rain, a portion of the hillside is slowly sliding toward a few residential houses on Cooper Drive. Firefighters cut and removed a portion of a fence that was being pushed by the slide into one of the homes to prevent further damage. The slide mass has grown significantly over the recent storms and now affects six properties, including the Congregation Shomrei Torah. Out of an abundance of caution, the City’s Chief Building Official red-tagged two Cooper Drive properties and worked with those residents to find alternative accommodations until conditions improve. The slide mass behind three additional properties is being closely monitored but no actions have been taken by the City at this time. Both the City and Congregation Shomrei Torah have engaged geotechnical engineers to evaluate the conditions and placed plastic sheeting and sandbags to protect against further rain infiltration into the soil to slow the slide mass movement. Once access to the area is safe and weather conditions have improved, the City will begin to remove the slide mass to reduce the current threat and work with property owners to evaluate the next steps. With more rain in the forecast, soil continues to reach saturation limits. If you notice any land or hillside movement, call 911. For minor falling debris or roadway flooding, call 707-543-3800. Visit SRCity.org/WinterStorm for storm-related information.

Posted by City of Santa Rosa Fire Department on Sunday, March 12, 2023

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.

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