Freeze warning issued for North Bay, followed by atmospheric river over weekend

The first atmospheric river of the rain season, expected to reach the North Bay on Saturday, could bring up to 4 inches of rain to the the valleys and as much as 6 inches to nearby mountain and coastal ranges by the middle of next week.

Combined with rainfall from earlier this week - about an inch in valley areas and almost two inches in the mountains - the two storm systems represent the first significant storm activity in the rain season, which officially began Oct. 1.

The atmospheric river comes on the heels of sharp cold snap, with nighttime temperatures that triggered the National Weather Service to issue a freeze warning between 10 p.m. Thursday and 9 a.m. Friday.

This weekend's rain storm will begin dumping water over Sonoma County by late Saturday morning and continue into Sunday afternoon, said Spencer Tangen, a meteorologist for the National Weather Service. Forecasters expect up to 2.5 inches of rain in North Bay valleys during that period, he said.

An additional bout of rain, up to 1.5 inches in the valleys, is expected to come Monday through Wednesday, he said.

“By the time it's all done, North Bay valleys could see 2 to 4 inches by Wednesday,” Tangen said.

An atmospheric rivers is a “plume of moisture” that comes from the ocean and which typically has its roots in the subtropics, Tangen noted. They tend to sit in place for a day or two and can produce significant rainfall.

Current weather models show this weekend's storm system may “wobble” and shift between the North Bay, the Santa Cruz Mountains and Big Sur. It is expected to hit the North Bay first, Tangen said.

But even as the atmospheric river makes it's shift southward, the North Bay will continue to see rain throughout the day Sunday, though not as heavy as Saturday night, Tangen said.

The freeze warning issued by the Weather Service for Thanksgiving night, alerted Sonoma County residents that they could see temperatures drop to the low 20s, with nighttime temperatures in North Bay valleys ranging from 22 to 30 degrees.

“The storm system that brought all the rain the other night has since drifted south and all the cold air that came with it is now over the Bay Area,” Tangen said Thursday afternoon.

When temperatures drop that low, people without access to heat or proper shelter are at the highest risk, the weather service said.

Lengthy exposure to such low temperatures could also be harmful to unsheltered pets and livestock. The sustained cold can also endanger sensitive outdoor plants if left uncovered and potentially damage exposed water pipes.

For most of Santa Rosa, temperatures were expected to be around 28 degrees. Valleys and lower elevations were expected to be the coldest, Tangen said.

The cold snap is “the tail end” of Tuesday's rainstorm, which has since moved south leaving cold air to settle over the Bay Area before the next front moves in on Saturday.

“When there's no clouds, heat rises and escapes out to space,” he said. “That's why on clear nights it's often colder than on cloudy nights.”

You can reach Staff Writer Martin Espinoza at 707-521-5213 or martin.espinoza @pressdemocrat.com. On Twitter @pressreno.