3 more atmospheric rivers headed for Sonoma County. What’s driving the storm train?

January is typically atmospheric river season in California, but since the start of the new year Sonoma County has been hammered by an almost nonstop series of them — an unusual occurrence, which experts are attributing to the jet stream that is surging over the Pacific Ocean.

Literally described as rivers in the sky, three more of these moisture-laden events are being carried by winds toward the Golden State, according to meteorologists, who expect more storms to begin Wednesday night.

Those storms will continue through Jan. 20, said Chad Hecht, a research and operations meteorologist with UC San Diego’s Center for Western Weather and Water Extremes.

While the presence of atmospheric rivers has barely raised alarms among weather forecasters, what has stood out is that they’re happening in such quick succession.

The Bay Area is usually hit by about a dozen atmospheric rivers during the winter, according to the National Weather Service’s Bay Area office in Monterey. But including the two that occurred after Christmas, the region — and Sonoma County in particular — have already been hit by at least six of them.

“If I had my wish, I would like to spread these out more,” Hecht said Tuesday.

Atmospheric rivers result from moisture forming above warmer, tropical regions. The recent ones may have originated as far away as Guam, Hecht added.

They can grow to as large as 375 miles wide and stretch up to 1,000 miles long, according to the National Oceanic and Atmospheric Administration. Winds push them through the atmosphere, and the moisture they carry turns into rain or snow once they are above land.

Four have pounded Sonoma County since Jan. 4, when upward of 2 inches of rain and strong winds wreaked havoc across the region, and felled a towering redwood near Occidental that landed on a mobile home and killed a 2½-year-old boy inside.

The second atmospheric river developed Saturday and also dropped at least 2 inches of rain onto this region. A third, on Sunday night, dropped more than 2 inches of rain on Sonoma County before moving on to Southern California, where it caused major flooding near Santa Barbara.

The latest atmospheric river showed up Monday night and, by Tuesday evening, had dropped 1⅓ inches on Santa Rosa, according to the National Weather Service.

Sarah McCorkle, a meteorologist with the National Weather Service’s Bay Area office, agreed that wind has been the dominant factor in the wave of atmospheric rivers.

“That’s what’s been abnormal,” she said.

The next atmospheric river is expected to arrive Wednesday and bring Sonoma County up to 3 inches of rain through Thursday, according to the National Weather Service.

Another 2 to 3 inches may fall over the weekend before smaller amounts of rainfall develop early next week.

Precipitation levels are expected to decrease because next week’s atmospheric rivers are headed south along the coast, before then turning east above land, McCorkle said.

Hecht called atmospheric rivers a “double-edged sword.“

“Not only can they be hazardous, they’re also quite beneficial because they’re one of the main meteorological phenomena that supplies California’s water,” he said.

The U.S. Drought Monitor shows Sonoma County remains in the severe drought category, and Hecht said it’s too early to say whether the region is out of the woods following these recent storms.

“In order to have a normal water year, you need more than what we had over the past couple weeks,” he said.

You can reach Staff Writer Colin Atagi at colin.atagi@pressdemocrat.com. On Twitter @colin_atagi.