Last week, as forecasters at the National Weather Service in the San Francisco Bay Area looked at a series of storms that would hammer the region, they predicted that some areas in the North Bay would see six to eight inches of rain between Saturday and Tuesday.

Instead, around a foot of rain fell at the wettest locations. Highways were flooded, landslides were triggered, and rivers swelled. By Tuesday afternoon, a house slid down a saturated hillside and fell into the Russian River in Sonoma County. Evening evacuation orders were issued, hours before the swollen river started spilling over onto a handful of roadways.

The storms, known as atmospheric rivers, are a common feature of West Coast winters. They’re thin ribbons of moisture carried by powerful winds, sometimes for thousands of miles, which can make forecasting them especially challenging.

“We got a little more rain than expected, and the rivers responded more than expected as well,” said Dylan Flynn, a meteorologist with the Weather Service. “A couple days ago we didn’t expect any of the rivers to flood.”

The rain could have been even more impactful if Northern California hadn’t seen an abnormally dry January. The first of the recent rains was mostly absorbed by the dry soil starved of rain, sparing the area from more severe flooding.

Another one of these moisture-rich storms is slated to sweep the region Thursday into Friday, bringing more rain and flooding concerns, but Mr. Flynn said this one is expected to be more “run-of-the-mill” than the last.