Warm Storm to Blanket California With More Rain This Week

Rainy conditions will pick up again across Northern California this week, starting Monday night and lasting through Wednesday morning.

The National Weather Service is forecasting solid precipitation in the North Bay, where rainfall of 1 to 3 inches is expected in Sonoma and Marin counties, with rates decreasing south of the Golden Gate. Ryan Walbrun, meteorologist with the Weather Service’s Bay Area office, said the rainfall will precede warmer, drier conditions later in the week.

“For the Bay Area, we do expect this to be a beneficial rain event,” he said. “We want to take the precipitation while we can here in the Bay Area.”

The rain comes amid paradoxical winter conditions: while this winter has been unusually warm, UC Berkeley’s Sierra Snow Lab recorded its third-highest accumulation of snow in a five-day period last week.

The incoming rain will fall from a large atmospheric river storm that will impact areas of Northern California, including high elevations in the Sierra Nevada, with its newly bolstered snowpack.

Heavy rainstorms over fresh powder can rapidly melt the snow. And that extra runoff into rivers can push water over the banks and flood. But Daniel Swain, a climate scientist with UC Agriculture and Natural Resources, said he does not yet see a cause for major concern, even though the storm will hit the mountains that received last week’s heavy snowfall.

“Now that might set off some flood-related alarm bells for some folks, but there’s a couple of major mitigating factors that are very likely going to prevent serious flooding during this event,” Swain said.

The first of the mitigating factors, Swain said, is that while warm and moist, the storm isn’t accompanied by a strong low-pressure system. This means that the moisture will primarily come in as humid air rather than rain.

The Sierra snowpack has the capacity to soak up the rain that does fall, which means less risk of flooding. In areas where 2 to 4 feet of snow accumulated on the ground, water is expected to saturate it and then refreeze.

“This atmospheric river is fortunately one that probably has a lot more bark than bite,” Swain said. The areas most at risk for flooding are along the North Coast, where rainfall will be the heaviest, and towns in the Sierra.

Despite all the snow from last week, much of the Sierra snowpack is still lower than average. Swain said that while last week’s snowstorms helped, much of the Western U.S. is starved for snow.

Swain said he expects the rain-on-snow activity to greatly increase the risk of avalanche activity in the backcountry, as heavy rains can make snowpacks heavier and trigger wet slab avalanches.