River of rain heading toward Northern California

A major storm is expected to blow into Northern California starting Thursday night and lasting into Monday. Known as an “atmospheric river,” the storm is funneling a stream of tropical moisture across the Pacific Ocean like a firehose. It likely will bring high winds to the Sacramento Valley and a threat of flooding on some major Northern California rivers, including the upper Sacramento River, as well as urban streams. Here’s a closer look.

Q: What is it?

A: An atmospheric river is a weather phenomenon in which a narrow band of strong wind sweeps eastward across the Pacific Ocean, gathering moisture as it goes. These storms often make landfall in California and historically have produced many of the state’s worst weather-related disasters.

A single atmospheric river can transport 10 to 20 times the flow of the Mississippi River. The state typically sees five or six of these storms every year, and while potentially damaging, they also provide as much as half of California’s annual rainfall.

The latest storm comes at an ideal time for a team of scientists gathered in Sacramento to study atmospheric rivers. Their effort, known as Calwater 2015, will explore how to predict when and where atmospheric rivers will hit land, the role of ocean evaporation in their strength, and how the ocean changes after an atmospheric river passes.

The project involves four research aircraft, which will fly through major storms while a ship outfitted with additional instruments cruises below on the ocean. The work is expected to provide a better understanding of how California gets its rain and snow, how human activities are influencing precipitation, and potentially will inform water management decisions relating to drought and flood.

Q: How big is this storm, and when will it hit?

A: The storm is predicted to begin with a chance of rain Thursday morning, then intensify during the night. The worst rainfall is expected throughout the day and evening on Friday.

Allen White, a research meteorologist who is part of the Calwater team, said it remains unclear whether the storm will manifest as a series of events or one continuous stretch of rain throughout the weekend.

“This is going to be a fairly intense atmospheric river because there are a couple of pulses associated with it,” said White, who works for the National Oceanic and Atmospheric Administration in Boulder, Colo. “It’s not just sweeping through like some atmospheric rivers do.”

Forecasts call for some mountain areas of Northern California to get 10 inches of rain or more through Monday. The Sacramento Valley, including the city of Sacramento, could see 2 to 5 inches of rain.

Strong winds approaching 30 mph are expected throughout the day Friday in the Valley, with gusts near 50 mph.

Q: Where will the storm hit?

A: One of the great uncertainties about atmospheric rivers is predicting where the main band of rainfall will strike.

Picture an uncorked garden hose turned on full blast: It tends to whip around. That’s what an atmospheric river does, too. One goal of the research effort is to figure out how to predict where an atmospheric river will make landfall in California, which will help predict where the worst flooding will occur and where the best opportunities exist to capture runoff.

For now, White could say only that the approaching storm is expected to strike California between San Francisco and the Oregon border. But that prediction could change as the storm gets closer.

Several Northern California rivers are projected to exceed flood stage during the storm, starting late Friday and into Saturday. On the coast, this includes the Eel and Navarro rivers. Inland, the upper Sacramento River also is expected to exceed flood stage at Red Bluff, Tehama Bridge and Ord Ferry. The Butte and Sutter bypasses along the river are expected to flood, but not the Yolo Bypass.

The storm is expected to be quite warm. Snow levels will start out at 6,000 feet, then rise to 7,000 feet as the storm continues. This means only minor snow-related impacts to mountain highway passes.

Q: What does it mean for the drought?

A: Because the storm is so warm, it will not contribute significantly to California’s mountain snowpack. That snowmelt is crucial to refill reservoirs, which provides a major share of the state’s freshwater supply during long, hot summers.

But after three years of drought, all major storms are welcome. The heavy rainfall runoff from this storm will help refill reservoirs, replenish groundwater and recharge streams, and also help rangeland, farm fields and natural habitats that are reliant solely on precipitation.

Shasta Reservoir, the state’s largest, could gain as much as 400,000 acre-feet of new supply, said Alan Haynes, service coordination hydrologist at the California-Nevada River Forecast Center, a branch of the National Weather Service. But because it is so large and so depleted by drought, this will merely push the reservoir just over half full.

“The reservoirs still have plenty of capacity,” Haynes said. “They’ll be able to take whatever this thing gives. What it won’t bring is the snow for melting later in spring and summer. So we’ll have a little bit of a problem there.”

Call The Bee’s Matt Weiser at (916) 321-1264. Follow him on Twitter @matt_weiser.

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