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California’s not done. Three more atmospheric rivers are on the way.


San Francisco is dealing with one of its wettest stretches on record. A whopping 10.33 inches of rain — equivalent to more than two months’ worth of water — has come down in 10 days, causing widespread flooding and scattered rockslides. The culprit? A series of atmospheric rivers, or tongues of deep tropical moisture buffeting the Golden State, which have also been accompanied by strong winds and high surf. And the waterlogged and windy pattern isn’t going away anytime soon.

At least three other storm systems, one of which appears to be especially significant, are on the way in the next seven days. The most significant is anticipated between Sunday night and Monday night, which will probably bring “widespread flooding, damaging winds, and dangerous beach and marine conditions.” according to the National Weather Service. Thereafter, signs point to a pattern that will keep the moisture fire hose aimed at the state through at least mid- to late-January.

“A series of potent weather systems will impact our area this weekend into next week,” wrote the National Weather Service serving the Bay Area in a forecast discussion Friday. “Please stay current with forecast updates as there is a direct threat to life and property from these impacts.”

California is being inundated with rain. Will it ease the drought?

While the deluges bring a welcome dent to the drought that’s gripped the state for years, they’re proving to be too much of a good thing. Flood watches are up for the northern two-thirds of California, with winter storm warnings in the Sierra Nevada.

Just last week, roughly 81 percent of California was gripped by a “severe” or worse drought, according to the U.S. Drought Monitor. That number has trickled down to 70 percent, but the bigger story has been the elimination of the top-tier “exceptional” drought. It’s unlikely the years-long drought will be busted, but a deeper dent in the deficit is at the very least probable.

Twenty-two trillion gallons of water could fall on California from the upcoming series of storms, said Michael Snyder, a meteorologist based in Seattle.

What is an atmospheric river?

Atmospheric rivers are narrow filaments of tropical moisture that can stretch thousands of miles or more. They’re often only a couple hundred miles wide, but may be transporting more than a billion pounds of moisture overhead every second. Much of that moisture remains as water vapor or condensate in the form of cloud cover, but some falls as rain or snow. Unsurprisingly, precipitation totals can climb quickly.

Most atmospheric rivers carry the bulk of their moisture a mile or so above the ground. That’s why the greatest totals of rain or snow are usually found in the higher elevations. Additionally, moist air forced up the mountains frequently cools to its dew point, becoming saturated and ridding itself of excess moisture in the form of heavy precipitation.

It’s not unusual for rainfall rates to approach or exceed half an inch per hour in the lowlands, with snowfall rates of 5 inches per hour or greater in the Sierra Nevada.

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At the University of California at Berkeley’s Central Sierra Snow Lab, 57.9 inches of snow have fallen in the past week, and there’s a lot more to come.

Why can’t California catch a break?

California sees atmospheric rivers every winter; they’re a frequent part of the West’s cold-season weather pattern. But it’s been an exceptionally wet stretch as of late. San Francisco received 11.6 inches of rain during December, including 5.46 inches on Dec. 31, which was the city’s second-wettest day since 1849.

A contributing factor has been a zonal, or west to east, jet stream pattern. That’s maintained a wet westerly flow into California. That means that atmospheric rivers, swirled east by low-pressure systems, have been barreling east into California rather than slipping northeast into Oregon or Washington.

That pattern appears poised to continue for weeks to come.

What’s next for California?

Another conga line of atmospheric rivers is on the way to California. The first may be modest, but the second, anticipated to arrive early in the workweek, appears to be more formidable.

Event No. 1: Saturday into Sunday

The first event is actually a pair of low-end atmospheric rivers. They’re being tugged into California as they wrap northward into a duo of dying low-pressure systems several hundred miles west of the Oregon-Washington border.

The initial atmospheric river will begin brushing against the coast of extreme Northern California on Friday night before dropping south overnight, aiming at the Bay Area as it wanes on Saturday. By the evening, another pulse of moisture from the second low-pressure system will pivot into the region.

Up to 5 inches of rain is expected in the Coastal Range, with 2 to 3 inches in the lower elevations north of the Bay Area. San Francisco may be on the fringe of the first episode, but could still see more than an inch of rain. Several feet of snow are expected in the highest elevations of the Sierra Nevada.

Event No. 2: Monday into Tuesday

A more significant atmospheric river — likely a 4 out of 5 on the scale from the Center for Western Weather and Water Extremes, which rates this phenomenon — will come in Sunday evening and continue into Tuesday. Very heavy rainfall is likely, with totals of 2 to 4 inches at the coastline and in the Central Valley. In the high terrain, where it’s too warm to snow, more than 4 inches of rain is probable and as much as 8 to 10 inches is possible.

In the Sierra Nevada, 2 to 5 feet of snow are expected, primarily above 7,000 feet. At the coastline, winds may gust to more than 55 mph, with 65 mph gusts expected in mountain summits above 6,000 feet.

“[W]idespread and potentially significant flooding is expected,” wrote the National Weather Service office serving the Bay Area. “Gusty winds are also anticipated in the higher terrain which could bring more trees down, causing power outages, travel issues, and add[ing] to debris in flooded waterways.”

Although rain should gradually ease on Tuesday, many waterways may swell over their banks. “Tuesday is probably the day where you’ll likely need to keep a really close eye on the weather as the potential for widespread flooding of rivers, creeks, streams and roadway and urban flooding will be at its highest during the next week, as all the runoff and heavy precipitation comes together resulting in a mess,” wrote the Weather Service office serving the area around Sacramento.

Event No. 3: Late next week

A third atmospheric river will affect parts of the West Coast late next week. While details are hazy, it appears significant precipitation is again likely.

Read More:California’s not done. Three more atmospheric rivers are on the way.