California’s megastorm: Rain, snow, flooding – and maybe drought relief

California entered 2017 hoping a wet winter could end the state’s six-year drought.

Be careful what you wish for.

Northern California and the San Joaquin Valley are bracing for potential flooding this weekend, as a massive weather system known as an atmospheric river builds off the coast. Forecasters say that by Monday, rainfall and river flows could reach totals not seen in more than a decade.

Already, some regions north of Sacramento are issuing voluntary evacuation orders and several northern counties, including Sacramento, are readying sandbag stations.

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The National Weather Service issued a flood watch Thursday, stretching from Saturday afternoon through Monday, for 24 northern counties, including the six-county Sacramento region. Officials warned of flooding in streams and creeks that normally aren’t much more than a trickle. The Sacramento River is expected to roar with its highest flows since 2006 and send a huge gush of water into the Yolo Bypass, the massive floodplain west of Sacramento engineered to prevent the city from getting swamped.

The area of the state likely to be hardest hit is farther south. If the storm follows its current trajectory, forecasters say it could send an impressive – and possibly dangerous – cascade of water to the drought-ravaged southern Sierra Nevada and San Joaquin Valley. Fresno, Madera, Mariposa, Tulare and Tuolumne counties are under a flash-flood warning.

Any water is welcome news to a region home to some of the most iconic images of California’s lingering drought: millions of dead trees in the mountains near Yosemite National Park, and dusty, impoverished farm communities whose wells have gone dry.

But the prospect of too much water too fast has officials on edge, nonetheless.

“We definitely welcome the rain but wish it were distributed more evenly,” said Reed Schenke, an engineer at Tulare County’s Resource Management Agency.

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 some 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.

Northern California’s rainfall has already been 76 percent above normal this season, and this storm could be particularly powerful. Michelle Mead of the National Weather Service said areas south of Interstate 80 could see their largest rainfalls in 15 to 25 years. “It’s going to hit the foothills quite hard and the eastern Valley pretty good,” she said.

The Grass Valley area could get up to 15 inches of rain by the time the storm passes on Monday.

The flood risks are exacerbated by warm temperatures in the mountains, which could send a rush of snowmelt downstream. While the storm is expected to start out cold, bringing several feet of snow to Sierra ski resorts, temperatures will quickly warm up.

“By the time Sunday rolls around it should be rain all the way up to 8,000 feet,” Mead said.

The rain likely will melt a portion of the precious Sierra snowpack, which on Thursday hit slightly above average levels for the first time this winter. And that won’t help the state’s water supply in the long term. A healthy snowpack acts as an additional set of reservoirs, bolstering the state’s overall water supply. In a decent year, the snowpack contributes about 30 percent of the state’s supplies, gradually refilling reservoirs and canals through summer and fall.

When winter precipitation falls as heavy rain rather than snow in the mountains, the state’s water network can get overwhelmed. With the ground already saturated, much of the rainfall will run off into rivers and streams – and too quickly to be safely stored in many reservoirs.

The weather service said rainfall totals may rival December 2005, the last time Sacramento’s Arden Arcade neighborhood flooded. In those storms, about 40 of 700 owner-occupied town homes in the Woodside complex were damaged when a nearby slough overflowed, some under more than a foot of water.

Matthew Robinson, a spokesman for Sacramento County’s Department of Water Resources, said officials are readying for similar localized flooding. Five sandbag stations are open across the county, including a just-opened station at the Wilton fire department. Officials will keep a close watch on the flood-prone Cosumnes River near Wilton.

The Sacramento County Department of Regional Parks announced that it had closed areas of the American and Dry Creek parkways. County park rangers were warning visitors of rising water levels.

Robinson urged residents to clear their gutters and drains, and to avoid the temptation to drive through flooded streets. “Turn around, don’t drown,” Robinson said. “If you see flooding on the road, don’t go through it.”

He also asked residents to sign up for the county’s Everbridge Alert System, which can deliver texts, calls and emails to residents about upcoming problems.

In Yuba County, a handful of businesses and residents along the Yuba River near Marysville received voluntary evacuation notices Thursday. County emergency services manager Scott Bryan said the river was expected to rise to 83 feet by early Monday, which would bring flooding.

U.S. Bureau of Reclamation officials increased releases from Folsom Lake, pouring more water into the American River in anticipation of huge amounts of water cascading into the reservoir from upstream.

“You need to have a certain amount of space available for exactly the situation coming up this weekend, which is a big storm with a lot of rain,” said Joe Forbis, chief of water management at the Sacramento office of the U.S. Army Corps of Engineers. “If you’re too full, you won’t be able to release water quickly enough.”

Other key reservoirs in the state still have plenty of room, particularly south of the Sacramento-San Joaquin Delta, stark evidence of the drought. New Melones Reservoir on the Stanislaus River, the state’s fourth-largest reservoir, was just one-quarter full Thursday. The hope is it could fill substantially if the central Sierra gets inundated with rain.

Operators of the state and federal pumping stations in the Delta, which deliver Northern California water to the San Joaquin Valley and millions of Southern Californians, also plan to ramp up pumping operations to capacity over the next few days. That, along with unusually heavy rain south of Sacramento, could bolster reservoirs and groundwater basins in the San Joaquin Valley and Southern California, bringing long-awaited relief to some of the driest areas of the state.

Ryan Sabalow: 916-321-1264, @ryansabalow

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