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Another wet winter, or more drought? It all starts this weekend in Northern California

Snow forecast for Northern California

A winter storm watch has been issued for the northern Sierra Friday night through early Sunday morning, November 3-5, 2017, with possible hazardous travel conditions.
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A winter storm watch has been issued for the northern Sierra Friday night through early Sunday morning, November 3-5, 2017, with possible hazardous travel conditions.

California’s rainy season gets underway this weekend with more than 2 feet of snow in the Sierra, warnings about dangerous post-wildfire runoff in wine country – and a heavy dose of uncertainty about whether the winter will bring lots of precipitation or a return to drought-like conditions.

The short-term forecast calls for a classic winter storm, with up to 30 inches of snow in Lassen Park and 12 to 18 inches at Donner and Carson passes and other regions of the Sierra. Most of the Sacramento Valley will get at least a half-inch of rain over the weekend, with heavier rainfall expected in Auburn, Placerville and Oroville, the National Weather Service said Thursday.

Light rain was expected to begin in the Sacramento area by Thursday evening, with heavier precipitation forecast for late Friday into Saturday. The weather service said a slight chance of rain was expected for Sunday and Monday. Another storm was expected to hit the region next Thursday.

Wet weather isn’t welcome everywhere. Up to an inch of rain was forecast for fire-ravaged communities like Santa Rosa and Redwood Valley, where officials feared that precipitation could do more harm than good. Because the Oct. 8 fires incinerated much of the vegetation that would normally soak up rainfall, Sonoma County issued a warning this week about “ash, debris and other pollutants” entering streams, blocking culverts and causing other problems.

“That’s something we’re watching, for sure,” said Mike Smith, a weather service meteorologist. “It’s definitely a concern at this point.”

It’s anyone’s guess whether California will get a repeat of last winter, when record rainfall in the north state prompted Gov. Jerry Brown to declare an end to the drought after five-plus years.

The National Oceanic and Atmospheric Administration said last month that La Niña conditions could return this winter. That suggests a relatively dry winter could be in store, especially in the southern half of the state, forecasters said. La Niña is a weather phenomenon linked to cooler-than-average temperatures in the surface of the eastern Pacific Ocean, near the equator.

But Jan Null, a private weather consultant in Saratoga, said the La Niña forecast is essentially meaningless. California went through La Niña conditions last winter – and wound up with the rainiest season on record in the northern half of the state. In 2015, NOAA predicted more rain due to a strong El Niño, yet the state saw below-average precipitation.

“It’s not a predictive tool,” said Null, who runs Golden Gate Weather Services.

His long-range forecast for the season? “Nobody knows,” he said.

Mike Anderson, the state’s climatologist, offered this explanation of the fickle nature of California’s winters: This season’s climate patterns resemble last winter’s, when California was nearly overwhelmed with rain – but are also similar to conditions in effect in the drought year of 2013. The main difference? In 2013 a high pressure system planted itself on California’s doorstep, diverting storm systems to Canada and Alaska. Last year it didn’t. It’s unclear what will happen this time around.

“It’s a highly uncertain seasonal outlook,” Anderson said.

Though Brown said the drought ended in April, many of California’s largest cities have responded to his recommendation to maintain restrictions on water usage. The city of Sacramento’s winter watering schedule, which began Wednesday, restricts outdoor watering to once a week, either Saturday or Sunday. The city also recommended that residents shut off their sprinkler systems completely through the winter.

Twice-weekly watering will be allowed again on March 1.

Even if it’s a dry winter, that won’t necessarily prompt another drought declaration. Major reservoirs are generally in good shape with ample “carry-over” water from last winter. The U.S. Bureau of Reclamation said Shasta, Folsom and other federal reservoirs are 75 percent full; that’s well above the historical average for the start of the rainy season.

The largest state-run reservoir begins the season with below-average water levels, however. Lake Oroville is being operated at low levels as a safety measure because of ongoing reconstruction work on Oroville Dam’s flood-control spillway, where final fortifications won’t be completed until sometime next year. Keeping the lake lower than usual might crimp supplies in 2018 for the agencies that store water there, although state Department of Water Resources officials said it’s far too soon to tell. The dam’s spillway fractured in two in February, triggering an emergency that forced the evacuation of 188,000 residents.

Repair milestone: The spillway repair at the Oroville Dam "is indeed ready to safely handle winter flows if needed,” says Grant Davis, director of the Department of Water Resources. Our drone fly-over shows the spillway on Wednesday, November 1, 2

Dale Kasler: 916-321-1066, @dakasler

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