Atmospheric rivers cut powerful swaths across the West
It’s shaping up as the biggest snowstorm to hit the central Sierra in two years.
Which is a dramatic way of saying: On Thursday, amid a serious drought, the central Sierra will finally experience a normal winter snow event.
The National Weather Service expects 2 to 3 feet of snow will fall on the highest peaks overlooking Lake Tahoe. At lake level, near Tahoe City, forecasters predict 8 to 16 inches of snow. The last time Tahoe City saw 10 inches of snowfall in a single day was Dec. 7, 2013, according to the National Climatic Data Center.
The storm won’t pack quite the same punch in the Sacramento Valley. Forecasters are predicting a quarter-inch to half-inch of rain in Sacramento, with somewhat heavier rainfall farther north.
Even with the precipitation that’s fallen on Northern California’s flatlands and peaks the past few weeks, it’s been a dry fall by historical standards. The Sierra snowpack is at 60 percent of normal for this time of year. So, while this week’s storm may seem comparatively big, that’s only because Northern Californians, in just a few dry years, have come to expect so much less.
“In a normal winter, this is a normal deal,” said Shaun Tanner, a meteorologist with the private forecasting service Weather Underground. “But in a period where we’ve had such a little amount of rain, this is a very big deal.”
California needs a lot of snow and rain. After four years of drought, its reservoirs are dry: Folsom Lake last week hit its lowest point since record-keeping began 40 years ago. The State Water Resources Control Board, which regulates water in California, already is discussing another year of tough water restrictions for tens of millions of residents. Farmers and environmentalists are bickering over water allocations from Shasta Lake, with the fate of an endangered run of Chinook salmon, not to mention millions of dollars worth of crops, in the balance.
“There’s a lot on the line this year,” said Kelly Redmond, deputy director of the Western Regional Climate Center. “People are watching the horse race this year with binoculars.”
Adding to the drama is the promise of a strong El Niño, a phenomenon linked to above-average water temperatures in the Pacific Ocean that often brings heavy rains, particularly to Southern California.
Redmond and other forecasters said that Thursday’s storm is probably not an El Niño event. A “classic” El Niño storm, he said, also would hit Southern California, which this storm is not expected to do, and would come later in the winter.
Instead, he and others said, Thursday’s storm is an old-fashioned atmospheric river, often referred to as a “Pineapple Express.” An atmospheric river is a massive water vapor pipeline, responsible for many of the major storms along the West Coast and about half of the rain and snow Northern California sees in a typical year.
Northern California has seen a few atmospheric rivers in the last two years, but they brought more rain than snow to much of the Sierra because temperatures were unusually warm. California’s vast and complex water delivery network relies on heavy snowpack in the Sierra to produce the melt that feeds government reservoirs through spring and summer. Without ample snowpack, the state’s major reservoirs start running low by late spring.
This storm, though, looks to be a cold one, with temperatures in Tahoe City falling into the teens. Michelle Mead, a National Weather Service meteorologist in Sacramento, said Thursday’s storm is expected to bring a lot of snow to the Coastal Range and Sierra, but not necessarily a lot of rain to the Central Valley.
“This is definitely going to be helping our reservoirs,” she said. “This is a nice, wet storm.”
The Sierra snowpack on Wednesday was at its highest point for this time of the year since 2012, but well below historical averages. “We’ve had storms about every seven to 10 days, but the duration of these events has been shorter,” said state climatologist Michael Anderson, adding that it’s not yet clear why that has occurred, or if it will continue.
If Thursday’s storm delivers as predicted, the Sierra snowpack could be near average for the water year so far, Redmond said.
Even if Thursday’s storm surprises forecasters and brings more rain than snow, it will still help the state’s reservoirs, said Jay Lund, director of the Center for Watershed Sciences at UC Davis. Reservoirs like Folsom Lake have plenty of room for water and little need to increase releases to prevent flooding.
“If the reservoirs were closer to full, I would say, ‘Oh please, let’s have it as snow,’ ” he said. “Right now, we’re just happy to have water, whether it’s liquid or snow.”
As the Sierra prepared for a big storm, federal, state and local water officials conducted a series of exercises Wednesday to prepare for the possibility of flooding and other natural disasters that could result throughout the state later this winter from the predicted El Niño storms.
A federal emergency response plan released Wednesday lays out a series of worst-case scenarios that could result from El Niño. Among the biggest threats: widespread flooding in the Sacramento Valley and mudslides in areas damaged by this year’s devastating wildfires. The 56-page plan offers steps for dealing with such problems, ranging from using a central hub for monitoring a potential crisis to deploying emergency management teams to critical sites.
“People are taking the threat seriously,” said Bob Fenton, regional administrator for the Federal Emergency Management Agency.
Thursday’s storm likely will not bring the kind of disaster the FEMA playbook addresses. It will, though, bring difficult conditions to much of the Sierra. “It’s going to be heavy and wet; it’s definitely going to be treacherous travel,” Mead said. “This is one of those instances where you don’t travel if you don't have to.”
Central Sierra snowpack
Snowpack in the Central Sierra remains below historical averages. Here’s a snapshot for Dec. 8 over the last decade.
Percent of normal
Source: California Department of Water Resources