TRUCKEE, Calif. — A powerful storm blasted the Sierra Nevada with waves of torrential rain and heavy snowfall on Sunday, leaving a vast swath of California bracing for potentially disastrous floods, avalanches and mudslides.
The latest weather comes just days after the mountains around Lake Tahoe and Yosemite National Park received several feet of snow over the span of a week. At Mammoth Mountain, a ski resort bordering Yosemite, the 11,000-foot peak got 84 inches of snow in just two days. This week’s forecast calls for several more feet of snow, as well as heavy rain, part of a meteorological phenomenon known as the “Pineapple Express,” which brings an atmospheric river of warm moisture north from the tropics.
The conditions that accompany the latest band of moisture hovering over Northern California bear some semblance to those of a 1997 storm that flooded the Yosemite Valley and led to a years-long, $250 million recovery effort.
Park rangers closed roads into the Yosemite preserve over the weekend, and local officials in mountain towns handed out sandbags for residents to reinforce their homes against the possible deluge.
The storm will continue to pound the Sierra Nevada range this week. Weather experts predict that colder temperatures could possibly turn the moisture from rain into heavy snow, bringing the potential for up to seven additional feet of snow in the mountains.
At the highest elevations, the cold air could translate to as much as 20 feet of snow on the peaks, according to forecasts from the National Weather Service.
Such high snow accumulation could mitigate California’s enduring drought by building up the Sierra snowpack. Farming is a crucial facet of the California economy, and the dry conditions and water shortages in recent years have hurt the state’s agriculture industry. The snowpack, which begins to melt in the spring, helps fill the reservoirs that are critical for growing crops during the summer months.
Frank Gehrke, the chief snow surveyor for the California Department of Water Resources, said the storm cycle — though potentially dangerous in the short term — could help quench the region’s drought conditions.
“This series of storms that we’re experiencing . . . are certainly going to have an impact on water supply, but we’ve got to wait and see how things settle out,” Gehrke said. “The ongoing concern is how warm or cold any particular event is. Warm can bring flooding, and a cold event can build the snowpack. That’s one thing we’re monitoring closely.”
As the storm settled over the mountains during the weekend, roads were closed and resorts halted operations. Visitors had to be kept off the slopes, as extremely high winds and low visibility coupled with thunder and lightning made skiing too dangerous.
“We haven’t seen a storm cycle like this in the last five years of really heavy snowfall,” said Lauren Burke, a spokeswoman for Mammoth Mountain resort. “With the amount of rain that’s in the forecast, flooding is definitely on the forefront of people’s minds.”
In addition to flooding, the prospect of massive snowfall has experts concerned about catastrophic avalanches. The Sierra Avalanche Center issued a Category 5 warning and ranked the probability of hazardous conditions as “extreme,” noting that “due to significant loading from rain and heavy wet snow, natural and human triggered avalanches are certain in the next 24 hours.”
“We’re worried about infrastructure, roads, houses in avalanche zones, and potentially seeing some very large — up to historic — avalanches,” said Steve Reynaud, an avalanche forecaster at the center. “There’s high probability that things can slide big. Things that we haven’t seen potentially in a 10- to 20- to 30-year cycle.”
Brian Kniveton, a Truckee-area resident, joined volunteers at the Squaw Valley Fire Department to fill sandbags as the Truckee River swelled and carried chunks of floating ice.
“I just felt like paying it forward and trying to help do my part to keep North Lake Tahoe a community who can rely on each other,” Kniveton said.
This region of California has seen extensive flooding, but it has been quite a while since a system has come through with this kind of potential. Twenty years ago, Yosemite’s largest recorded flood was generated by a rainfall event not unlike what the park experienced this weekend. All of the park’s major floods resulted from a simple combination of warm rain falling on heavy snowpack.
In the 1997 storm, torrential rain melted the snowpack and the Merced River burst over its banks on New Year’s Day. Water levels in Yosemite Valley peaked at 16 feet, inundating park infrastructure. Electrical, water and sewer systems were ruined, according to the park’s recovery report. The major roads into and out of the park were washed out, leaving more than 2,000 guests and employees stranded as they watched the floodwater pour into the valley.
“Every cliff was a waterfall,” a Yosemite spokesman told The Washington Post’s Ann Grimes in 1997. “Yosemite’s cliffs put Niagara Falls to shame.”
It took three days — during which time the water continued to rise — for the stranded parkgoers and residents to be evacuated by convoy. Downstream, 100,000 people were ordered to evacuate California’s Central Valley. On Jan. 3, 1997, the Merced River reached a record 23.43 feet at Yosemite’s Pohono Bridge, where flooding begins at 10 feet. The resulting damage was so significant that park officials closed Yosemite to the public until March 14, and even then, it was only partially reopened.
The federal government allocated more than $250 million to recovery and flood prevention projects, which weren’t fully completed until 2012.
This week’s cycle of storms began when high pressure — which has all but dominated California weather for the past five years — shifted east. Its absence allowed waves of low pressure to wash onto the West Coast.
Wind flows counterclockwise around low pressure, and swirling air draws warm moisture north from the tropics; the result is what’s known as an atmospheric river. Due to this particular phenomenon’s origins in the Pacific Ocean around Hawaii — and its ability to quickly beam storms toward the West Coast — meteorologists call it the “Pineapple Express.”
“We think of it as a fire hose, because that’s basically what it looks like,” said Jim Anderson, a meteorologist at the National Weather Service in Hanford, Calif.
That fire hose of moisture poured down over much of California as rain and snow in two sessions — one at midweek and a second, stronger wave during the weekend.
Between New Year’s Day and Thursday, the Squaw Valley ski resort north of Lake Tahoe racked up 83 inches of snow on its peaks. Areas west of the peaks were inundated with nearly 10 inches of rain in 48 hours.
Highways through the Sierra Nevada, including Interstate 80 at Donner Pass, were closed during the heaviest snow. When they reopened, six-foot walls of snow towered on the shoulders, and traffic crept through the wintry tunnels.
Sauerbrey reported from Truckee. Fritz and Shapiro reported from Washington. Mayumi Elegado in Truckee contributed to this report.