Massive Sierra snow has California facing floods

A vehicle drives past a truck struck on a log near Rancho Murieta, California. Rivers are rising and winds are whipping up as a massive storm arrives in Northern California. Photo / AP

A powerful storm blasted the Sierra Nevada with waves of torrential rain and heavy snowfall, 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 centimetres of snow over the span of a week.

At Mammoth Mountain, a ski resort bordering Yosemite, the peak got 210cm of snow in just two days. This week's forecast calls for several more cm 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, US$250 million recovery.

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 2m more of snow in the mountains. At the highest elevations, the cold air could translate to as much as 6m of snow on the peaks, according to forecasts from the National Weather Service.Park rangers closed roads onto 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.

Such high snow accumulation could mitigate California's enduring drought by building up the Sierra snowpack. Farming is a crucial aspect 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 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 snows has experts concerned about catastrophic avalanches. The Sierra Avalanche Centre 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. "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.