Danger lurking: While huge snowpack is welcome now, more rain or warm spell brings potential for flooding
The gargantuan California snowpack, over twice the normal size for this time of year in some parts of the Sierra, just keeps growing. On Tuesday, yet another storm unloaded several feet of snow in the Lake Tahoe area, completely burying the Sugar Bowl Resort office.
Ideally, the snowpack gradually melts during the spring and summer, releasing water when reservoirs aren’t capped by flood control limitations and can maximize storage. All the snow right now is fantastic news for the state’s enduring drought.
“We’re happy about that,” said Dan Feldman, an atmospheric scientist at Lawrence Berkeley National Laboratory.
But the overabundance also presents potential flood risks.
“There is so much more snow water stored up there than usual,” said Daniel Swain, a climate scientist at UCLA and the Nature Conservancy, during an online presentation Monday.
A spring heat wave, for example, could drive an early melt that results in flooding. A warm atmospheric river aimed at snowcapped mountains could also rapidly melt snow and
overload watersheds.
While individual runs of a few weather models hint at the possibility of warm, tropical moisture arriving in the Sierra Nevada next week, most models disagree — and it’s far too early to know for sure, experts reiterate.
Still, in parts of California, climate change is rewriting how frequent — and devastating — these events could be.
“In a warmer climate, you might get more rain-on-snow at those higher elevations,” said hydrologist Keith Musselman,
an assistant professor at the University of Colorado Boulder and the Institute of Arctic and Alpine Research. “And that can cause rivers to rise and react in ways that we haven’t seen historically.”
A rain-on-snow event occurs when heavy rain falls on snowpack. If conditions line up, the result is runoff that includes both rainfall and melted snow, which can overwhelm rivers and flood downstream areas.
That’s what happened during California’s 2017 winter. In
January, cold atmospheric rivers brought snow to relatively low parts of the Sierra Nevada.
Subsequent storms in February, fueled by a warm atmospheric river, melted this snow and sent extreme runoff into the state’s second-largest reservoir, Lake Oroville. Damage to the reservoir’s spillways and fears of catastrophic failure forced the evacuation of 188,000 downstream residents. Repairs totaled over $1 billion.
During rain-on-snow events, it isn’t necessarily water from raindrops that powers the melting. Instead it’s moisture, in the form of water vapor. When this vapor meets a frigid snow-covered surface, it shifts from gas to liquid.
“That releases a great amount of energy,” Musselman said, far more than what comes from raindrops directly hitting snow.
There are other factors at play. Conditions need to be warm enough for the snowpack to melt. The past few months have been cold, meaning that frozen snow atop California’s peaks could resist an immediate thaw even if storms do arrive.
Blustery conditions during severe storms can also accelerate thawing.
“Wind is kind of slamming this moist air against the snowpack and making it turbulent and ensuring that the snow is getting a lot of vapor pushed onto it,” Musselman said. “That really drives the melt.”
The places that most frequently encounter flooding due to rain-onsnow events are at low to intermediate elevation, around 3,000 to 5,000 feet. One way to think about where these elevations are is to imagine a road trip from the Bay Area up to Tahoe, Musselman said.
“As you drive up the highway towards Donner Pass, Donner Summit, you start to see snow on the ground — you’re starting to get into this transition area,” Musselman said.
The rain-on-snow transition regularly sees a mix of rain and snow. When it’s unusually cold, like this year, these mid-elevation locales can build a deep snowpack that’s prone to rapid melting if the next storm brings rain.
Alternatively, a rain-onsnow event can occur when snow typically found at high elevation receives an unusual visit from a big, warm storm.
Scientists expect this latter scenario to pop up more often due to climate change. While rain-onsnow events are expected to become less common at lower elevations — due to less snow thanks to warmer temperatures — rain is expected to fall more frequently at higher elevations, where storms can trigger snowmelt.
The future is also expected to bring a rise in extreme weather behavior. That could mean a big snowstorm quickly followed by an extremely wet, warm atmospheric river — a compound extreme weather event unlike what’s been seen historically, Musselman said.
Fortunately, the current snowpack is relatively cold. And though individual weather model simulations hint at warm, moist conditions in the coming week, ensembles are not pointing to a warm atmospheric river on the horizon. But it may be prudent to remain cautious.
“Given how large the snowpack is, even low probability warm (atmospheric river) events are something I think that we should probably be thinking about seriously,” Swain said. “They would have the potential to produce major impacts from a flood perspective.”