Mountains in the southwestern U.S. are welcoming record-breaking snowpacks this year, but new research shows they are melting at a record pace too.

Winter storms have provided the Sierra Nevada snowpack with some of the highest preliminary snow levels in 40 years, according to federal resource managers.

Snowpack levels across Nevada are currently 144% of normal. The deepest snowpack in Nevada was last observed at Mt. Rose Ski Area with a snowpack depth of 134 inches, about 214% of normal, when compared to its 64 inch average depth for this time of year, according to weather station data.

Those yields are the result of a weeks-long atmospheric rivers — streams of moisture in the atmosphere that transport water from the tropics — that brought heavy rain and high-elevation snow across part of the West, leading to drought improvements in California, the Pacific Northwest, the northern Rockies and the Great Basin, say federal resource managers.

Much of the excess moisture, however, is tied up in snowpacks and its effects on soil moisture, water runoff, and groundwater recharge remains to be seen.

Snowpack is different from snow depth. It’s a measure of how much water is sitting in the snowpack, meaning that if the entire snowpack melted at once, that’s how much water would be running off a mountain. And while new snow could add to the West’s stockpile in some areas, there is no guarantee Nevada will avoid dry conditions this year.

“Last year brought record snow to the Sierra in December, but accumulation from January through March was record dry and led to a below normal spring runoff. While chances of such a historic dry period again this year are low, current numbers should be viewed with cautious optimism since snow water amounts are still only about half of what was measured at the end of big winters such as 2019, 2017 and 2011,” said NRCS Water Supply Specialist Jeff Anderson earlier this month.

Snow-capped mountains provide natural reservoirs of frozen water that slowly melt into watersheds throughout the spring and summer months, a process water managers in the West rely on to sustain water supplies, but climate change and extreme weather conditions are impacting that cycle.

Rapid snowmelt eliminates those natural snowpack reservoirs when water is most needed during the warm season.

In a study published earlier this month, researchers at the Desert Research Institute analyzed how extreme spring heatwaves accelerated melting rates of mountain snowpacks across the West in 2021, leaving water managers unable to accurately predict future water supplies. 

“Many of the sites we used had about 30 to 40 years of data and a lot of them were showing record snow melt rates compared to other years,” said Dan McEvoy, an associate research professor and regional climatologist for the Desert Research Institute.

Researchers found that in April 2021, record-breaking snowmelt rates occurred at 24% of all mountain snowpack monitoring sites in the region, further compounding the impacts of extended drought conditions. 

Researchers also found that above average snowpacks in the Pacific Northwest in 2021 still melted early despite the greater levels of snow.

“It was interesting to see that even though some of these places had above average snowpacks during the peak time of year, one, they still melted out early, and two, those regions went into drought later that summer,” McMvoy said.

Beware the spring heat waves

The first week of April in 2021 saw maximum temperatures 4-6 degrees Celsius above average, driving the most widespread record snow melt on the Rocky Mountains, which supplies water to the Colorado River. 

With reservoirs reliant on the Colorado River below expected levels based on early-season snowpack predictions, less water flowed to downstream users, said McEvoy.

While this last round of rain has helped return smaller reservoirs to historical averages, many larger reservoirs — including Lake Mead and Lake Powell— still remain low due to long-term drought in the West, according to a report from the U.S. Department of Agriculture’s Natural Resources Conservation Service last week.

More snow will be needed through the winter to ensure there’s enough water stored in the snowpack to meet demands this summer and fall, say federal water managers.

Several factors contributed to the rapid rate of snowmelt in the spring of 2021, said McEvoy. For one, record high minimum temperatures prevented snowpacks from refreezing at night, leading to substantial loss. 

Researchers say these spring heatwaves are part of a long-term trend of spring warming across the West.

“There’s a high likelihood of these events happening more in the future,” McEvoy said.

A loss of moisture sooner in the season can lead to drier soil that absorbs more spring snowmelt before it can run off into streams and reservoirs, a process known as aridification. 

Rapid snow melt also means rangelands and vegetation are exposed to spring heat longer making them drier and more vulnerable to wildfires, said McEvoy. The link between reduced mountain snowpacks and increased wildfire potential is consistent with previous research, said McEvoy.

“We hope to forecast snowmelt events in advance to give some warning and give water resource managers a chance to understand any implications for what that might mean,” McEvoy said.

Still, storms resulting from the atmospheric river relieved drought conditions to some degree across the West. In Idaho, severe and moderate drought improved in some areas, and stream flows showed improvement. In Utah, extreme and severe drought areas received 300% more precipitation than normal over the last 30 days, according to federal water managers. Heavy rain also helped erase areas of abnormal dryness in parts of Washington, Oregon, western Wyoming, western Colorado, Arizona and New Mexico.

“It was a pretty exceptional three to four week period,” McEvoy said. “There are places across the Sierra and in the Northern Great Basin that have the most snow for this time of year that they’ve had in 30 or 40 years, and that’s saying a lot. It’s very encouraging.”