Last month’s massive snowstorm in the Sierra Nevada followed a dry start to winter. Such extremes in precipitation may become the norm.
By Andrew Chapman, EOS
In late February 2024, residents of California and Nevada communities surrounding Lake Tahoe were starting to wonder whether winter would ever come. Warm temperatures and rain had left shovels and snowblowers untouched, local skiers discouraged, and water and disaster managers wondering what the year’s water supply and fire hazard would look like.
“Our snowpack needed a huge boost.”
Then, on the last day of February, the storm of the year blew in, dumping more than 10 feet (3 meters) of snow in some places and bringing fierce winds, making national headlines. “A quarter of the seasonal snowpack fell on northern Sierra Nevada in a matter of a couple of days,” said Michael Anderson, California’s state climatologist. “It’s a huge boost,” he said. “And our snowpack needed a huge boost.”
Experts say that such extreme swings in weather—known as weather whiplash or its wintrier cousin, snow whiplash—are likely to become the norm in the Sierra Nevada. Quickly moving between extremes poses challenges for residents of the Lake Tahoe area, who need to dodge road closures and remove snow from their properties, and also for the people who manage the state’s resources because unpredictable weather can make it more difficult to prepare for drought and fire risks. “It’s becoming a much more dynamic and active environment to live in,” Anderson said.
Normal, but Not
Winters in the northern Sierra Nevada have long been known to bring a “boom or bust economy” to resort towns in the region because dry periods followed by big storms are normal, said Benjamin Hatchett, an atmospheric scientist at Colorado State University who lives in South Lake Tahoe. The most recent weather event “was our textbook, classic, big cold storm.” However, this storm was on a different scale—only one storm every 10 years will produce this much snow.
Climate change is likely to drive this boom or bust economy to the extremes.
The biggest snowstorms in the Sierra Nevada over the past 50 years have several features in common. They aim directly at the mountains; have strong wind speeds, especially in the upper atmosphere; and have lower pressure than other storms. These features favor a lower freezing elevation, so precipitation is more likely to be snow.
These storms also spend time over the Pacific Ocean, picking up moisture that later falls as precipitation. The ocean is already a massive source of moisture for storms, but every 1°C (1.8°F) of atmospheric warming causes 7% more moisture in the atmosphere. So in a warmer future there will be a greater chance that when the other factors common to big snowstorms come together, all that extra water will condense into rare, but intense, snowfall in the mountains.
“These kinds of weird extremes could certainly be more common.”
The extra moisture in the atmosphere can also intensify atmospheric rivers. These narrow bands of water vapor travel from tropical regions and are associated with warmer and wetter storms.
“These kinds of weird extremes could certainly be more common” as the climate changes, said Adrian Harpold, a snow hydrologist at the University of Nevada, Reno.
On the other hand, though occasional storms could produce greater snowfall than average, general warming due to climate change may mean temperatures will be too high for snow. “With every little bit of warming, we stack the deck towards more rain and less snow,” Hatchett said.
That means less snowpack to slowly melt and feed streams throughout the spring. Any snow that does fall is more likely to melt earlier in the season, leaving drier conditions on the ground.
The wobble in snowpack levels from year to year can also worsen the severity of wildfires. Low-snow years parch the soil throughout the summer, making the forest more likely to catch fire. But perhaps counterintuitively, high-snow years can also lead to worse wildfires because the added moisture causes growth in vegetation, leading to more tinder for fires in subsequent drier years. “We often see like a 2-year delay between a big winter and a bigger fire year,” Hatchett said.
Adjusting to Extremes
This weather whiplash means that the state will need to be more careful about managing its water resources. Sierra snowpack provides about 30% of California’s water supply. In the past, water managers wouldn’t worry about collecting runoff behind the state’s dams until the spring because consistent snowpack meant they were sure to see the slow filling of reservoirs during the spring melt.
But as seasonal snowpack thins, it ceases to be a reliable source of fresh water. Managers will need to consider how they collect water throughout warming periods that can melt snow with punctuated, unpredictable storms. “That’s just a recipe for a challenge,” Harpold said.
Though scientists assert that climate change will eventually push the Sierra Nevada into snowless winters, sporadic, walloping storms in the near future might be enough to save the snowpack. Unfortunately, such storms are hard to predict.
Anderson said that the next step in his work is to help develop better forecasting on such extreme weather events to help guide water management throughout the winter—saving water while also avoiding flooding.
—Andrew Chapman (@andrew7chapman), Science Writer
