Dec 11, 2017


Morgan Levey
Climate Change Medill


Climate scientists are finding that it’s not just warming temperatures that are causing the transition from snow to rain but also the altering intensity and variability of winter storms in the Sierra Nevada.

“It appears that the storms themselves are getting a lot warmer,” said Benjamin Hatchett, a hydrometeorologist at the Desert Research Institute in Reno, devoted to studying natural and human-induced environmental changes. According to a new paper published in the journal Water by Hatchett and a team of climate scientists, the combined effects of rising sea surface temperatures and warming background air temperatures make conditions more favorable for warmer and wetter storms landing in the Sierra Nevada. Prolonged periods of dry weather between storms only exacerbates the water resource problems caused by rainier winter storms. The rainier winter downpours cause greater risk of flooding and leave little snowpack to stream into the water supply during summer.


Hatchett analyzed recorded snow-level data over the past decade and discovered that the snow for winter storms has risen approximately 1,200 feet in altitutde in the atmosphere between the beginning and end of the last ten years. Meanwhile, since 1951, the snow fraction or percent of precipitation that falls in a given time as snow, has steadily declined by about three percent each year.

“Over 10 years that’s 30 percent less precipitation falling as snow, so that can make a significant impact on our water resources,” Hatchett said. The last 10 years has been the steepest decadal decline in snow fraction over the past 67 years.


Since the Sierra Nevada is a maritime mountain range, it’s highly influenced by its proximity to the Pacific Ocean. A major component of climate change is warming sea-surface temperatures, making conditions preferable for atmospheric rivers, or long, narrow filaments of water vapor in the atmosphere that “favor wetter, warmer, higher snow-level types of storms,” Hatchett says. With increased evaporation potential from the warming, atmospheric rivers pick up energy and moisture across the ocean and make landfall with more intensity and variability.