Early spring heatwaves may be melting Pacific Northwest water supply

Increasingly frequent spring heatwaves are prompting premature snow melts across the Pacific Northwest, jeopardizing a key water source for area residents, a new study has found.

Successive stretches of unseasonable heat have been occurring earlier in the year in a region that depends on snowpack for summer water, according to the study, published on Wednesday in npj Climate and Atmospheric Science.

“Short-term events like heatwaves have had an under-appreciated impact on accelerating snow melt, and cumulatively, they can amplify each other,” lead author Luke Reyes, a doctoral student in Washington State University’s School of the Environment, said in a statement.

Heat domes, weather events in which the atmosphere traps hot ocean air, triggered record temperatures that rose to nearly 122 degrees Fahrenheit across the Pacific Northwest in late June 2021.

So extreme was this event that British Columbia endured severe floods in several snow- and glacier-fed watersheds, as well as a large rock avalanche, the authors noted. 

While the researchers had intended to examine the snowmelt caused by that single event, they ended up finding that much of the region’s snowpack was already gone before the heat dome arrived.

Their subsequent analysis — conducted using high-resolution snowpack and temperature data — indicated that high-elevation snow had begun melting during a series of heatwaves from April through early June.

During these incidents, temperatures were just 7.2 to 12.6 degrees Fahrenheit above normal, according to the study.

Looking back at records from 1940 through 2021, the researchers saw that springtime heatwaves doubled in frequency, intensity or both since the mid-1990s.

Such short-term events have been understudied, as these single-digit temperature surges typically last just a few days, according to the study.

Meanwhile, mountain snowpack was for decades thought to withstand temporary heat spikes in the spring by staying sufficiently cold at higher elevations, the authors noted.

But the new research demonstrates that this buffering capability may have declined as heatwaves have become more frequent and intense, they added.

“The data suggests that we don't necessarily need to be worried about a very rare event like the heat dome,” co-author Marc Kramer, an associate professor of environmental chemistry at Washington State, said in a statement.

Of more concern, he continued, is the notion “that heat waves are becoming far more prevalent and are more likely to be driving a lot of snowpack loss in the future.”

The extreme early melting in 2021 occurred even though that was a La Niña year, a weather phenomenon that usual means more precipitation and deeper snowpack in the Pacific Northwest, according to the study.

While snowpack that spring was 135 percent greater than normal in the higher-elevation snow zone, by the end of June, it was gone, the researchers explained.

This trend, the authors warned, is not a good sign for 2024, when the region is expected to endure increased dryness due to El Niño conditions.

“We have a coming El Niño year and next year, and there may be some amplification effects,” said Kramer.

“If we have less snow to begin with, the snowpack is going to be all that much more vulnerable to these heat anomalies earlier in the season,” he added.