Rising nighttime temperatures melting snowpack earlier in the year

About 14 percent of the Spokane River basin’s snowpack melted away over the past couple of weeks.

Sunny skies weren’t responsible for the rapid snow loss. The culprit was nighttime mountain temperatures that were 10 to 15 degrees warmer than normal.

“We’re not cooling off enough at night to freeze things up,” said Scott Pattee, water supply specialist for the Natural Resources Conservation Service.

Researchers say warmer nights are a trend consistent with climate change, and one that could have serious long-term ramifications for Northwest snowpack and summer water supplies.

Mountain snowpack can withstand spring days with temperatures in the 50s and 60s if it drops below freezing at night, Pattee said. But around the Inland Northwest, warmer nights during late March and early April triggered an early beginning to snow melt.

On Friday, the low temperature at Mullan Pass was 46 degrees – a full 20 degrees warmer than normal for the 6,000-foot-elevation mountain pass near the Idaho-Montana border.

“If we melt our snow off early, we’re not going to have anything left for summer,” Pattee said. “We depend on the slow melting of high-country snowpack to take us into August.”

Nick Bond, Washington’s state climatologist, said warmer nights have been documented worldwide as concentrations of greenhouse gases increase in the atmosphere. In the Pacific Northwest, the trend is most pronounced in the summer and winter, he said.

Warmer summer nights are often associated with increased humidity, Bond said. In the winter, the trend is related to downward radiant heat.

“Snow is so reflective that it doesn’t absorb a lot of solar radiation, so sunlight doesn’t have too large of an effect in melting snow,” he said.

But snow does absorb the increased infrared radiation associated with climate change.

“Infrared radiation is particularly important to the snow melt,” he said. “A warm, moist atmosphere – especially with lots of clouds – is when you’ll get a lot of melting.”

On Monday, snowpack for the Spokane River basin was about 81 percent of normal for this time of year. March storms helped build up mountain snow accumulations. Though the Spokane basin is in much better shape than last year, the snowpack has lagged behind normal for most of the winter, Pattee said.

In addition to less snow, “we just never got the cold snaps that deep freeze the snowpack,” he said.

Without the cycle of hard freezes, the snowpack is primed to melt off earlier, Pattee said.

According to the National Weather Service, recent weather patterns are likely to continue into June. The three-month forecast calls for an increased chance of warmer-than-normal temperatures, with a slight increase in the chance of below-normal precipitation.

Last summer’s drought was a wake-up call for the Northwest in understanding how temperature affects water supply, Bond said.

“It wasn’t a drought caused by lack of precipitation,” he said. “It was a temperature-caused drought. It rained a lot more than usual in the mountains, and we didn’t have the snow.

“We’d never seen anything quite like last year, and we’re grateful we’re not getting a repeat,” Bond said. But, “it’s perhaps a taste of what will be happening more often in future decades.”

This story was updated to correct the spelling of Nick Bond’s name.