Stream flows in the Little Spokane and North Fork Coeur d’Alene rivers both dropped by 23 percent between 1948 and 2006, according to the “Missing Mountain Water” study.

Last weekend’s doozy of a storm followed a classic Northwest weather script.

Winds gusting to 40 mph blew moisture-rich air from the ocean into the Cascades and Northern Rockies, dumping snow on the mountains while leaving lower elevations bare.

The winds – called “winter westerlies” – are vital to a region that depends on mountain snowpack for its water supply. But a new study suggests that climate change is disrupting the winds, with stark implications for future water availability.

“Those winds are being slowed down by climate change,” said Charlie Luce, a research hydrologist at the Rocky Mountain Research Station in Boise. That means fewer storms will reach the mountains, or smaller water droplets will drift over the peaks as fog instead of falling as snow, he said.

Either scenario would mean additional headaches for Northwest policymakers preparing for an altered climate.

Warmer temperatures already are expected to shift some Northwest precipitation from snow to rain and cause the snow that does accumulate to melt earlier in the year. The net effect is reduced runoff during the spring and summer, when the water is needed for irrigation, hydropower, fisheries and other uses. Complicating matters, Luce’s study suggests there will be far less water to begin with.

The “Missing Mountain Water” study was published last week in Science magazine by Luce and researchers from the University of Idaho and the U.S. Forest Service.

“I think it points out potential new risks,” Amy Snover, director of the Climate Impacts Group at the University of Washington, said of the study.

Meeting competing demands for water during dry years is already a challenge in the Northwest, she noted. And as the study points out, many questions remain about snowpacks because historically there hasn’t been good climate monitoring at high elevations, Snover said.

Philip Mote, an atmospheric scientist and director of the Oregon Climate Change Research Institute, found Luce’s conclusions about westerly winds interesting, but said he’s not confident the study can be used to predict sparser snowpacks. It’s an area ripe for additional research, he said.

Luce agreed that additional research is needed but also said he hopes to get water policymakers’ attention with the initial results. “We recognized a puzzle that was out there and started chipping away at it,” he said.

The research team set out to probe a discrepancy in data from earlier studies: Stream flows across the Northwest dropped significantly between 1948 and 2006, even while precipitation levels remained fairly steady. When the researchers looked at where the precipitation gauges were, they realized measurements were primarily taken at low-elevation sites. They started to suspect that declines in high-elevation precipitation were responsible for diminished stream flows.

Others had tied the decline to increased evaporation from warmer temperatures. But according to calculations done by the researchers, the drop-off in stream flows was too large to be attributed to evaporation, Luce said.

Instead, the team turned its attention to mountain snowpacks and the influence of westerly winds. The speed for winter westerlies has dropped by about 10 percent over the past 60 years, a decline too great to be explained by natural fluctuations in ocean cycles, such as El Nino events, Luce said.

Westerly winds are caused by temperature differences between the ocean and land, Luce said. Additional carbon in the atmosphere is heating British Columbia’s land mass – which juts out to the northwest – faster than the ocean directly south of it. The land is still colder than the ocean during the winter. But the shrinking temperature gap decreases the pressure gradient that causes the westerly winds to blow across the Northwest, Luce said.

Weaker winds reduce the supply of water vapor reaching the mountains, and “this has big consequences relative to planning,” Luce said.

Across the Northwest, there’s been interest in building new reservoirs as a partial mitigation to climate change. Water managers are looking for ways to hold back the earlier runoff to meet summer demands. Instead of naturally storing the water in snowpack, they’re looking at storing it behind dams.

But if less water vapor is reaching the mountains, Luce said the situation could be much more complex. The water to fill those reservoirs may not be there, he said.