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NW Fishletter #391 March 4, 2019

[3] Report: Stable Snowpack Not Likely To Continue In Coming Decades

According to a Feb. 25 drought outlook webinar from the National Integrated Drought Information System, the Columbia Basin appears to be dodging the bullet expected to come with climate change--a large drop in the snowpack, as measured on April 1.

The outlook expects a replenished snowpack throughout much of the basin, thanks largely to February's cold weather and above normal precipitation in the Snake River and lower Columbia River basin, where it was most severely lacking at the end of January.

Although forecasters predict a warm and dry spring, for now the snowpack is much closer to normal than it was on Feb. 1, especially in the Snake River and lower Columbia, Karin Bumbaco, Washington's assistant state climatologist, said during the webinar.

In the first three weeks of February, the snowpack's snow water equivalent in the lower Snake River basin went from 88 percent of normal to 121 percent, and the middle Snake went from 92 percent of normal to 128 percent.

The Columbia River basin is still below normal, but big gains were made in the lower Columbia, which jumped from 66 percent of normal to 92 percent, and the middle Columbia, which went from 67 percent of normal to 95 percent.

Upper reaches of the Columbia River saw only slight a slight increase, from 86 percent of normal to 88 percent, she said.

While February was unusually cold and snowy throughout much of the Pacific Northwest, it was within the region's natural variability for both temperature and precipitation, Bumbaco added.

That natural variability is also what's been keeping snowpack throughout the West relatively stable for the last 35 years, researchers say. And once those variabilities change, scientists believe the snowpack will begin to disappear at a much faster rate.

Nick Siler, assistant professor in Physics of Oceans and Atmospheres at Oregon State University and lead author of a new study comparing snowpack with oceanic and atmospheric climate data, presented his findings during the drought webinar. Written with colleagues from the University of Washington and Lawrence Livermore National Laboratory, the study was published Jan. 11 in Geophysical Research Letters.

Siler noted that while snowpack throughout the West has declined between 15 and 30 percent since the 1950s, Western states have largely been spared the disappearing snowpack over the last 35 years.

"It's not that the region hasn't been warming," he said, noting that temperatures in the West have increased nearly 2 degrees Fahrenheit since 1984. This raises the question of why snowpack hasn't been declining as much as expected, he said.

Researchers looked at snowpack measurements at 329 SNOTEL sites across the West, leaving out the sites where the average snow water equivalent on April 1 was less than 10 inches, since they do not contribute significantly to summer streamflows, Siler said.

In all, 64 percent of the sites examined saw decreased snowpack on April 1, and 36 percent saw increased snowpack. "But none of these trends are statistically significant," he added. Only four sites had statistically significant declines in snow.

To determine whether the lack of response by snowpack is due to climate change, the researchers identified the atmospheric circulation patterns that correlate most strongly with each SNOTEL site, and then removed the variability that could be attributed to those circulation patterns. "The question, really, is: Are the sea surface temperatures and circulation part of the climate's response, or part of natural variability?" Siler said.

After looking at 86 variations from climate models, the researchers did not find that the trends observed over the last 35 years were represented by trends in any of the models. "Basically, these trends are likely not due to CO2 forcing, or human influence, but rather part of natural variability," he said, adding that there's a chance that the atmospheric changes are due to something unknown that the models are not capturing.

But assuming the climate models are correct, this "suggests that this phase of favorable variability isn't going to last forever, which suggests the declines are likely to accelerate in the coming decades."

Whether this year's snowpack will provide average runoff through the summer is still very much in question. With another month before the snowpack-building period is over, forecasters still aren't sure what to expect.

Mike Halpert, deputy director of NOAA's Climate Prediction Center, said in the webinar that although El Nino had been predicted since early winter, it didn't actually arrive until a couple weeks ago. Because it's so late it's unlikely to play an important role in the weather this spring, he said.

The question now is whether El Nino will persist through the year, as it did in 2015, and become a consideration for next winter, or whether it will react more like the weak El Ninos of the early 1990s, which persisted for a few months before dying out.

In terms of the drought outlook through May, Halpert said, most of the region is showing no sign of drought, except for a large area in north-central Oregon and a small area in north-central Washington, where drought is predicted to persist.

"Oregon had drought last year, and is still recovering from that," climatologist Bumbaco explained. She said current drought conditions in Oregon are considered severe-to-extreme. But more telling is the improvement in drought conditions over the last two months throughout most of the Pacific Northwest, she said.

So far this water year--which began Oct. 1--the Northwest has been warmer than normal, and precipitation has been below normal, she said. But February helped bring both closer to normal, she said. eastern Washington and Montana have seen temperatures that are 11 to 12 degrees below normal, while western Washington has been hit with snow.

This was Seattle's snowiest February on record, with a total of 20.2 inches, Bumbaco said, and also the fourth snowiest month on record, in Seattle.

Portland, Ore., got 6.1 inches of snow in February, which was not a record. In south-central Washington, the storms and cold hit 15 dairy farms near Sunnyside, where 1,800 cows valued at $4 million died as a result of the storms. -K.C. Mehaffey

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Publisher/Editor-in-Chief: Mark Ohrenschall, Editor: K.C. Mehaffey
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