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NW Fishletter #379, March 5, 2018

[5] Water-Supply Outlook Encouraging For Columbia, Snake Rivers

A combination of above-normal snowpack in the upper Columbia River Basin and forecasts for a cool, wet spring prompted an encouraging outlook for water supply in the main-stem Columbia and Snake rivers for April to September.

The monthly update was presented in a Feb. 1 webinar by Steve King, senior hydrologist for the National Weather Service's Northwest River Forecast Center. He predicted that the upper Columbia, southern British Columbia and western Montana will experience a summer water supply ranging from normal to "quite a bit above," compared to the agency's 30-year average.

The forecast has improved from a month ago, and includes predictions of water volume at 107 percent of normal at Libby Dam on the Kootenai River, 109 percent of normal on the Columbia at Mica and Grand Coulee dams, and 122 percent of normal at Albeni Falls Dam on the Pend Oreille River.

The forecast for The Dalles Dam on the lower Columbia is now at 107 percent of normal, up from January's prediction at 96 percent.

The upper Snake River is also showing above-normal forecasts, with Palisades Dam at 112 percent and Jackson Lake at 128 percent of normal. Volume in the lower Snake, at Lower Granite Dam, is at 106 percent of normal. Some areas are still forecast for below-normal flows, like Lucky Peak Dam on the Boise River, predicted at 80 percent of normal.

The numbers are based on current precipitation, temperature, snowpack and runoff observations, combined with climate forecasts.

King noted that 60 percent of the regional water supply comes from Canada and western Montana. "So, fortunately from that standpoint, the [snowpack] numbers are pretty healthy," he said.

Whether the forecast holds depends partly on the accuracy of current climate outlook for the next three months. While predictions show warmer-than-normal temperatures in February, the three-month outlook calls for wetter and cooler conditions compared with the 30-year average.

King said the forecast for cooler temperatures over the next three months is "something we haven't seen much of this winter, so we'll see how that shakes out." -K.C. M.

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