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NW Fishletter #379, March 5, 2018

[6] Northwest Water Supply Outlook Ranges From Well Above To Well Below Normal

Predictions for April-through-September water supply across the Pacific Northwest are all over the place after another month that brought more snow to the north, but left Oregon and southern Idaho much drier than normal.

At a March 1 briefing, NOAA Northwest River Forecast Center's senior hydrologist Kevin Berghoff said the upper Columbia River Basin is predicted to see above normal summer flows ranging from 178 percent of the 30-year average at the Clark Fork River above Missoula, Mont., to 115 percent of normal at Grand Coulee Dam.

The lower Columbia is also projected for above-normal flows--111 percent of normal at The Dalles Dam. The Snake River is predicted to have flows ranging from 122 percent of normal at Jackson Lake Dam to 106 percent of normal at Lower Granite Dam.

But some Snake River tributaries are anticipated to contribute far less water to the system this summer, with only 81 percent of normal flows forecast for Lucky Peak Dam, and 45 percent at Owyhee Dam.

Also of concern, Berghoff said, is the early runoff occurring everywhere in the region except Canada. "The early runoff is a sure sign we're losing our snowpack too fast--something we'll continue to watch through the season," he said.

Warmer temperatures in January and February have already adversely affected snowpack in some areas, Berghoff said. But snowpack in the upper Clark Fork, at 158 percent of normal, and Bitterroot, at 132 percent of normal, are ranked in the top 10 for the last 47-year records, he noted. Meanwhile parts of Oregon and southern Idaho are well below normal, such as the Willamette River basin, with about 61 percent of its normal snowpack.

This winter's precipitation in the upper Columbia has been between 100 and 127 percent of normal so far, since October, while precipitation in the Snake River Basin has ranged from 75 percent to 90 percent of normal, Berghoff said.

He said the Snake exception is the Clearwater River, "which tends to be more similar to the upper Columbia in precipitation and temperature." The season's total precipitation there so far is about 121 percent of normal. -K.C. M.

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