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NW River Forecast Center: Above Normal Snowpack In Basin’s Northern Tier, Not So Much Moving South
Posted on Friday, February 02, 2018 (PST)

A La Nina weather influence off the Pacific Ocean has been felt, but not at all like it manifested in the Columbia Basin last winter.

 

That became clear during a webinar sponsored by the Northwest River Forecast Center in Portland Thursday with a focus on snowpack, and how temperatures and precipitation have combined over the last few months across the Columbia Basin.

 

Overall, the runoff forecast is expected to be above-average on the Columbia River this year. But the most striking graphics displayed showed how last winter’s La Nina-driven weather was widespread in the basin, with abundant precipitation and much cooler temperatures in the southern tiers of Oregon and Idaho.

 

That didn’t happen this year. “We are above normal in our northern tier basins and not quite in our lower tier basins,” said Steve King, service and coordination hydrologist with the Northwest River Forecast Center. “It is opposite of last year.”

 

And it appears weather in January has had the biggest effect on the overall picture, he added. “The one that stands out is the month of January, which is 3 degrees above normal (on average over most of the basin). That’s obviously had an effect on snowpack. We are quite a bit lower than normal this season in Oregon, but with healthier numbers in the north.”

 

King noted that 60 percent of the basin’s water supply comes from Canada or western Montana, and “from that standpoint, it looks pretty healthy. We are wetter in the northern tier and drier in the southern tier.”

 

That trend is forecasted to continue for at least the next 10 days, and possibly through February.

 

But King said “there’s still a lot of time” for conditions to change. “It’s only Feb. 1 and we frequently see a lot of things change in the spring.”

 

As it currently stands, snowpack measurements are above average by healthy margins across the Northern Rockies in Idaho and Montana, but they taper down into the mountains feeding the Snake River Basin, and there is an obvious decline in snowpack percentages from north to south in the Cascade Mountains.

 

Those conditions are reflected in runoff forecasts, for April through September of this year:

Columbia River in Canada, Mica Dam, 109 percent of average; Kootenai River, Libby Dam, 107 percent; Coeur d’ Alene Lake, 107 percent; South Fork Flathead River at Hungry Horse Dam, 123 percent; Pend Oreille River at Albeni Falls Dam, 122 percent; Columbia River, Grand Coulee Dam, 109 percent.

 

On the Upper Snake River, Jackson Lake Dam, 128 percent; Palisades Dam, 122 percent; Snake River near Heise, 117 percent; Boise River at Lucky Peak Dam, 80 percent; North Fork Clearwater River at Dworshak Dam, 107 percent; Lower Snake River, Lower Granite Dam, 106 percent.

 

And at The Dalles Dam, the big barometer for water supply in the Columbia Basin, the forecast is for 107 percent of average April through September of this year.

 

On the west side of the Cascade Mountains, the water supply numbers for the same period steadily drop from north to south, starting with the Skagit River near Concrete, Wash, at 103 percent of average, the Green River forecast at 89 percent, the Cowlitz River at 95 percent, the North Santiam River at 80 percent, the Willamette River at Salem at 72 percent, and the Rogue River at Applegate Reservoir, 58 percent.

 

Northwest River Forecast Center, Water Supply Forecasts:  https://www.nwrfc.noaa.gov/ws/index.html?version=20170308v1

 

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The Columbia Basin Bulletin, Bend, Oregon. For information or comments call 541-312-8860.
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