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Snowpack Lagging; For Now, April-Sept. Water Supply Forecast At The Dalles Dam 95 Percent Of Normal
Posted on Friday, January 11, 2019 (PST)

With precipitation and snowpack lagging to varying degrees in the Columbia Basin, the first water supply and streamflow forecasts for 2019 are reflecting those conditions.


The Northwest River Forecast Center in Portland conducted its first briefing of the year on Jan. 3, re-capping the 2018 water year for the region, outlining current conditions and the outlook for the water year that is underway.


Hydrologist Kevin Berghoff said that so far, there has been below-normal precipitation across the Upper Columbia and Snake River basins, resulting in snowpack that is well below normal. Well below normal snowpack conditions persist in the Cascades, particularly in Oregon.


Based on those conditions, the April through September water supply forecast ranges from 88 to 90 percent of the historic average on the Upper Snake; 79 percent on the Boise River; 99 percent on the North Fork Clearwater; 90 percent on the Lower Snake; and 95 percent of normal on the Lower Columbia River at The Dalles Dam.


In the Cascades, current projections for April through September streamflows range from a high of 99 percent of normal on the Skagit River near Concrete, Wash., to a low of 81 percent of normal on the Willamette River at Salem, Ore.


The Columbia Basin’s overall snowpack picture is beginning to resemble characteristics of last year’s conditions, with the healthiest snowpack in the Upper Columbia and Snake River Basins and the driest areas being concentrated in central and southwest Oregon.


“If you recall last year there was a fairly stark dividing line between basins that had above-normal snow water equivalent and basins that were deficient,” Berghoff said.


With some exceptions, snowpack conditions appear to be developing in a similar way, with recent warm and dry weather impacting seasonal precipitation measured from last Oct. 1 through Dec. 31.  in the Columbia River basin above Grand Coulee Dam, three-month precipitation is at 88 percent of normal, compared to 102 percent of average for the same period last year.


Precipitation in the Snake River Basin is at 85 percent, compared to 98 percent last year; and precipitation in the Columbia River Basin above The Dalles is at 85 percent compared to 98 percent last year.


For weeks now, NOAA’s Climate Prediction Center has had an active El Nino Watch, based on above normal sea surface temperatures in eastern Equatorial Pacific Ocean. Those sea temperatures have yet to “couple” with atmospheric conditions.


“The late winter and early spring tend to be most favorable months for coupling, so forecasters still believe weak El Nino conditions will emerge shortly” and continue into spring, the CPC said in its monthly forecast issued this week.


Berghoff highlighted historical data showing how weak El Nino systems do not necessarily translate to warm and dry conditions in the Pacific Northwest. Strong El Nino systems, however, are more likely to produce those conditions in the Northwest.


“You could make the case that there’s just about an equal number of El Nino years with above-average precipitation as below average in weak El Nino years,” Berghoff said.

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