A slow start to the snow accumulation season north of the U.S.-Canadian border has served to drag down -- to about 97 percent of normal -- predictions of how much water will gush from Columbia River basin mountains by summer’s end.
The Northwest River Forecast Center’s monthly “final” forecast predicts, in the most likely scenario, the water supply as measured at The Dalles Dam on the lower Columbia from January through July will be 104 million acre feet. That would be just below the 1971-2000 average. The forecast for April through September is also 97 percent of average.
The latest NWRFC forecast assumes 100 percent of normal precipitation for the rest of the winter and spring. All the runoff from the mid- and upper Columbia and Snake River courses past The Dalles.
The forecast is buoyed by rosy prediction of 110 percent of average runoff from the upper Snake, as measured at Lower Granite Dam on the lower Snake. The water volume expected to run past Lower Granite from January through July is 33 MAF.
British Columbia, at least through early January, is saddled with a modest snowpack in the Columbia’s headwaters. The water volume forecasts for Canadian measuring points range from 90 to 93 percent of normal. About 60 percent of the overall basin water supply comes from British Columbia/western Montana in the Columbia’s upper reaches, the NWRFC’s Steve King said during a Monday discussion of the forecast.
The water supply forecast for Grand Coulee Dam on the mid-Columbia in central Washington is 94 percent of normal or 59 MAF for January through July.
It’s possible that the upper basin’s snowpacks could catch up. The National Weather Service’s Climate Prediction Center has issued “a very bold forecast” for a wetter than average January-March across the Northwest, King said. The CPC forecast is for a greater than 33 percent chance of above normal precipitation for the region as a whole and a probability of more than 40 percent throughout Washington state, in northern Oregon, and in north and central Idaho.
“There’s lots of season to come,” King said.
La Niña conditions, which tip the odds toward cooler, wetter winters in the Northwest, are expected to continue well into the Northern Hemisphere’s spring 2011.
A moderate-to-strong La Niña continued during December 2010 as reflected by well below-average sea surface temperatures across the equatorial Pacific Ocean, according to a Jan. 6 “diagnostic discussion” posted on the CPC’s web site.
“The current ENSO model forecasts have not changed significantly compared to last month. La Niña is currently near its peak and is expected to persist into the Northern Hemisphere spring 2011 at a lesser intensity,” the Jan. 6 discussion says. The climatic phenomenon is known as the El Nino/Southern Oscillation.
“Thereafter, there remains considerable uncertainty as to whether La Niña will last into the Northern Hemisphere summer (as suggested by the NCEP [National Centers for Environmental Prediction] CFS and a few other models), or whether there will be a transition to ENSO-neutral conditions (as suggested by the CPC CON and a majority of the other models).”
“We’re still expecting near normal conditions for the year,” Steve Barton of the U.S. Army Corps of Engineers told the Technical Management Team Monday. “Everything near the border and south is near normal. Canada does have some ground to make up.”
The Corps and Bureau of Reclamation operate the dams in the Columbia-Snake mainstem hydro system. TMT’s federal, state and tribal members discuss dam operations that might be implemented to benefit salmon and steelhead stocks that are listed under the Endangered Species Act.
Precipitation totals show the same north-south differential. In the Columbia basin above Grand Coulee precipitation was 93 percent of normal Jan. 1-10 and 94 percent of normal so far in the water year (Oct. 1 through Jan. 10). It could be higher but for the fact that the Columbia above southern British Columbia’s city of Castlegar has only received 77 percent of normal precipitation so far during this water year.
The Snake River basin above Ice Harbor Dam has received 120 percent of its average precipitation through Jan. 10.
The Columbia basin above The Dalles has received 101 percent of its normal precipitation.
Snowpack “snow-water equivalents” are the best, as a percent of average, in the upper Snake region in south-central and southeastern Idaho, according to the National Resources Conservation Service’s SNOTEL Snow/Precipitation Update Report. The area that includes the Henrys Fork, Teton, Willow, Blackfoot and Portneuf drainages had a snowpack SWE at 114 percent of normal through Jan. 11 and the Raft, Goose, Salmon Falls and Bruneau area was at 123 percent of normal.
The Owyhee-Malheur snowpack SWE is at 118 percent of normal. The Owyhee originates in northern Nevada, cuts across the southwest corner of Idaho and the southeast corner of Oregon and empties into the lower Snake. The Malheur originates in southeast Oregon and likewise drains into the lower Snake.
The Clearwater-Salmon snowpack SWE was only 90 percent of normal through Tuesday. Both rivers originate in central Idaho. The NWRFC’s latest forecast, however is for runoff of 109 percent of normal during January-July in the Salmon at Salmon, Idaho, and 102 percent of normal in the Clearwater at Orofino, Idaho.
The NWRFC’s water supply forecast for the North Fork of the Clearwater, which fills Dworshak Dam’s reservoir in west-central Idaho, is 100 percent of average. The Corps’ own forecast completed early this month for Dworshak water supply is 124 percent of average. The Corps forecast is nudged up by the La Nina conditions while the NWRFC forecast is not.
Western Montana snowpack is, for the most part, at or above average. The Kootenai River drainage in Montana is at 97 percent SWE, the Flathead River at 112 percent and the lower Clark Fork River drainage at 97 percent while the upper Clark Fork is at only 86 percent. The Idaho Panhandle mountains have only 84 percent of their average SWE.
The NWRFC’s Jan. 7 forecast predicts the Kootenai River’s flows into Libby Dam’s reservoir will be 93 percent of average during January-July. The Corps’ forecast is for inflows of 96 percent of average during the April-August period.