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February Gives Runoff A Boost: April-Sept. Water Supply Now Forecasted At 98 Percent Of Normal
Posted on Friday, March 09, 2012 (PST)

Columbia-Snake river water supply forecasts for the approaching warm months remain “below normal, but not drastically so,” according to the Steve King of NOAA’s Northwest River Forecast Center in Portland.


Forecast have nudged upward, for the most part, across the Columbia River basin in recent weeks and are expected, given a 10-day forecast of above average precipitation, to continue that trend in the near term.


The NWRFC’s most recent “Ensemble Streamflow Prediction” generated forecast, based on precipitation, snowpack, runoff and other data compiled through Wednesday, says that in the most likely scenario a total of 97.133 million acre feet will flow down from the Snake and upper Columbia and past the lower Columbia’s The Dalles Dam from April through September. That would be 98 percent of the 1971-2000 average annual runoff volume.


After a relatively wet February, the forecast is up from a 94 percent of average forecast a month earlier. Most of the volume overall, and the improvement, is due to relatively bountiful mountain snowpacks to the north in portions of British Columbia, Idaho and Montana.


“They are the saviors for the region,” King said of river basins such the Clark Fork, Kootenai and the uppermost Columbia. The water volume forecast released Thursday estimated runoff past Libby Dam on the Kootenai River in northwest Montana would be about 102 percent of average from April through September, up from an estimate of 83 percent issued Feb. 7.


The U.S. Army Corps of Engineers’ early March forecast estimates runoff from Libby Dam will be 96 percent of average during the April-August period. The Corps operates the dam.


The Clark Fork above Missoula, Mont., is predicted to furnish 104 percent of its average runoff in the NWRFC’s most likely scenario.


The top water supply forecasts are from Canada’s Columbia headwaters. Flows into Mica Dam’s reservoir are expected to be 114 percent of normal from April through September, which would be 14.2 MAF. That forecast is actually down from a prediction released a month earlier of 117 percent of normal.


The forecast for Columbia flows as measured at the international border is 104 percent of average for the April through September period.


Flows downstream at Grand Coulee Dam in central Washington are predicted to be 104 percent of normal.


King said that after a dry December, precipitation began to pick up in January and continued near annual averages in February in most of the region’s high points – along the Rockies and the Cascades in February. An exception was the area above Mica.


Another exception, pretty much throughout the fall and winter season, has been the southern tier of the basin – southern Oregon and Idaho. The forecast flows on the lower Snake River at Lower Granite Dam in southeast Washington are expected to be 89 percent of normal. Much of the upper Snake River, while originating in Wyoming, is in southern Idaho.


Some of the upper Snake feeders have a scanty snowpack to rely on, at least so far. The Henrys Fork, Teton, Willow, Blackfoot and Portneuf drainages have, collectively, snowpack with a snow-water equivalent that is only 79 percent of average, according to data retrieved through Wednesday by the National Resources Conservation Service from its electronic SNOTEL measuring stations. The Little and Big Wood river basins in Idaho were at 77 percent. The Weiser, Payette and Boise SWE measured 81 percent of average through the date.


And further downstream, the Owyhee and Malheur river basins had SWE at 66 percent of average through March 7. They feed into the lower Snake from Oregon along the Idaho-Oregon state line.


The Clearwater River, which flows into the Snake at the Idaho-southeast Washington boundary, has 84 percent of its average SWE for March 8. The latest forecast predicts that North Fork of the Clearwater inflows to Dworshak Dam’s reservoir will be 95 percent of average during April-September, up from 94 percent a month earlier. An early March forecast produced by the Corps expects April-July water supply from Dworshak will be 96 percent of average. The Corps operates the dam.


According to the NWRFC, jet streams off the Pacific began to stream more directly into the Columbia River basin beginning in the second week of February with weak to moderately strong systems moving onshore.


But overall, the month was relatively benign weather-wise. February precipitation was 85 percent of normal in the Columbia basin above Grand Coulee, 83 percent of normal in the Snake basin above Ice Harbor Dam and 86 percent of normal above The Dalles.


Seasonal precipitation (October through February) was 92 percent of normal above Grand Coulee, 93 percent of normal above Ice Harbor and 94 percent of normal above The Dalles.


Streamflow forecasts improved 5 to 10 percent in most northern basins and 1 to 5 percent on Snake River drainages during February. 


Snow accumulations during the month were best in the northern tier basins in Washington, north Idaho, Montana and in British Columbia with SWEs ranging from 95 to 120 percent. The uppermost Snake, such as Jackson Lake in Wyoming, March 1 SWEs at 100 to 110 percent of average. The runoff forecast for Jackson Lake for the April-September period is 107 percent of average.


Eastern Oregon and the southern Snake basins in Idaho experienced average to below average snow accumulations during February. SWEs in those areas ranged from 70 to 80 percent of average. West of the Cascades SWEs ranged from 100 percent in the north to 60 percent in Oregon as of March 1.


Most of the Columbia-Snake basin had below average streamflow during the month of February. Most basins ranged from 70 to 85 percent of the 30-year average. The exceptions were on the Clark Fork river in Montana, the uppermost Snake basins and on the Yakima River in Washington where February streamflows ranged from 100 to 110 percent.


The Yakima-Ahtanum basins are bright spots overall with a SWE of 108 percent of average. The Yakima River runoff forecast, as measured at Parker, for April-September is 101 percent of average.


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