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NOAA Forecast Center’s April-Sept. Streamflow Prediction Now Pegged At 92 Percent Of 30-Year Average
Posted on Friday, February 03, 2012 (PST)

Despite a mid-January burst of precipitation, snowpack in the Columbia River basin remains below “normal” as the region heads into wintertime’s home stretch.


The NOAA Northwest River Forecast Center’s most recent “Ensemble Streamflow Prediction” water supply forecast, based on data compiled through Tuesday, predicts that runoff past the lower Columbia’s The Dalles Dam this year from April through September will be 92 percent of the 30-year average (1971-2000) or 90.4 million acre feet.


The average runoff volume past The Dalles is 98.65 MAF . Last year’s total was highest in the past 52 years, 132.942 MAF, according to NWRFC records.


The strength of the 2011-2012 Columbia River basin snowpack is to the north. Inflows to Mica Dam’s reservoir near British Columbia’s Jasper National Park is expected to produce 120 percent of its average April-September runoff, 15 MAF. That would be the fourth highest total in the 1960-2012 period. A bit further downstream, British Columbia’s Revelstoke Dam’s reservoir is expected to see 115 percent of its average runoff, according to the NWRFC forecast.


But elsewhere in the upper Columbia the water volume forecasts are not so rosy. The Kootenai River at Libby Dam’s reservoir just south of the border in northwest Montana is expected to see only 82 percent average runoff. Downstream on the Columbia at central Washington’s Grand Coulee Dam the Aril-September flow is expected to be 61.2 MAF, which would be 94 percent of normal, according to the new NWRFC forecast.


The Snake arm of the Columbia/Snake wishbone is predicted to produce 85 percent of its average runoff, 19.45 MAF April-September, as measured at Lower Granite Dam in southeast Washington.


Snowpacks are mostly subpar across the U.S. part of the Columbia basin, according to data compiled by the National Resource Conservation Service’s automated SNOTEL sites. The exception as of Thursday was the Kootenai River basin within Montana, which has 100 percent of its average snowpack “snow-water equivalent. Elsewhere in the upper Columbia, Montana’s Flathead drainage had 86 percent of its average SWE, the upper Clark Fork River 93 percent, and the lower Clark Fork 98 percent. The Idaho panhandle snowpack was at 90 percent of its average SWE.


A bright spot so far is the Yakima/Ahtanum river basin in central Washington at 102 percent of average SWE.


Snowpack above Palisades Dam on the Snake in southeast Idaho was at 89 percent of normal this week. The Big and Little Lost river drainages in southeast Idaho through Wednesday had among the basin’s lowest SWE readings at 66 percent.


Over the past two weeks, sea surface temperatures in the central tropical Pacific cooled slightly, reversing a recent warming trend, according to a Feb. 1 update from Australia’s Bureau of Meteorology. Other indicators of La Niña, such as the Southern Oscillation Index trade winds, and cloudiness over the equatorial Pacific Ocean have generally remained steady, below their December peak but clearly exceeding La Niña thresholds.


NOAA’s Weather Service and others note that La Nina conditions, which can alter weather conditions worldwide, generally tilt the odds in favor of wetter, cooler winters in the Pacific Northwest. Last year La Nina was strongly in place; and the Northwest late winter and spring, was extremely wet. Columbia-Snake river basin snowpacks generally continue to build through February and March.


Climate models surveyed by the Bureau of Meteorology indicate a gradual decline in the strength of the La Niña over the coming months, with most models suggesting a return to neutral conditions in the late spring.


The National Weather Service’s Climate Prediction Center’s 6-10-day forecast calls for drier than normal conditions across the Northwest. Its February-April forecast says odds favor a wetter than normal late winter and spring.

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