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Upper Columbia 183 Percent Of Average Precipitation In April; Continued Below Normal Temps Expected
Posted on Friday, May 06, 2011 (PST)

A still-wet spring has served to push up forecasts of how much water will be on tap this spring and summer for fish, irrigators, power generators and others depending on the Columbia-Snake river system.

 

The Northwest River Forecast Center’s April 27 “early bird” forecast says that, in the most likely scenario, an estimate 120 million acre feet will pour down from the Snake and mid and upper Columbia rivers from April through September and through the lower Columbia’s The Dalles Dam. That would be 119 percent of the 30-year (1971-2000) average of 98.65 MAF and the seventh highest output in the past 41 years, according to the NWRFC. The Dalles is the second to last hydro project downstream in the Columbia-Snake river system.

 

The forecast considers existing mountain snowpack, runoff to-date, observed precipitation through April 25 soil moisture conditions and other available data and assumes normal precipitation for the balance of the spring-summer season.

 

The month of April continued a trend of above-average precipitation across much of the Columbia River basin. The upper Columbia, upstream of central Washington’s Grand Coulee Dam, has experienced 183 percent of its average precipitation April 1-25 and 121 percent of average so far in the water year (Oct. 1 through April 25), according to the NWRFC. The area in British Columbia, northern Idaho and northwest Montana typically provides nearly two-thirds of the Columbia basin runoff overall. The precipitation averages are also based on the 1971-2000 period.

 

The flow into the reservoir backed up by northwest Montana’s Libby Dam is, according to the early bird forecast, expected to be 8.27 MAF or 125 percent of normal while Flathead River flows past Columbia Falls, Mont., are expected to be 143 percent of average.

 

The Flathead joins the Clark Fork River, which mingles with the Pend Oreille River which flushes into the Columbia near the Canadian border. The Kootenai River basin, which feeds Libby Dam’s Lake Koocanusa, received 185 percent of its average precipitation April 1-25 and is at 120 percent for the water year.

 

The Snake River drainage is also stocked. Flows as measured at Lower Granite Dam on the lower Snake River are expected to total 31 MAF in April-September, which would be 128 percent of average, according to the early bird.

 

The forecast for flows into Dworshak Dam’s reservoir is 3.43 MAF, which would be 130 percent of average. Dworshak is on the North Fork of the Clearwater in west-central Idaho. The North Fork feeds into the Clearwater and then meets the Snake just above Lower Granite Dam. The Clearwater basin received 226 percent of its average precipitation April 1-25 and was at 130 percent of average for the water year.

 

Precipitation above Ice Harbor Dam, which is also on the lower Snake in southeast Washington, was been 192 percent of average April 1-25 and 129 percent of average so far in the water year.

 

The Columbia-Snake basin above The Dalles was 189 percent of average April 1-25 and 123 percent of average Oct. 1 through April 25.

 

The 90 days ending on April 25 were cooler than normal across the entire Pacific Northwest, according to the “Climate Outlook” prepared for the Climate Impact Group’s Spring 2011 newsletter, Pacific Northwest Climate CIGnal. CIG is located in Seattle at the University of Washington, where it is part of the Center for Science in the Earth System at the Joint Institute for the study of the Atmosphere and Ocean in the College of the Environment.

 

The newsletter’s Spring 2011 can be found at:

http://cses.washington.edu/cig/outreach/newsletter.shtml#anchor1

 

The temperature departures from the 1971-2000 normal (in Fahrenheit) for the western United States, indicate that much of the Pacific Northwest was between 2 and 4 degrees cooler than normal. Northern Montana was even cooler, with temperatures up to 8 degrees below normal during the same period.

 

The cooler and wetter than normal conditions combined to help build snowpacks that are all at least 125 percent of average across much of the Pacific Northwest in terms of water content as of April 27, according to the Climate Outlook. A few basins in central Idaho, where the 90-day precipitation was below normal, have snowpack that is closer to normal.

 

NOAA’s Climate Prediction Center April 20 outlook for May through July says that the odds tilt towards a continuation of below normal temperatures for western Oregon, most of Washington, northern Idaho, and most of Montana.

 

The probability of below normal temperatures is higher for the Olympic Peninsula, northern Washington, and northern Montana than the other areas. The CPC forecast for the rest of Oregon and southern Idaho have equal chances of below, equal to, or above normal temperature for May, June and July. In other words, the probability is split evenly split into a 33 percent for each of the three outcomes.

 

The CPC outlook for most of the Pacific Northwest has equal chances of below, equal to or above normal precipitation. The outlook does indicate that northeastern Montana has chances exceeding 33 percent for above normal precipitation during the period.

 

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