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Highest Basin Water Supply In 12 Years; April 1-11 Precipitation 203 Percent Above Average
Posted on Friday, April 15, 2011 (PST)

The Columbia River basin is poised to enjoy what will likely be its largest water supply since 1999 in terms of April through September runoff past The Dalles Dam on the lower river.


The Northwest River Forecast Center’s monthly “final” water supply forecast released April 7 says that, in the most likely scenario, a total of 107 million acre feet will rush through The Dalles Dam, which passes all of the water pouring down from mountain snowpacks into the Snake River and mid- and upper Columbia.


Such a total would be the 12th highest in the past 41 years. The Dalles Dam total hasn’t been that high since 117.7 MAF was recorded during the April-September period in 1999, which was the seventh highest total on the 41-year record, according to the NWRFC. The record dating back to 1997 was 140.9 MAF in 1997, which was 143 percent of average.


Fish and other water users have been blessed this year with a hard-finishing winter and a wet, so far, spring. The March 7 final forecast predicted runoff past The Dalles would be 99 percent of normal. But March precipitation was 159 percent of normal (1971-2000) in the Columbia River basin upstream of Grand Coulee Dam, which is located in central Washington; 170 percent of normal above the lower Snake’s Ice Harbor Dam, and 173 percent of normal above The Dalles.


From Oct. 1, the beginning of the “water year,” through March precipitation was 118 percent of normal above Grand Coulee, 124 percent of average in the Snake basin above southeast Washington’s Ice Harbor Dam and 119 percent of normal above The Dalles.


Adding to the water wealth is a wet start to April. Precipitation above The Dalles was 203 percent of average April 1-11, according to data compiled by the NWRFC.


The latest NWRFC water supply forecast assumed that future precipitation would be 110 percent of average through April and normal for the rest of the season. That jibes with the NOAA Weather Service Climate Prediction Center’s forecast of a greater than equal chance of above normal precipitation in April and an equal chance of above normal, normal or below normal precipitation in April-June.


An April 7 CPC “La Nina advisory notes that “La Niña weakened for the third consecutive month, as reflected by increasing surface and subsurface ocean temperatures across the equatorial Pacific Ocean.


“While there is confidence in ENSO-neutral conditions by June 2011, the forecasts for the late summer and beyond remain highly uncertain,” according to the advisory. “At this time, all of the multi-model forecasts suggest ENSO-neutral conditions will persist from June through the rest of the year. However, the spread of individual model forecasts and overall model skill at these lead times leaves the door open for either El Niño or La Niña conditions by the end of 2011.”


A La Niña is characterized by cooler than average sea surface temperatures in the equatorial Pacific Ocean, stronger than normal easterly trade winds, and a westward shift in the region of intense tropical rainfall.


The El Nino(La Nina)/Southern Oscillation conditions can influence weather around the world. In the Northwest, those influences are strongest from October to March; by summer, Northern Hemisphere wind patterns are such that they effectively trap ENSO-related disturbances in the tropics.


El Niño winters in the Pacific Northwest tend to be warmer and drier than average with below normal snowpack and streamflow. La Niña winters tend to be cooler and wetter than average with above normal snowpack and streamflow.


Relatively strong La Nina conditions held sway this past fall and winter. And whether or not they are the cause, those seasons have been wet across the Columbia-Snake basin almost without exception. A small corner of the north Oregon coast has had below average precipitation.


The April final forecast predicts that runoff past Grand Coulee Dam in central Washington will be 107 percent of average or 68.7 MAF from April through September. That’s up from a 103 percent forecast made a month earlier. All of the runoff from the upper Columbia in parts of British Columbia, Idaho, Montana and Washington flow past Grand Coulee.


The forecast includes a 109 percent of average prediction for inflows to Libby Dam’s reservoir, about 7.22 MAF. That comes close to matching the U.S. Army Corps of Engineers’ recent forecast of 7.191 MAF for the April-August, which would be 123 percent of the 1975-2009 average. The NWRFC forecasts are compared to the 1971-2000 average runoff volumes.


The forecast for the Snake River basin, as measured at Lower Granite Dam in southeast Washington, is 116 percent of average for the April-September period, or 27.9 MAF. That’s an increase from 100 percent of average forecast in the March 7 final.


Forecast inflows to the Dworshak Dam reservoir are 116 percent of average, 3.24 MAF, according to the NWRFC final. The Corps forecast is 3/387 MAF for the April-July period, which would be 126 percent of the 1929-1999 average. The west-central Idaho’s North Fork of the Clearwater River is backed up by Dworshak. The North Fork feeds into the Clearwater and then into the Snake, which joins the Columbia in southeast Washington.


Snowpacks across the region are well above normal, particularly in the upper Columbia drainages of northwest Montana. The snow-water equivalent as of Wednesday at Kootenai River SNO-TEL sites were 139 percent of normal for that date. The SNO-TEL sites are electronic measuring mechanisms operated by the Natural Resources Conservation Service. Elsewhere in Montana the Flathead River drainage was at 142 percent of normal, the upper Clark Fork at 127 percent, the Bitterroot at 119 percent and the lower Clark Fork at 131 percent.


North Idaho panhandle SNO-TEL sites average 119 percent of normal. Columbia River tributaries in Washington above the Methow River confluence are at 123 percent of average SWE, according to the April 13 data.


The Clearwater-Salmon river drainages have snowpacks at 117 percent of average SWE through April 13. The Clearwater and Salmon feed the lower Snake River.


SNO-TEL sites above the Palisades reservoir in the Snake’s headwaters in Idaho have snowpack at 131 percent of average through April 13.


The lowest SNO-TEL readings were 106 percent of average in the Big and Little Wood river drainages in south-central Idaho and the Yakima-Ahtanum drainages in central Washington.


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