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Water Supply Forecast Gets Big Jump, April-Sept. Runoff Now Projected At 104 Percent Of Normal
Posted on Friday, March 18, 2011 (PST)

Whopping precipitation totals across the Columbia-Snake river basin have helped push the forecast of the spring and summer water runoff volume to 104 percent of normal, which would be the best outpouring since 1999, according to data compiled by the Northwest River Forecast Center.


The NWRFC’s monthly “mid-month” forecast released Thursday predicts that runoff past The Dalles Dam from April through September will total 103 million acre feet, which would be 104 percent of the 1971-2000 average of 98.65 MAF. The Dalles, located on the lower Columbia, passes water running down from both the Snake and the mid- and upper Columbia.


The 1999 total runoff of 117.7 MAF was 119 percent of normal. Runoff during the past decade has been mostly below normal, though 2006 did produce runoff totaling 101.65 MAF, which was 103 percent of average.


The latest NWRFC forecast takes a big jump from the March 7 monthly “final” forecast of 99 percent. It considered observed precipitation through February and assumed future precipitation would be 110 percent of average through March and average thereafter.


The March 17 forecast took into consideration observed precipitation through March 14 and assumed precipitation through the end of March would be 95 percent of average.


The first 14 days of March saw precipitation in the area of the upper Columbia River basin above central Washington’s Grand Coulee Dam at 188 percent of normal. That pushed precipitation for the season (Oct. 1 through March 14) to 107 percent of normal.


The uppermost part of the Columbia basin, above Castlegar, British Columbia, got 160 percent of its average precipitation for March 1-14. That pushed its season total from 95 percent of average on Feb. 22 to 103 percent through March 14.


The Snake River basin above Ice Harbor Dam in southeast Washington got 140 percent of its average precipitation March 1-14 to push its seasonal total to 115 percent of average.


Overall, precipitation above The Dalles was 183 percent of average in March.


The March 17 mid-month forecast predicts, in the most likely scenario, that runoff past Grand Coulee from April through September will be 69.2 MAF or 108 percent of average. That’s up from the March 7 forecast of 103 percent.


Runoff past Lower Granite Dam on the lower Snake River is now forecast to be 104 percent (25.2 MAF) of average for the April-September period. The March 7 forecast was 100 percent of average.


The mid-month forecasts are intended to be a quick look at runoff accumulation trends. They use only about half the precipitation reports used in the monthly final and employ a short-term assessment of future precipitation made with the help of meteorological models. There are no updated snow or runoff values used directly in the model input, but all available snow water equivalent reports are used subjectively by the forecaster during the review of forecast results.


Most snowpacks in the basin have snow-water equivalents through March 17 that are well above average. They range from 130 percent of average in the Flathead River basin in northwest Montana and 124 percent of average in the Kootenai River basin located in Montana to 90 percent of average in the Chelan-Wenatchee-Entiat river basins in central Washington and the Big and Little Wood River basins in south-central Idaho.


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