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Moist March Pushes Water Supply Forecast To 13th Highest In 41 Years; 107 Percent Of Average
Posted on Friday, April 01, 2011 (PST)

A late winter-into-spring wet spell across the Columbia-Snake basin has served to solidify predictions that the region will enjoy an above average water supply pouring down from the mountains in the coming months.


The monthly “early bird” forecast released Thursday by the NOAA Weather Service’s Northwest River Forecast Center predicts that, in the most likely scenario, 106 million acre feet of water will flow past The Dalles Dam on the lower Columbia from April through September. That would be 107 percent of the 1971-2000 average and the 13th highest in the 41 years of record. All of the water draining down from the Snake and the upper Columbia courses past The Dalles.


And with much of the slow accumulation season past, the chances of a greatly changed result (from the forecast to actual outcome) has shrunk. A worst-case scenario (very dry from now on) modeled by the NWRFC predicts runoff past The Dalles would be 93 MAF or 94 percent of average. A best-case scenario (extremely wet) pegs runoff at 119 MAF or 121 percent of average.


An above average runoff volume would be only the second of the new millennium. In 2006 the water volume past The Dalles was 101.657 MAF or 103 percent of normal.


The March 31 forecast includes observed precipitation through the March 27 and estimates that precipitation will be 120 percent of normal for the first half of April and 100 percent of normal for the rest of the period.


The Dalles forecast is up from a Feb. 24 early bird forecast of 98 percent of average.


Precipitation at 120 percent of average could be termed dry when compared to March. Overall, precipitation in the upper Columbia basin above Grand Coulee Dam in central Washington was soaked with 158 percent of its average for the March 1-27 period, and 115 percent of average so far in the water year (Oct. 1-March 27). The new water volume forecast predicts that 70.1 MAF (110 percent of average) will flow past Grand Coulee during the April-August period.


The Snake River basin upstream of Ice Harbor Dam in southeast Washington got 188 percent of its average precipitation March 1-27 and is at 123 percent for the season. The March 31 early bird forecast predicts that runoff past Lower Granite Dam from April through September will be 114 percent of normal or 27.4 MAF. That’s up from the Feb. 24 forecast of 100 percent.


Recent weeks’ precipitation has helped to lift snow-water equivalent above 100 percent of normal for all 22 groupings of SNO-TEL measuring sites in the Columbia basin for the first time this winter. The last to break the 100 percent barrier, which it did this week, is the Chelan, Entiat, Wenatchee area in central Washington. As of Thursday the SWE there was 101 percent of normal. The area was at only 72 percent of normal on Feb. 15.


The Yakima, Ahtanum area south-central Washington is at 105 percent of normal SWE, also up from 72 percent on Feb. 15.


Northwest Montana has the highest SWEs, as a percentage of average. The Kootenai River drainage in Montana was at 125 percent, the Flathead at 131 percent, the upper Clark Fork at 117 percent, the Bitterroot at 110 percent and the lower Clark Fork at 120 percent of average SWE.


The latest runoff forecast for Libby Dam reservoir inflows is 7.45 MAF, which would be 110 percent of average.


The Clearwater, Salmon river SNO-TEL sites in central Idaho now show a SWE that is at 110 percent of average, the Owyhee, Malheur area is at 128 percent and the Grand Ronde, Powder, Burnt, Imnaha had a SWE reading Thursday that was 116 percent of average. All of the snowpack runoff in those streams flow into the lower Snake


The NWRFC’s forecast predicts that North Fork of the Clearwater River flows into Dworshak Dam’s reservoir will be 3.13 MAF, which would be 112 percent of normal.


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