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Late Snow Bumps Up Latest Water Forecasts by Bill Rudolph NW Fishletter, April 15, 2003

The April "early bird" update of the January-July water supply forecast for the Columbia River above The Dalles bumped up four percentage points from last month's early forecast, to 77 percent of average. That forecast, released March 27, included results from the big regional snowfall in early March. The April "final" forecast released ten days later racheted the water supply up a couple more notches to 79 percent.

The current forecast for the Columbia River Basin above Grand Coulee is about 84 percent of average, up 6 percent from last month's early forecast. Overall March precipitation above Coulee was 200 percent of average, according to the National Weather Service. It was nearly that high for the Columbia River above The Dalles, at 175 percent of normal. The improved forecast has boosted the "probable" forecast for April-July inflows at Lower Granite Reservoir to 17.1 million acre-feet from 14.7 MAF last month.

An analysis by the US Army Corps of Engineers using the mid-month forecast (16.6 MAF) said flows would average about 79 kcfs for the spring. At the time, BPA's Scott Bettin said that translated into a 65 percent chance of spill this spring. Since then, hydro and fish managers agreed to start the spill program at lower Snake dams.

The hydro BiOp calls for target flows in the 85- to 100-kcfs range at Granite if the April-July flows are estimated between 16 MAF and 20 MAF. Below 85 kcfs, the BiOp does not require spill at three of the four lower Snake dams.

Other basins also saw extremely high precipitation in early March, as tropical moisture came ashore early in the month and led to flooding in some areas. Montana's Flathead Basin was 200 percent of average for March with an 89 percent of average snow-water equivalent. Idaho's Clearwater Basin has received enough precipitation lately to have a snow-water equivalent 94 percent of average.

The situation in the Yakima Basin has also improved, and is now about 74 percent to 90 percent of average snow-water equivalent. The April-September water supply forecast now stands at 78 percent to 81 percent of average.

March has stood in extreme contrast to February, when precipitation throughout the Northwest was extremely low: 54 percent of average above Coulee and 69 percent of average for the Columbia River above The Dalles. The Snake River above Ice Harbor fared better in February, with 89 percent of normal precipitation.

Observed precipitation for the October-April period is now at 90 percent of average for the Columbia River above Coulee and 94 percent of average for the Columbia above The Dalles