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Latest Water Supply Forecast Up Slightly by Bill Rudolph NW Fishletter, March 21, 2003

The March "mid-month" January-July water supply forecast for the Columbia River above The Dalles has bumped up five percentage points from the earlybird forecast to 75 percent of average. The latest forecast, released March 20, includes results from the big regional snowfall in early March; the earlier forecast did not.

Much snow has fallen in March, but much has also melted. The current forecast for the Columbia Basin above Grand Coulee is about 81 percent of average. Overall precipitation for the first 17 days of March above Coulee was 234 percent of average, according to the National Weather Service, and was nearly that high (188 percent) for the Columbia above The Dalles.

February precipitation throughout the Northwest was extremely low, only 54 percent of average above Coulee and 69 percent of average for the Columbia above The Dalles, but 89 percent of normal for the Snake River above Ice Harbor.

Observed precipitation from the October to March period was 78 percent of average for the Columbia above Coulee and 85 percent for the Columbia above The Dalles.

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 has received 280 percent of average and northern Idaho's Clark Fork Basin was 243 percent of average for the month so far. The state's Clearwater Basin received 4.2 inches of precipitation by March 17, a whopping 274 percent of average--which added up to a basinwide snow water equivalent of 91 percent, much higher than most other Columbia subbasins.

Central Oregon was hurting, with Hood River and Willamette River drainages showing only slightly above 50 percent of their average snow water equivalents.

Washington's Yakima Basin was running about 73 percent to 85 percent of average snow water equivalent, with the April-September water supply forecast at 72 percent to 77 percent of average. -B. R.