Persistent rain and snowfall across the Columbia-Snake river basin during the first 18 days of April has resulted in a spike in a prediction of how much water will rush down through the system from April through September.
On April 7 the Northwest River Forecast Center’s monthly “final” forecast predicted that runoff from the mid and upper Columbia and the Snake would total 107 million acre feet, which would be 108 percent of the 1971-2000 average, as measured at The Dalles Dam on the lower Columbia. The forecast by the National Weather Service’s NWRFC included observed precipitation through March, snowpack data and other data, and predicted rainfall would be 110 percent of average in April and normal thereafter.
On Thursday, the Center issued its mid-month forecast, which predicts runoff past The Dalles would be 119 MAF or 121 percent of average. That would be the ninth highest total in the past 41 years of record. The forecast includes observed precipitation through April 18, and assumes precipitation at 125 percent of average through April, then normal for the rest of the season.
That observed April 1-18 precipitation in the area of the Columbia-Snake River basin upstream of The Dalles was 202 percent, and is now at 122 percent of normal for the water year (Oct. 1 through April 18). The Columbia’s upper reaches above the central Washington’s Grand Coulee Dam received 213 percent of average precipitation April 1-18.
The April-September water supply forecast for Grand Coulee was 107 percent of normal or 68.7 MAF as of April 7. That forecast this week was hiked to 75.6 MAF, which is 118 percent of normal.
The mid-month forecast for Lower Granite Dam reservoir inflows is 30.8 MAF or 128 percent of normal. That’s up from a 27.9 MAF, 116 percent of average prediction in the final forecast.
Some of the National Resources Conservation Service’s SNO-TEL snowpack measuring devices have impressive readings. The snowpack in northwest Montana’s Flathead River basin is at 156 percent of normal and the Kootenai River basin in Montana is at 153 percent.
The lowest snow-water equivalent among the snowpacks monitored in the basin is 109 percent for the Big and Little Wood basins in south-central Idaho and 115 percent of average in the Yakima-Ahtanum in central Washington as of Thursday. All the rest are at 120 percent of average or better.