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Forecast for normal runoff into Lake Chelan

Chelan County PUD
News Release
3/4/2008

The March 1 runoff forecast for the Lake Chelan drainage basin will likely be in the range of 98 percent to 103 percent of average for the April 1 to July 31 time period.  The low forecast of 98 percent of average runoff would result in 514,600 second-feet-day (SFD) or 1,020,450 Acre-Feet (AF) of water, and 103 percent of average runoff would result in 540,900 SFD or 1,072,600 AF of water.  Long-range forecasts are calling for below average precipitation and temperatures from March through May.  This is a significant change from last month, when precipitation was forecasted at 125 percent of normal for March and 140 percent of normal for April. Anecdotal evidence suggests above-average low-level snow in the Lake Chelan basin.  This could result in actual runoff meeting or exceeding the upper end of the forecasted range.  Large amounts of low-level snow could also lead to significant early inflows as the weather begins to warm.               

Moisture content in the soil (soil prime) appears to be slightly lower than average due to less than average precipitation during November 2007.  The average snow water equivalent at the above listed sites is 105 percent of normal.  Snowpack densities range from 38 percent-40 percent, which is slightly above average and approximately 6 percent lower than last year.   

The elevation of Lake Chelan on March 1, 2008 was 1,083.7 feet, which is 2 feet lower than the expected average under the new license and 6 feet lower than last year.  There are two reasons the lake is so much lower this year than last.  The main reason is heavy rains and subsequent inflow in early November 2006 increased the lake level by almost 3 feet over a two-week period.  The secondary reason is the lake was drafted approximately 0.8 feet lower at the end of September 2007 than 2006 due to implementation of the new operating license.  Given the current lake level there should be plenty of water for refill by July 1. 

The generation forecast for Lake Chelan is guardedly optimistic.  If below-normal precipitation and temperatures occur as forecasted, generation will need to be reduced through June to ensure refill.  If average precipitation and temperatures occur, generation at full capacity will be likely until late April.  If above-average precipitation and temperatures occur, generation may not be curtailed this spring.  

The Northwest River Forecast Center’s March 2008 Early Bird Forecast for the Columbia River at Grand Coulee is 98 percent of average runoff for January to July.  At The Dalles the forecast for January to July runoff is 9 percent of average.  Forecasts for other local area basins range from below to above average.  The Wenatchee River is forecast for 110 percent of average runoff April to September.

At right is a color coded diagram of the Columbia River Basin that shows current snow water equivalent in percent of normal.