Wednesday, May 11, 2011

April’s weather brings expectations for high runoff

April’s cool, moist weather held off the snow melt and even added to the mountain snowpack according to the latest snow survey conducted by the Natural Resources Conservation Service. This summer’s water supply is forecast to be plentiful for Idaho’s water users but the spring runoff is expected to be high. 

 “The water supply analysis completed this week shows that April’s precipitation and cool temperatures added significant amounts of water to the snowpack,” said Jeff Burwell, Idaho NRCS State Conservationist. “While this contributes to an ample water supply, it increases concern over how the runoff season will unfold.”

The whole state received above average precipitation in April. Precipitation ranged from 110% of average in regions of central Idaho to 250% of average in the Northern Panhandle. However, lingering cool spring temperatures delayed the snow melt creating a potentially threatening runoff season.

“Usually the mid-elevation snowpacks begin melting in April - at least 25% of the snowpack melts off,” said Ron Abramovich, NRCS Water Supply Specialist. “Not this year. Below normal temperatures prevented snow melt in the mid-elevation range and kept the headwater streamflow levels below normal.”

“The May 1 mountain snowpack is above average ranging from 125 to 190% of average. And, now there is a shorter runoff season,” said Abramovich. “This means more streamflow in a shorter time period.”

The timing and magnitude of peak streamflows depend on spring temperatures, consecutive hot days, non-freezing night temperatures, and if rain falls when the snow is melting. Reservoir operators across Idaho are drawing down reservoir levels to increase water storage space.

“Our current Water Supply Report forecasts river levels and volumes to be above average through the summer,” Burwell added. “Whether you are a river runner or a water manager, expect extremely variable conditions.”

View May’s full report on snowpack, precipitation, runoff and water supply predictions at and click on the ‘Water Supply’ link.

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