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April-September Basin Runoff Predicted To Be 10th Best In 52 Years; La Nina Dissipates In April
Posted on Friday, May 11, 2012 (PST)

With the spring sun finally starting to impose its will on snowpack, it’s clear the Columbia-Snake River basin will be enjoying a fully stocked water supply.

 

According to the latest NOAA National Weather Service forecast the second largest runoff volume since 1999 will flow past The Dalles Dam on the lower Columbia River this year during the April-September time frame – supplying water for power, irrigators, fish and recreation.

 

“This would be another solid runoff year for The Dalles,” Steve King of NOAA’s Northwest River Forecast Center said of the 2012 prospects.

 

The most recent NWRFC forecast, issued Tuesday based on data compiled through Monday, says runoff as measured at The Dalles Dam will be 110 percent of the 1971-2000 average for the time period for the January-July period, which would be the 14th best total in the past 52 years during that time frame.

 

The runoff for April-September, 115.665 million acre feet is forecast to be 117 percent of average and the 10th best on the 52-year record. The 1999 total was 117.711 MAF, the eighth highest total.

 

A relatively wet late winter and spring continued into April with 141 percent of average precipitation in the part of the Columbia River basin above central Washington’s Grand Coulee Dam, and 114 percent of normal for the October-April period, according to NWRFC data.

 

The area upstream of the lower Snake River’s Ice Harbor received 113 percent of its average precipitation for the month of April, and is at 106 percent of normal for the October-April season. That area includes the upper Snake in Idaho and parts of Wyoming and Nevada. Ice Harbor is in southeast Washington.

 

The area above The Dalles which takes in the Snake basin and mid- and upper Columbia soaked up 127 percent of its average April precipitation, and 110 percent for the season.

 

Precipitation thus far in May has been near average across the basin, with forecasts for below average in the coming weeks and months. The National Weather Service’s Climate Prediction Center says the odds favor lower than average precipitation from May-July across the Idaho, Montana, Oregon and Washington.

 

The “La Nina” conditions, which have gotten credit to some degree for the past two years’ wetter-than-normal winter weather in the Northwest, have subsided. La Nina conditions in the equatorial Pacific can favor wetter and cooler conditions in the Columbia basin.

 

“La Niña dissipated during April 2012, as below-average SSTs [sea surface temperatures] weakened across most of the equatorial Pacific Ocean and above-average SSTs persisted in the east,” according to a May 3 CPC diagnostic discussion.

 

“… oceanic and atmospheric patterns indicate a transition from La Niña to ENSO-neutral conditions. The current and evolving conditions, combined with model forecasts, suggest that La Niña is unlikely to re-develop later this year,” the CPC discussion says.” A majority of models predict ENSO-neutral conditions to continue from April-June (AMJ) through the June-August (JJA) season.

 

“However, at least half of the dynamical models predict development of El Niño conditions by JJA. Still, from JJA onward there is considerable forecast uncertainty as to whether ENSO-neutral or El Niño conditions will prevail, due largely to the inability to predict whether the warmer SST will result in the ocean-atmosphere coupling required for a sustained El Niño event. The official forecast calls for ENSO-neutral conditions through JAS, followed by approximately equal chances of Neutral or El Niño conditions for the remainder of the year,” according to the CPC. El Nino conditions tilt the odds in favor of drier, warmer wintertime conditions in the Pacific Northwest.

 

King, during a Tuesday “webinar” explaining the new forecast, said the available spring-summer water volumes were at this point in time relatively set, regardless of the next few months’ precipitation totals.

 

“There are healthy snowpacks up there already” and the region “doesn’t necessarily need more” in order to get a normal/average water supply.

 

The latest forecast predicts that the Snake River water supply will be about 95 percent of average for January-July. The forecasts take into account runoff to-date, soil moisture content, precipitation, snowpack snow-water equivalents and other factors, including near-term 10-day weather forecasts.

 

The upper Columbia is more amply stocked. The NWRFC forecast for runoff past Grand Coulee for January-July is 115 percent of average, which would be the 11th best total in the past 52 years.

 

In northwest Montana, the NWRFC predicts that Kootenai River flows past Libby Dam will be 121 percent of normal from April-September. The Kootenai ultimately empties into the Columbia in British Columbia.

 

In west-central Idaho, the forecast of for 115 percent of average flows into Dworshak Dam’s reservoir from April-September

 

For more information about the Pacific Northwest’s climate, precipitation and water supply see Climate Impacts Group latest postings at http://cses.washington.edu/cig/fpt/cloutlook.shtml

 

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