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Basin Water Managers Expecting Slightly Below Average Runoff From Now To September
Posted on Friday, June 17, 2016 (PST)

Water managers in the Columbia Basin are expecting a slightly below average runoff in the months leading to September, but so far it has been gradual, mostly due to cooler temperatures.


It is regarded as a welcome contrast to last year, when there was a rapid, early runoff due warm temperatures.


Things will surely heat up this summer, however, and the latest forecasts were calling for those conditions to develop within 10 days, members of the multi-agency Technical Management Team were told Wednesday as part of the June water supply forecast.


The runoff for combined basins upstream from The Dalles Dam are projected to be about 86 percent of the historic average going into September at the dam.


Runoff forecasts vary in upstream Columbia Basin waterways. At Montana’s Libby Dam the runoff is expected to be 110 percent of average going into September.


At Dworshak Dam, the runoff is projected to be 86 percent of average. Runoff above the Columbia River’s Grand Coulee Dam is expected to be 100 percent of average through September.


TMT participants noted that water temperatures, rather than flows, will soon be drawing attention, because of temperature influences on migrating salmon.


“The good news right now is that temperatures are pretty low on the Snake and Clearwater” rivers, one participant said, noting that a warming trend could get underway in the next 10 days.


Water temperatures throughout much of the Columbia Basin are above historic averages for this time of year, but still considerably lower than temperatures at this time last year.


Meanwhile, the NOAA Climate Prediction Center says that La Nina conditions have developed in the Pacific Ocean, prompting predictions for the West Coast and most of the rest of the country throughout summer.


The latest analysis of powerful oceanic influences in the Northern Hemisphere, issued June 9 by the Climate Prediction Center in College Park, MD., says that the El Nino weather pattern dissipated during the spring and signs of La Nina influences have been taking shape.


In addition to the expected warmer-than-average trend, higher-than-average precipitation is expected in parts of the Rockies and the northern Plains as a result of La Nina weather patterns coming off the Pacific Ocean.


Some of those influences may already have been taking shape. For example, record rainfall was recorded in parts of western Montana during the month of May. On the west side of Glacier National Park, which feeds the easternmost headwaters of the Columbia Basin, 6.4 inches of rain fell during the month, a new record since the National Weather Service started keeping track in 1948.


The average rainfall for May in the same area, by contrast, is 2.64 inches.


NOAA says there is about a 75 percent chance of La Nina conditions to persist throughout the coming fall and winter.


As of early May, the National Weather Service recorded precipitation measurements that were 101-113 percent of average in the broader Columbia Basin. Snowpack runoff was 114-142 percent of average in May, largely due to a rapid depletion of snowpack throughout the region due to temperatures.


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