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Moisture Streaming Into Northwest Gives Columbia Basin ‘Snow/Water Equivalent’ Big Boost
Posted on Friday, January 20, 2012 (PST)

Water supply forecasts, ski hill snow totals and backcountry snowpack have nudged up over the past week with sudden downpouring, after what has been a slow start to the wintertime water accumulation period.

 

Jet streams off the Pacific Ocean have in recent days assumed a more common winter pattern, bringing moisture into the region.

 

Ten days ago, the NOAA Weather Service’s Northwest River Forecast Center predicted that runoff past The Dalles Dam on the lower Columbia River would be 86 percent of the 30-year average (1971-2000) from April through September in 2012.

 

The average is 98.65 million acre feet; the NWRFC “ensemble” forecast issued Jan. 9 – based on light snowpack, particularly in the southern parts of the Columbia/Snake river basin in Oregon and Idaho -- predicted runoff would be 84.71 MAF.

 

But after several days of precipitation across much of the region, and a 10-day forecast of more to come, that forecast has jumped to 92 percent of average, or about 90.97 MAF, according to a water supply forecast issued Thursday, Jan. 19, by the NWRFC. The April-September runoff past The Dalles includes water streaming down from the Snake River basin and the upper Columbia.

 

Snowpack totals have ticked up considerably over the past week. SNOTEL automated monitoring stations operated by the National Resources Conservation Service in central Idaho’s Clearwater and Salmon river drainages had an average “snow/water equivalent” of 82 percent of average on Jan. 20, which was up from 65 percent of average a week earlier. The average for northeast Oregon’s Grande Ronde, Burnt, Powder and Imnaha river drainages jumped from 54 to 70 percent of average from last week to this.

 

The snowpacks feeding the Malheur and Owyhee rivers, which drain into the Snake at the Idaho-Oregon border, jumped from 26 percent of average snow-water equivalent on Jan. 13 to a somewhat less dismal 49 percent this morning (Jan. 20).

 

In south-central Idaho, the snowpack above Palisades Reservoir on the upper Snake increased from 63 percent of average snow-water equivalent to 79 percent over the past week.

 

In the north part of the Columbia River basin snow totals mounted as well over the past week. The Kootenai River basin in northwest Montana jumped from 83 percent of average to 88 percent; Montana’s Flathead River basin eased up from 68 percent of average to 76 percent.

 

In central Washington, the Yakima/Ahtanum basins saw s/w equivalents increase from 80 percent of normal of 90 percent thanks to recent precipitation.

 

The best-stocked are the northernmost portions of the Columbia River basin in British Columbia. The NWRFC’s Jan. 19 runoff forecast into Mica Dam’s reservoir is 111 percent of average. Downstream, Revelstoke Dam is expected to pass 108 percent of its average runoff for the April-September period, according to the latest NWRFC forecast.

 

Meanwhile, the relentless storm battering west of the Cascades is creating havoc and threatening records – and more rain appears on the horizon.

 

The Marys River already is a record stage, according to Kathie Dello of the Oregon Climate Service at Oregon State University. It reached 21.41 feet Thursday morning, breaking the old record of 20.9 feet, and causing flooding in Corvallis and Philomath.

 

“Many streams in western Oregon are now classified as ‘high,’ or above the 90th percentile,” Dello said. “What makes this so unusual is that a few days ago, most of these same streams – especially in southwest Oregon – were near record low levels.

 

"We needed the rain,” she added, “but not all at once.”

 

The Marys River data goes back 72 years for the gauge, according to Gordon Grant, a research hydrologist with the Pacific Northwest Research Station of the U.S. Forest Service. When it floods, he says, it usually creates problems.

 

“Once the Marys gets out of its banks, even a small increase in the flood stage represents a huge volume of water because the floodplain is so flat,” said Grant, who is a courtesy professor of geosciences and forestry at OSU. “The crest of the flood is largely driven by when the precipitation starts to taper off. The rain is projected to begin tapering off this afternoon, but we are still projected to get another 1-2 inches.

 

“So expect to see the Marys continue to rise through much of today,” he added. “Once the rains diminish, we can expect the floodwaters to stick around for a while – a day or more – because the valley and floodplains are quite flat and do not drain quickly.”

 

The rainfall has been significant. On Wednesday, 4.02 inches of rain fell at Hyslop Farm outside of Corvallis – the third highest 24-hour total in the station’s 101-year history. The only other days with more than four inches of rain occurred on Nov. 19, 1996 – when massive flooding hit Oregon – with 4.45 inches; and on Jan. 28, 1965, with 4.28 inches.

 

The Coast Range has been especially hard hit with five to seven inches of rain common in many areas. This is causing flooding of the rivers draining those hills, including the Siuslaw, Marys and Luckiamute rivers.

 

“We’re getting what is called an ‘atmospheric river’ or ‘pineapple express’ event, with warm, wet weather coming from the southwesterly direction,” Dello said. “These tend to be big rain events here in Oregon, though it is still cold enough at higher elevations to snow in the mountains, which was desperately needed.”

 

The U. S. Fish and Wildlife Service announced Thursday the closure of all refuges in the Willamette Valley National Wildlife Refuge Complex due to flooding and high water. This closure includes Ankeny, Baskett Slough and William L. Finley national wildlife refuges. The closure will continue until water recedes and staff and visitors can safely return.

 

 

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