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Despite Recent Dry Days, January Precip Pushes Basin Water Supply Forecast To 101 Percent Of Normal
Posted on Friday, January 28, 2011 (PST)

A wet start to January helped to push the Columbia River basin’s water supply forecast to slightly above average, despite barren skies that have dominated much of the region in recent days.


The Northwest River Forecast Center’s “early bird” forecast issued Thursday predicts that the runoff volume past the lower Columbia’s The Dalles Dam from January through July will be 101 percent of normal, which is four percentage points higher than the Jan 7 “final” forecast.


The forecast for the April through August and April through September periods is 98 percent of normal, which is up one point since the Jan. 7 forecast. That’s a reflection of the fact that recent warm, rain-on-snow weather served to push a big chunk of the runoff volume downstream already.


The new forecast input includes observed precipitation through the Jan. 24 and assumed 80 percent of normal precipitation for the next two weeks and 100 percent of normal after that. It also considered updated snow totals. The averages are based on the 1971-2000 period.


Since the start of the water year, and in January, most areas of the basin have received above average precipitation, perhaps thanks to La Nina conditions that have been in place in the equatorial Pacific since this past summer. La Nina climatic conditions seem to tip the odds towards wetter and warmer than normal winters in the Northwest.


The Columbia-Snake river basin above The Dalles Dam received 140 percent of its average precipitation Jan. 1-24 and 114 percent of normal from Oct. 1 through Jan. 24.


The Snake basin above Ice Harbor on the lower Snake received 108 percent of its normal precipitation Jan. 1-24 but 126 percent of average since Oct. 1.


The upper Columbia started slow but got a soaking in January. The precipitation in the Columbia basin above Grand Coulee Dam in central Washington receive 165 percent of its average precipitation during those first 24 days of January and is at 109 percent for the season. The uppermost part of the Columbia basin, above Castlegar in British Columbia, got 150 percent of its average precipitation early this month but is at 93 percent of average for the season.


The January precipitation north of the border did help lift water supply forecasts there. The NWRFC prediction for Canadian measuring stations jumped from the 90-93 percent of average runoff range in the Jan. 7 final to the 94-96 percent range in the early bird forecast released this week.


“The upper elevations got a 10-20 percent increase in snow-water equivalent” during the early January deluge, the NWRFC’s Rick van der Zweep said of the Canadian snowpacks.


The Jan. 27 water supply forecast for Grand Coulee is 97 percent of average, up from 94 percent in the Jan. 7 estimate.


The inflows to Libby Dam’s reservoir in northwest Montana are now expected to be 98 percent of normal, up from the 93-percent prediction three weeks earlier. Montana’s Flathead basin is now expected to deliver 106 percent of its average annual runoff.


The upper Snake snowpack building process started strong, but did not benefit quite as much as the upper Columbia from early January’s inclement weather. The Jan. 7 expectations were for a water volume past Lower Granite Dam on the lower Snake at 110 percent of average; that dipped to 109 percent in this week’s forecast.


The dry spell over the past several days has pushed down snowpack SWE, as a percentage of average to-date, across much of the basin. As an example the combined central Oregon Deschutes-Crooked-John Day basin SWE dropped from106 percent of average through Jan. 15 to 88 percent Jan. 27, according to SNOTEL data posted online by the National Resources Conservation Service.


The Lower Columbia-Hood readings were the lowest in the basin at 68 percent of average as of Jan. 27, down from 101 percent on Jan. 15. The Willamette SWE is at 73 percent of average, down from 97 percent.


In central Washington, the Yakima-Ahtanum SWE fell from 96 to 82 percent and the Chelan-Entiat-Methow dropped from 83 to 76 percent of normal from Jan. 15 to Jan. 27.


Most of the upper Snake SNOTEL readings dropped a few points too because of the recent window in what is supposed to be the snow accumulation season. The exception is the SWE station that feeds Palisades in south-central Idaho, which jumped from 114 percent of average Jan. 15 to 121 percent on Jan. 27.


Central Idaho did OK too. The SWE was in the Clearwater-Salmon drainages was 104 percent of long-term average through Jan. 27, up from 101 percent as of Jan. 15.


Most parts of the upper Columbia saw a jump in SWE during that recent 12-day period, including the Flathead, upper Clark Fork, Bitterroot and Columbia (below the border) upstream of the Methow River basin. The exception there was the Kootenai River basin in Montana, which saw its SWE decrease from 109 to 107 percent of normal from Jan. 15 to Jan. 27.


The rain and snow will likely begin again, but few are predicting that to happen in a big way anytime soon. An outlook released Thursday by the National Weather Service’s Climate Prediction Center says the odds are against even average precipitation across most of the basin over the next 8-14 days.


But the CPC says that La Nina conditions remain entrenched. Its February-April forecast says the odds are good that most of eastern Oregon and Washington, all of Idaho except for the southernmost tier and all of Montana will experience above normal precipitation.


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