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Basin’s April-Sept Runoff Forecast Now At 90 Percent; Rosiest Scenario Only Gets It To Normal
Posted on Friday, January 13, 2012 (PST)

What a difference a day makes.


On Monday the Northwest River Forecast Center issued a 2012 water volume runoff forecast for April-September as measured at the lower Columbia River’s The Dalles Dam that figured to be 86 percent of average due to relatively paltry precipitation totals across the basin, and particularly in the south.


On Tuesday, thanks to an improved 10-day weather forecast that includes more expected precipitation, that forecast jumped to 90 percent of the 30-year average.


Still, after a dry start to the water accumulation season, odds are slimming with each dry day that the Columbia River basin will have available a full (average) water supply during the spring-summer months, according to senior hydrologist Steve King of the federal Weather Forecast Center’s Northwest River Forecast Center.


A positive, though brief, start to the water/snow accumulation season in the Pacific Northwest has degenerated (unless you are a sun seeker) into a fair weather episode. High pressure ridges off the coast in recent weeks have served to deflect storms that would normally soak and cover the states of Idaho, Montana, Oregon and Washington in midwinter.


“Conditions changed dramatically in November and through December,” King said during a Tuesday “webinar” briefing on the Columbia River basin’s water supply situation. A Northwest map that was all green and blue in October (indicating normal to above normal precipitation) -- except for parched south and southwestern Oregon --  turned to yellow, orange and red (all below average) during November and December.


Precipitation totals in December, usually one of the region’s wettest months, were 49 percent of average in the upper part of the Columbia River basin upstream of central Washington’s Grand Coulee Dam and only 47 percent of normal above northwest Washington’s Ice Harbor Dam on the lower Snake River. The upper Columbia drains western British Columbia as well north Idaho and western Montana. The Snake’s origins are in Idaho, as well as Wyoming and Nevada.


“It’s pretty darn dry,” particularly the upper Snake River region and the southern reaches of the Columbia River basin, King said. As of Thursday morning, the snow-water equivalent in the Owyhee and Malheur river drainages that feed the lower Snake were at 24 percent of average for that date in time, according to SNO-TEL electronic monitoring stations maintained by the National Resources Conservation Service.


The Raft, Goose, Bruneau and Salmon Falls snowpacks in southern Idaho and Wyoming averaged 39 percent of normal and Idaho’s Weiser, Payette and Boise drainages had snow-water equivalents at 56 percent of normal through Jan. 11.


“It’s drier as you go south,” King said of current snowpack conditions. “There’s very low numbers in western Oregon and the Snake.


The picture is considerably rosier in the northern end of the Columbia River drainage. The Kootenai River in Montana had snow-water equivalents at 84 percent of normal at midweek, the Flathead River weighed in at 68 percent of normal, and the Idaho panhandle was at 74 percent.


British Columbia’s mountains are, relatively, flush. The streams that flow into Mica Dam’s reservoir in the northernmost reaches of the Columbia basin are now predicted to produce 105 percent of average runoff from April through September.


High pressure off the coast helped keep precipitation out of the Pacific Northwest for much of the month of December, though that pattern did break briefly during the last week of the month. Warm weather that had built aloft helped bring unseasonably warm temperatures to the region during that period.


While a predicted relocation of the jet stream in the next week may bring better chances for snow, a persistent dry weather pattern across Montana has shifted water users’ expectations from a repeat of last year’s record snowpack and flooding to the possibility of average or even below-average stream flows in 2012.


“It is still fairly early in the snow season,” said Gov. Brian Schweitzer. “As of now, we are below normal for snowpack.”


At the midpoint of the water year, snowpack for Montana’s major river basins is about 80 percent of normal. In just the past week, the state has lost from 2-8 percent of the snow-water equivalent from around the state as the rate of new snowfall in the mountains failed to keep pace with the average historic rate of accumulation.


Stream flow forecasts issued Jan. 5 by the NRCS are a bit more optimistic, with a Montana projection of 86 percent of average for the coming water-use season. Last year at this time, snowpack levels were 112 percent of average.


The dry mid-winter weather strikes in the face of what have been, and continue to be, “La Nina” conditions, which tend to tilt Northwest winter weather patterns toward the cool, wet side. During December 2011, below-average sea surface temperatures (associated with La Niña continued across the eastern and central equatorial Pacific Ocean, according to a Jan. 4 update by NOAA Weather Service’s Climate Prediction Center.


“A majority of models predict a weak or moderate strength La Niña to peak during the December-February season, and then to continue into early Northern Hemisphere spring season before dissipating during the March to May period,” the most recent CPC “La Nina Advisory” says.


But those conditions bring no guarantee of plentiful wintertime precipitation in the Northwest. Similar starts to the winter, in terms of precipitation from Oct. 1 through December, occurred in 2003, 2005 and 2010, and all three were low runoff years” at 82, 76 and 79 percent average, King said.


Even stronger La Nina conditions prevailed last winter, which got off to a strong start, dozed in later January and February, and then broke loose with near record precipitation across the Columbia River basin in late winter and spring.


The result was near record snowpack and water supply. The runoff past The Dalles on the lower Columbia was 132.9 MAF from April through September in 2011, which was the fourth largest total in the past 52 years, according to the NWRFC.


The most recent NWRFC “ensemble” forecast is for runoff down from the Snake and upper Columbia river basins of 88.8 MAF, which would be 90 percent of the recent 30-year average and ranked as the 33rd most in the past 52 years.


With the winter just starting, the water supply situation could well improve during January-April. NOAA’s Weather Service predicts the odds are for above average precipitation across the region during the next three months.


According to the most recent NWRFC forecast the rosiest scenario would nudge runoff totals just barely up to normal. While La Nina condition do tilt the odds towards wetter winters in the Pacific Northwest, oft times the result can be a dry year, King said.


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