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January-July Early Water Supply Forecast: Will It Be First Above Average Runoff Since 2006?
Posted on Thursday, December 23, 2010 (PST)

It’s very early yet, but the water accumulation season is off to a strong start due to a wet autumn that soaked soils across the Columbia River basin and built mountain snowpacks that are, for the most part, above average in terms of moisture content for this time of the year.


The initial water supply forecast produced this season by the National Weather Service’s Northwest River Forecast Center predicts that water funneled down from the Snake and upper Columbia river basins from January through July will total 111 million acre feet as measured at The Dalles Dam on the lower Columbia River. That would be 103 percent of the average for the period 1971 through 2000 and, if the forecast comes true, would be the first above average runoff year since 2006.


The mid-month forecast issued Dec. 16 is based on observed precipitation through the Dec. 13 and precipitation assumed to be 110 percent for the rest of December and normal for remainder of the period.


For the mid-month forecasts only about half the precipitation reports used in the monthly final forecasts are available. There are no updated snow or runoff values used directly in the model input but all available snow water equivalent reports are used subjectively by the forecaster during the review of forecast results, according to the NWRFC. For each month from January through July the center issues mid-month, early bird and final forecasts.


The strength of the mid-month forecast is in the Snake River basin. Runoff past Lower Granite Dam on the lower Snake from January through July is forecast to be 109 percent of normal at 32.8 MAF.


The December mid-month forecast for the upper Columbia -- through what is roughly one-third of the precipitation season -- is 102 percent of average or 64.1 MAF. The forecast is weakest north of the border with estimates of January-July water supply at 95, 94 and 90 percent respectively for Micah, Arrow Lakes and Birchbank in British Columbia.


Some of the other December mid-month forecasts include Libby (northwest Montana) inflows at 101 percent of average; Lake Pend Oreille (north Idaho) inflows at 109 percent, North Fork of the Clearwater inflows to Dworshak reservoir in west central Idaho at 109 percent and the Yakima River at Parker in central Washington at 112 percent.


Year-to-date precipitation totals for each of the 23 Columbia basin reporting areas were above average, according to the Natural Resources Conservation Services Columbia River Basin SNOTEL/precipitation Update Report through Dec. 21.


And the measurement of snow-water equivalent in the snowpack is generally above average, with SNOTEL electronic measurements ranging from 145 percent of average (Raft, Goose, Salmon Falls, Bruneau) to 109 percent of average (Weiser, Payette, Boise) through that date for streams draining into the upper Snake River. The Owyhee/Malheur snowpack has a SWE of 132 percent of normal through Dec. 21.


Idaho’s Clearwater/Salmon SNOTEL stations weigh in at 97 percent of average SWE through Dec. 21 and the Grand Ronde/Imnaha/Burnt/Powder is at 93 percent of average.


The only other below average SWE readings were the Bitterroot (95 percent) in northwest Montana, Lower Clark Fork River Basin (99), the Idaho Panhandle (86), the Columbia above the Methow (95) in north-central Washington, the Chelan/Entiat/Wenatchee (88) in central Washington and the Yakima/Ahtanum (95).


Snowpacks might have been more robust if not for a few warm, wet periods that served to diminish lower level snowpacks in some areas, according to Rick van der Zweep, a NWRFC hydrologist.


But, he said, much winter is yet to come so forecasts can change drastically, upward or downward.


“With the December mid-month you have huge error bounds,” van der Zweep said. The f’recasts' level of certainty improves, generally with each passing month.


A Dec. 9 “El Nino/Southern Oscillation” update by the National Weather Service’s Climate Prediction Center says that during November 2010, the ongoing La Niña, which began early this summer, was reflected by below-average sea surface temperatures across the equatorial Pacific Ocean. The subsurface oceanic heat content (average temperatures in the upper 300m of the ocean) also remained well below-average in association with a shallower-than-average thermocline across the central and eastern equatorial Pacific. Enhanced low-level easterly trade winds and anomalous upper-level westerly winds continued over the equatorial Pacific.


“Collectively, these oceanic and atmospheric anomalies reflect a moderate-to-strong La Niña,” the update says. Impacts from such climatic conditions in the United States include an enhanced chance of above-average precipitation in the Pacific Northwest.


The CPC’s three-month “outlook” issued Dec. 16 forecasts a greater than 40 percent chance of above normal precipitation throughout Washington and Montana, across northern Oregon in central and northern Idaho. That forecast for January-March sees an equal chance of below-, near-, or above-normal precipitation in southern Idaho and Oregon.


La Niña is expected to peak during November-January and to continue into the spring of 2011, according to the CPC update.


“Thereafter, the fate of La Niña is more uncertain,” the update says. Most forecasts indicate a return to ENSO-neutral conditions during the spring and early summer, but a few “suggest that La Niña could persist into the summer. Historically, there are more multi-year La Niña episodes than El Niño episodes, but other than support from a few model runs, there is no consensus for a multi-year La Niña at this time.”


A late spring drenching and the strong start to the new water year have all but eliminated drought conditions in the Northwest except for a small patch in south-central Oregon, according to the federal U.S. Drought Monitor’s Dec. 14 update. That area in Oregon had shrunk by more than half than portrayed in a Nov. 16 Drought Monitor map, and drought areas in southeast and southwest Idaho were wiped off the map by recent precipitation.


Since Oct. 1 the mid- and upper Snake regions have been pelted. The Snake River basin above southeast Washington’s Ice Harbor Dam has absorbed 136 percent of its normal precipitation and is at 174 percent of average for the first 20 days in December.


The Columbia River basin above Grand Coulee in central Washington is at 98 percent of average since the water year began Oct. 1 but has received 119 percent of its average precipitation for the Dec. 1-20 period, according to data compiled by the NWRFC. Holding down the upper Columbia average is the fact that the Columbia River above Castlegar, British Columbia, has only received 75 percent of average precipitation since Oct. 1 and only 80 percent in December.


“Most of the storms have pretty much hit from the Canadian border south” so far this fall-winter, van der Zweep said.


But the interior Columbia-Snake basin (everything above The Dalles Dam) has been wet overall with 137 percent of average precipitation in December and 112 percent of average so far in the water year.


Some of the high water marks so far this year have been 160 percent of average on the Snake River plain and 141 percent in the upper Snake in Idaho, 191 percent in the Owyhee/Malheur river basins in the southeast Oregon-southwest Idaho area and 142 percent of average in the Upper Deschutes-Crooked river basins, 133 percent in the Hood/lower Deschutes area and144 percent in the Upper John Day basin in central Oregon.


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