Early Season Forecasts Expect 2008 Water Volumes Near Average 

Columbia Basin Bulletin

December 20, 2007  

Despite a relatively light early-season snowpack across the Columbia River basin , water supply forecasters are predicting that the region's streams will provide near or above average volumes for fish, hydro generation, irrigation and other uses next spring and summer.

That technically derived faith is fortified by fall precipitation that has soaked soils and blanketed the highest reserves with snow, and by continuing signals that La Nina conditions will prevail throughout the winter season.

A Dec. 13 "mid-month" water supply forecast issued by the NOAA Weather Service's Northwest River Forecast Center predicts an outcome with 104 million acre feet gushing past the lower Columbia's The Dalles Dam from January through July, 97 percent of the recent 30-year average.

That initial forecast mirrors what happened last winter season, strong input from the north (a prediction of 102 percent of average as measured at central Washington's Grand Coulee Dam) and subpar water supplies from the east (a predicted volume at 93 percent of average past the lower Snake River's Lower Granite dam next year).

The mid-month forecasts are admittedly cursory, taking into account a short-term assessment of future precipitation. No updated snow or runoff values are used in the statistical modeling. They are intended to show trends in water supply volumes. It included observed precipitation through Dec. 10 and assumed normal precipitation for the rest of the month.

A "final" monthly forecast will be issued after Christmas that involves a more thorough analysis and pools efforts of the NWRFC and the Natural Resources Conservation Service. The statistical regression modeling inputs include snow water equivalent, monthly precipitation and in some cases previous streamflow volumes.

An "ensemble prediction system" forecast issued Tuesday by the NWRFC predicts a wetter future, water volumes in April through September at 106 percent of average at The Dalles, 111 percent at Lower Granite and 104 percent at Grand Coulee. ESP modeling method utilizes a physical based modeling system to simulate soil moisture, snow pack, regulation, and stream flow. The ESP accesses current hydrologic status such as moisture content, and uses historical meteorological data to create equally likely sequences of future hydrological conditions.

A separate modeling technique developed by the Columbia River Inter-Tribal Fish Commission's Kyle Dittmer is predicting the January-through-July volume to be 119.5 MAF, about 111 percent of average at The Dalles . The hydrologist-meteorologist uses sunspot counts, the Multivariable ENSO Index, and 20 analog water years to generate expected trends in temperature and precipitation.

Dittmer's modeling analyzes the most recent three month's climatic data and uses the La Nina/El Nino index to predict a most likely outcome, matching patterns witnessed during those previous years.

NOAA's Climate Prediction Center in a three-month outlook issued today says that a moderately strong cold event or La Nina continues to prevail. Those below normal surface water temperatures in the tropical Pacific can affect weather around the world. They tend to make more likely a cooler, wetter winter in the Northwest.

Columbia Basin weather, particularly of late, fits the typical La Nina pattern, above average precipitation in the northern portions of the basin and lesser amounts to the south, Dittmer said. But that's where winter snowpack accumulation really counts, unless you're relying on Snake River water.

"If the Upper Columbia is above average, you'll definitely get above average" runoff overall, Dittmer said. On average about two thirds of the basin's water supply comes from the upper part of the basin in British Columbia and Montana .

The early water supply forecasts are subject to considerable change. As an example, the series of storms bringing precipitation across the basin this past week could serve to lift the NWRFC monthly final forecast. The mid-month assumed average precipitation. The NWRFC ESP forecasts are updated weekly as conditions change. Its early bird, mid-month and final regression forecasts are updated monthly.

Snowpacks now are "a small percentage of what they will be in April" on average, according the NWRFC's Rick van der Zweep.

"I'm seeing moisture between now and the end of the year," Dittmer said of the near term.

The basin's snowpack remains below average in terms of snow-water equivalent, but has surged in recent days.

The snowpack in Idaho's Snake River basin above Palisades Reservoir jumped from 69 percent of average on Dec. 10 to 76 percent as measured this morning via the NRCS's SNO-TEL system. The Weiser, Payette, Boise river drainage snowpack increased from 71 percent of average to 84 percent over the past 10 days.

To the north, the Kootenai snowpack increased from 74 to 86 percent over that time frame and the Flathead's from 65 to 71. Oregon 's Willamette River snowpack increased from 43 to 74 percent of average; the Grande Ronde's from 77 to 90 percent.

That trend figures to continue through Christmas.

"This is actually looking pretty juicy," Dittmer said of storms predicted to invade the Northwest over the next several days.

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