Water supply forecasts for next summer have been pushed down due to the combination of depleted soils in much of the Columbia River basin and the prospect of El Nino conditions holding sway across the Northwest over the next nine months.

Forecasters' numbers vary to some degree but all come to the same conclusion, said the University of Washington's Alan Hamlet during Tuesday's Climate and Water Resources workshop in Seattle.

Hamlet is a UW research scientist and member of the Climate Impacts Group, which periodically holds workshops and meetings to provide natural resource managers, Northwest policymakers and other interested persons up-to-date information on the impacts of climate variability and climate change on the region.

CIG's fall forecast meeting provides an opportunity to hear how forecasted climate conditions for winter 2009-2010 may affect regional climate and streamflow conditions in the Columbia River basin and western Washington and Oregon in summer 2010. The meeting also provides a forum for sharing the latest research on climate impacts and climate forecasting.

A second fall forecast meeting is planned Oct. 22 in Boise. For more information about CIG and the workshops, go to:

http://cses.washington.edu/cig/

A below average 2008-2009 snowpack accumulation resulted in below average runoff down through the Columbia-Snake river system this spring and summer. And a hot, dry summer served to create drought conditions in some areas. All of the Northwest, except for southern Idaho is now "abnormally dry" going into the wet season, according to a U.S. drought monitor posted online by the NOAA's Climate Prediction Center.

Those sapped soils are waiting to gulp a bigger-than-normal share of the coming season's precipitation, which could turn out to also be below average.

The CPC's mid-September forecast for October through December says there is "a slight tilting of the odds towards drier and warmer conditions" in the Northwest, CIG's Eric Salathe told workshop participants.

Much of that forecast is based on weak El Niño conditions typified by warmer than normal sea surface temperatures in the tropical Pacific. Such conditions can affect weather worldwide and tend to result in less precipitation and higher temperatures in the Northwest.

"It's been flat lately," Salathe said of equatorial anomolies that first emerged in June. "Right now it's not showing any sign of intensification but it's expected to do so.

History has shown that El Nino does not go away when signals linger this late into the season. Most forecasters expected the development of a weak-to-moderate strength El Niño through the fall with the likelihood of at least a moderate strength El Niño this winter.

"We don't really expect a reversal," Hamlet said.

The effect most strongly felt in the Pacific Northwest is usually the precipitation variable whereas temperature is the variable influenced most by El Nino across the rest of the U.S. northern tier, Salathe said. El Ninos often deflect to the north and south jet streams that normally pepper the Northwest with snow and rain during the winter.

A UW streamflow forecast for the next nine months "shows basically below normal conditions," according to researcher Francisco Munoz-Arriola. The forecast predicts Columbia River flows, as measured at The Dalles Dam, will be from 8 to 12 percent below normal.

NOAA's Northwest River Forecast Center used its Ensemble Prediction System modeling (ESP) to produce a forecast of 2010 streamflows that is also below the average. The model simulates soil moisture, snowpack, streamflow and regulation. For the forecast presented Wednesday it also considers CPC predictions for the winter that hinge on El Nino conditions.

The NWRFC's ESP forecast is for January-July streamflows past The Dalles Dam on the lower Columbia at 91 percent of average, 91 percent at the mid-Columbia's Grand Coulee Dam and 90 percent of average at the lower Snake River's Lower Granite Dam.

Researchers stressed that the long-range forecasts being made now are very early, and made with little to go on except current conditions and the specter of El Nino. The outcome during the winter of 2008-2009 was "was significantly different than we forecast last year," the NWRFC's Steve King said.

Most forecasts were for a relatively normal winter. But the outcome was a streamflow at The Dalles Dam that was only 84 percent of the 1971-2000 average. Grand Coulee flows were 79 percent of normal; runoff past Lower Granite 96 percent of average.

This winter's weather depends in part on how El Nino manifests itself.

The CPC's October-November-December temperature forecast is for a greater than 33 percent chance of above normal temperatures in Oregon, Washington and Idaho, which would be a continuation of the warmer than normal conditions observed this summer. The 30 days ending Sept. 28 saw temperature departures west of the Cascades of less than 4 degrees F above the 30-year mean, while temperatures over eastern Oregon and most of Idaho were in excess of 4 degrees F above the mean.

Precipitation during the 30-day period ending Sept. 28 was below normal precipitation over Oregon, Idaho and southern and eastern Washington, according to CIG.

The CPC forecast also includes increased probability of above normal temperatures for a broader region of the western contiguous United States and major portions of Alaska. The precipitation forecast for the same period is for a greater than 33 percent chance of below normal precipitation in all but the easternmost portions of Washington and Oregon, with the probability exceeding 40 percent in the western half of Washington and Oregon.

Winter seasonal climate prediction is more skillful in years of significant El Nino/La Nino episodes like the once expected to develop this year, according to CIG.

The seasonal forecasts should be interpreted as the tilting of odds towards general categories of conditions, and should not be viewed as a guarantee that the specified conditions will be realized, according to CIG.