Forecast Models Face Uncertainties, But Line-up With La Nina 

Columbia Basin Bulletin

October 5, 2007  

One phrase -- La Nina -- kindles hope for people across the Columbia/Snake river basin who rely on hydroelectric power, grow crops, manage fish or otherwise rely on a plentiful water supply.

Those people are hearing that phrase often this year as experts note the presence of a surface water cooling trend in the equatorial Pacific, a slipping into La Nina conditions. Such conditions have historically increased the odds that the Northwest will have a cooler, wetter winter.

There's a greater than 40 percent probability for Washington , Oregon , and Northern Idaho , and only a slightly increased probability for southern Idaho , that the October-December period will be wetter than average, according to NOAA National Weather Service's Climate Prediction Center .

Climate researchers warn that long-term forecasting involves many uncertainties.

The year 2001 is "an example of how an ENSO-based forecast can be wrong," said Alan Hamlet of the University of Washington 's Climate Impacts Group. The forecast that year was for a cool phase Pacific Decadal Oscillation and La Nina ENSO conditions. It turned out to be one of the driest years on record.

But the signs look good following a year in which fall/winter precipitation and snowpack fell short of the norm and spring/summer runoff from the Columbia basin's mountains was only about 89 percent of average.

The majority of the El Nino/Southern Oscillation forecast models are now suggesting a moderate to strong "cool" or La Nina event to persist through the winter. That historically has meant the increased likelihood of above average April-September flows at The Dalles the following spring and summer.

Hamlet noted that of the 13 La Nina years that immediately followed a El Nino phase, only one produced below average precipitation in the Northwest.

The other 12 were above average "and most of them were really wet years," he said while acknowledging that the significance of that phenomenon had not been scientifically vetted.

Still, "it's an amazingly large signal," Hamlet said.

A wetter 2007-2008 cold season would help refill a depleted reservoir system and recharge soils drained during a long, dry summer of 2007. The westernmost part of the basin was not so greatly affected. But east of the Cascades temperatures were above normal, with the warmest area being southern Idaho, which experienced its third warmest summer on record (since 1895) with summer temperatures ranging from 3 to 5 degrees F above the 1971-2000 average, according to CIG's Sept. 26 climate update.

Summer precipitation varied across the Northwest with above normal precipitation for the Washington coast and for Oregon 's central and High Plateau regions. But the east slopes of the Washington Cascades and northern and central Idaho received less than 50 percent of normal summer precipitation, according to the update.

A classic La Nina winter would serve to recharge reservoirs and soil moisture, but doing so would sap river flows. Dry initial soil conditions in the basin are expected to reduce natural flows from January through July next year by about 8 percent, according to Hamlet, one of the presenters at the Climate Impact Group's Climate and Forecast Workshop held Tuesday in Olympia.

Overall, streamflows would be "near normal when combining those two things," Hamlet said.

The West-wide Seasonal Hydrologic Forecast System developed at the University of Washington , in its initial forecast for the season, projects April-September streamflows next year at The Dalles to be about average, with outputs about 5 percent above average from the north and 5 percent below average from the east and southeast (the Snake).

ESP (Ensemble Prediction System) water supply forecasts produced by the Northwest River Forecast Center shows much the same trend, the Center's Steve King said during his workshop presentation. A Sept. 23 volume forecast predicts that 102.7 million acre feet of water will flow past The Dalles from January through July, 96 percent of the 30-year average, 107 MAF.

The basic NWRFC ESP modeling uses a physical based modeling system to simulate soil moisture, snow pack, regulation, and streamflow. It then accesses the current hydrologic model states, and uses historical meteorological data to create equally likely sequences of future hydrological conditions to generate probabilistic forecasts of seasonal water supply. It looks at the 10-day forecast and assumes more or less normal conditions beyond that.

This year the NWRFC modelers added an ENSO component which Sept. 23 pushed The Dalles forecast closer to average, 105.4 MAF for the January-July period.

A week of drenching weather pushed the forecast past the average. An Oct. 2 update sets the forecast flow at 108.3 MAF. That forecast takes into account recent rain's effects on soil moisture, snowpack and other variables. It also adds in a new short-term forecast for continued wet weather, King said.

The latest NWRFC climate adjusted ESP forecast pegs January-July flow past the lower Snake River 's Lower Granite Dam at 30.9 MAF. The average is 30 MAF.

The CPC predicts a strong start to the season with above average precipitation across the Northwest through December.

A forecast developed this week by Columbia River Inter-Tribal Fish Commission meteorologist-hydrologist Kyle Dittmer says to expect to higher than normal precipitation through the fall and early winter.

The CRITFC October 2007 forecast for Portland , Hood River , and tribal lands, is near normal temperatures. But precipitation will be 110 to 130 percent of normal in Portland , Hood River and the Warm Springs, Umatilla, Nez Perce, Yakama, Spokane , Coeur d'Alene and Colville reservations.

Dittmer predicts near normal temperatures (-0.5 to +1 deg F) and above normal precipitation (110-130 percent) for November and below normal temperatures (-1 to +0 deg F) and above normal precipitation (90-120 percent).

Dittmer said his latest analysis, using past La Nina years, forecasts runoff at The Dalles (January-July) to be 104 percent of average.

"I see a pattern where the cold and snow will dominate in January and February," he said. "I think that the forecast community is in agreement about La Nina, but it is a question of how La Nina will manifest itself and the distribution of the effects."

The Australian Bureau of Meteorology, which closely monitors surface and subsurface water temperature in the equatorial waters to the north, this week declared La Nina officially established.

"With the exception of the Southern Oscillation Index (SOI), all ENSO indicators showed an intensifying La Niña during September. Computer models forecast the La Niña to last until early 2008, making it almost certain that 2007 will be considered a La Niña year," according to the Bureau's web page.

"Most critically, the near-equatorial Pacific Ocean has continued to cool both on and below the surface, the Trade Winds remain stronger than normal across the western to central Pacific, and cloudiness in the equatorial Pacific is reduced. Together, these indicators suggest the atmosphere and ocean are reinforcing each other; a critical component in sustaining La Niña conditions for any period of time."

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
NOTE: In accordance with Title 17 U.S.C. section 107, any copyrighted
material  herein is distributed without profit or payment to those who have
expressed  a  prior interest in receiving this information for non-profit
research and  educational purposes only. For more information go to: http://www.law.cornell.edu/uscode/17/107.shtml

Source:  http://www.cbbulletin.com/Free/240463.aspx