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Winter Forecast Conference: Below Normal Temperatures, Above Average Precipitation?
Posted on Friday, November 04, 2011 (PST)

Each used a different combination of tools, climate indices and calculations, but all five meteorologists offering forecasts during a conference Oct. 29 in Portland agreed that “La Nina” could well influence what sort of upcoming winter the Northwest and other parts of the globe will experience.


Four of the five predicted that the winter of 2011-2012 will be wetter than normal, though none predicted a repeat of 2010-11 when stronger La Nina signals prevailed. Snowpacks across the Pacific Northwest reached near record levels when a cool, wet late winter and spring settled on the region.


The forecast presenters were among more than 350 people gathered last Saturday for the 19th Annual Winter Forecast Conference sponsored by the Oregon Chapter of the American Meteorological Society. The session was held at the Oregon Museum of Science and Industry.


La Nina conditions are occurring and expected to gradually strengthen and continue this winter, according to regularly updated forecasts from National Weather Service’s Climate Prediction Center. Those La Nina conditions, which include cooler than normal sea surface temperatures across must of the equatorial Pacific, seem to increase the likelihood that the Northwest will be colder and wetter than normal.


This winter CIG suggests that there are significantly increased odds of above average precipitation; odds favoring near normal or cooler than normal temperatures and that there are significantly increased odds of an above average snowpack, according to Dave Elson, lead forecaster for the Weather Service’s Portland office.


“La Nina, that’s really what drives this forecast,” Elson said. The agency says the region can “expect an active weather pattern this winter.”


Columbia River Inter-Tribal Fish Commission hydrologist/meteorologist Kyle Dittmer used a host of tools, including indices such as the El Nino/Southern Oscillation and the Multi-variate ENSO Index, and the Pacific Decadal Oscillation, Sea-surface temperature departure forecasts, and comparisons of past years’ La Nina outcomes (analog years), including water year volume forecasts. And he even factors in sunspot cycles (though they are currently in a relatively neutral mode), which he says can influence global weather patterns.


He predicts the Columbia-Snake river basin will be “slightly below normal temperature-wise for the season” and should see above average precipitation.


Dittmer said the region can expect some variability, which might include heavy rain events west of the Cascades, flooding, arctic blasts and high wind events. And rainy Portland should expect a few snow events from December to early March, he said.


His water supply forecast for January-July 2012 as measured at the lower Columbia’s The Dalles Dam is 117 million acre feet, or 109 percent of the recent 30-year average. In October 2010 Dittmer predicted that 2011 runoff would be 129 MAF, 120 percent of normal, and that early season forecast was in the ballpark. The observed, unregulated runoff was 141.7 MAF from January through July.


Former Oregon State Climatologist George Taylor, as well as Pete Parsons and Jim Little, likened the pattern being experienced now to one that began in the winter of 2007-2008 played out in 2008-2009. That pattern saw La Nina conditions building in the 2007-2008, lapsing to neutral conditions during the late spring-summer, then rebuilding the following winter.


“That’s about as good of a match as you can get,” Parson said of the strategy of identifying start-of-year conditions from the past that might apply to the current year’s forecast. The winter of 2008-2009 started out strong with an early season dump of snow even into lower level sites such as Portland but ended up being a relatively average precipitation year overall. Parsons and Little do forecasts for the Oregon Department of Forestry.


Taylor said he expected November to be relatively benign with a transition in December into cooler, wetter conditions. He forecast December through February to be “very active” with above-average mountain snowfall for the winter; significant precipitation totals with possibility of flooding in the west, cooler than average temperatures and a good chance of low elevation snow, especially in January.


Parsons’ forecast followed the same track with above average temperatures early and weather turning stormy in December and January, followed by a cold February.


“Cold periods will have a better chance of being accompanied by snow this winter. Most mountain snowpacks should be above normal by late January-February,” Parsons’ forecast said.


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