CORVALLIS, Ore. - So far this December, Portland has been drier than Tucson, Ariz., Corvallis has been as cold as Buffalo, N.Y., and central Oregon has experienced less precipitation than it did in July.
Despite the presence of a second consecutive La Niña event, this has been anything but a normal end to the year, according to analysts at the Oregon Climate Service at Oregon State University.
"When all is said and done, Oregon's precipitation in 2011 may not be all that unusual," said Kathie Dello, deputy director of the Oregon Climate Service, "but how we got to where we're at has been a little bit different. We had the second wettest spring on record in Oregon this year, but things have really dried out this fall and we didn't have our first 90-degree day until September.
"However, last winter we had a La Niña as well, and a lot of people got concerned because it was drier than normal in January and February," she added. "But then things really kicked into high gear with spring rain; so it's a little early to panic."
The reason for the recent spate of cool, dry weather - atypical for La Niña years - has been a high-pressure system that has plopped down over the Pacific Northwest and diverted all of the storm systems to the north and south, Dello said.
The clear, calm conditions have made most Oregon nights rather cold and there is enough moisture in the air to create foggy conditions - especially in the Willamette Valley from Eugene to Portland. The lack of wind and other weather activity, coupled with the sun being at its lowest point, has kept the fog from dissipating and created an air inversion. When that happens, fog settles into the valley up and down the Interstate-5 corridor and lowers the temperature several degrees.
"On some days in the last few weeks, it was warmer at Timberline Lodge on Mount Hood than it was in Portland or Salem," Dello said.
Portland has had only .14 inches of rain December, and the storms that have diverted to the north and south have carried our precipitation to other regions. Tucson, Ariz., in contrast, has had more than two inches of rain this month.
Portland is not alone. Bend/Redmond has had only a trace of rain in December, even less than the 0.6 inches the region got in July.
The National Oceanic and Atmospheric Administration forecasts that La Niña conditions should persist through the end of winter, but what that means remains to be seen, Dello pointed out.
"La Niña can tilt the weather toward being cooler and wetter, but all La Niña events are not created equally," she said. "There was a La Niña in 2008 during which Portland got almost 20 inches of snowfall in December, the La Niña of last year was different, and this year has started off unusually dry for any December, regardless of a La Niña event.
"Last year it was drier than normal in January and February before getting really wet in the spring, and that could happen again," she added, "but no one knows for sure."
Dello said parts of Oregon may see rain this Christmas weekend and the Willamette Valley could be hit with freezing fog, which creates some of the most dangerous driving conditions each winter. Freezing fog occurs when the water droplets are below freezing, but don't actually form ice until they come into contact with a solid surface - like roads or windshields.
The Oregon Climate Service has a variety of weather- and climate-related links on its website and offers historical data for Oregon: http://ocs.oregonstate.edu/
The OSU program also has a Twitter account (twitter.com/orclimatesvc) where Dello updates followers with data on record temperatures, extreme conditions, and anniversaries of unusual events, like last fall's tornado in Aumsville.
Click photos to see a full-size version. Right click and save image to download.
Kathie Dello, 541-737-8927