Winter forecasters: Were they right?

Last November, I wrote about the Oregon Chapter of the American Meteorological Society's annual Winter Weather Meeting, held at the Oregon Museum of Science and Industry in Portland. At that event a group of meteorologists and climatologists shared their opinions about the kind of weather that Old Man Winter might dish out to the Pacific Northwest, and Oregon in particular, in 2008-09.

Well, winter's over now and it's time to render a verdict on how they did. First, a brief forecast review is in order.

Each of the four prognosticators agreed that the normal ocean temperatures in the tropical Pacific last fall should result in weather patterns that typically occur when neither El Niño nor La Niña is in effect. This neutral condition often produces temperatures and precipitation close to normal values over the course of a winter. According to Steve Todd, head of the National Weather Service office in Portland, noticeable periods of variability frequently occur during this neutral phase. Todd added, "...and we also tend to have some of our bigger Valley snow events."

Pete Parsons, meteorologist for the Oregon Department of Agriculture, said that the odds of a severe cold snap throughout the Pacific Northwest go up during a neutral winter. Kyle Dittmer, hydrologist and meteorologist for the Columbia River Inter-Tribal Fish Commission, foresaw a cool and wet start to winter, with conditions warming and drying slightly as the season progressed. Finally, George Taylor, of Applied Climate Services and former state climatologist, said "winter will come early and leave early this year."

What actually happened?

The season was indeed dominated by sharply defined "spells" of weather. For most of November through the early part of December, the broad-scale weather pattern over our region featured an upper-level ridge that diverted Pacific storms northward into British Columbia. (Ridges usually bring warm and dry weather; troughs cool and wet.) This produced above-normal temperatures and very little precipitation. From November 7-13, however, the ridge was weak enough to allow heavy rain and flooding to occur in parts of Western Washington.

Then, in December, the ridge migrated westward into the eastern Pacific and amplified, producing a deep trough over the West Coast. This allowed modified arctic air from the Yukon to be drawn southward into Washington, Oregon and even Northern California.

The leading edge of the frigid air arrived in Central Oregon late on December 12 and quickly turned a light rain to snow. On December 16, Redmond reported a temperature of -12 degrees F, establishing a new low for the date. One day later Sisters recorded a low temperature of -13 degrees F. Snow depths of from one-and-a-half to two feet were common in the Sisters area. The cold and snow persisted through the first week in January.

Remarkably, Portland accumulated almost two feet of snow during this period, making it the third snowiest winter there since 1950!

As the second week in January was drawing to a close, the pattern flipped again. A strong ridge aloft once again settled over the region with its accompanying warming temperatures and scant precipitation. Freezing levels of 10,000 to 12,000 feet were not unusual during this time.

What happened next? You guessed it. By the end of the first week in February another trough appeared. Only this time temperatures weren't as cold and snowfall wasn't as deep.

Based on Sisters weather data, temperatures were slightly colder and wetter than average during the four-month period December through March. The snow pack in the Central Oregon Cascades is about 100 to 120 percent of normal according to Marilyn Lohmann, hydrologist at the National Weather Service office in Pendleton, thanks to frequent precipitation and low snow levels in March.

Our winter definitely did not come early. In fact, it was a bit late. And maybe the season was a touch cooler and wetter than expected. But on the whole the weather wizards were quite accurate and earn a B-plus in my grade book.

Ron Thorkildson is a retired meteorologist.