Oregon Chapter
American Meteorological Society

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5/3/2011
Winter came on strong at the end
Late-season snows rebuilt a snowpack that had gotten a little light in the middle of the season. photo by Ron Thorkildson
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Late-season snows rebuilt a snowpack that had gotten a little light in the middle of the season. photo by Ron Thorkildson

By Ron Thorkildson


Early last fall weather forecasters across the region were eyeing changes that were taking place in the tropical Pacific Ocean. A strengthening easterly trade wind was pushing warm surface water westward, allowing colder and deeper water to rise to the surface in eastern sections of the Pacific.

A potent La Niña was brewing that local meteorologists and climatologists believed would impact our upcoming winter.

At the American Meteorological Society's winter weather meeting, held in October at the Oregon Museum of Science and Industry in Portland, all the forecasters delivered the same message; the Pacific Northwest was headed for a cooler and wetter than normal winter.

The Central Oregon Cascades began building a snowpack during the third week in November as an early cold snap dropped the temperature in Sisters to -9 degrees Fahrenheit on Thanksgiving morning. December temperatures were near normal, but twice as much precipitation fell than the month usually sees. As 2010 drew to a close, the earlier forecasts concerning our winter seemed spot-on.

Then something happened.

When the year changed from 2010 to 2011, the weather changed, too - and abruptly. A ridge aloft built over the western U.S. that shunted incoming Pacific storms far to the north into British Columbia and southeastern Alaska. OK, these things do happen occasionally, even during a La Niña event. We'll be back to our usual stormy pattern in no time, right? Wrong!

Not counting the first four days of the month, January's average temperature was about seven degrees above normal. And except for some warm rainfall midmonth that melted away the high desert's modest accumulation of snow, it was dry.

The protective ridge persisted far longer than anyone could have guessed. So what went wrong?

The El Niño/La Niña phenomenon is a periodic climate pattern that occurs across the tropical Pacific Ocean. But they aren't the only players in the tropics.

In 1971, while examining wind flow patterns over the Indian and tropical Pacific oceans, researchers Roland Madden and Paul Julian discovered an eastward progression of enhanced tropical rainfall. Known as the Madden-Julian Oscillation, or MJO, these zones of greater rainfall begin in the Indian Ocean along the equator, move eastward into the western Pacific and usually die out in the eastern Pacific some distance from the equator. These weather-altering journeys usually take 30 to 60 days to complete. Some atmospheric scientists believe that when it is near the end of its cycle, the MJO may affect weather patterns along the U.S. west coast.

Chris Karafotias, operational weather and streamflow forecaster for the Bonneville Power Administration in Portland, has considerable knowledge of the MJO and incorporates it in his forecasts. He thinks the MJO probably played a role in the six-week dominance of the ridge that tempered what could have been a more severe winter.

The upper level ridge finally broke down in mid February, allowing Pacific storms to once again sweep across the Pacific Northwest. And at this writing they haven't stopped coming. The mountains now have plenty of snow. According to Marilyn Lohmann, hydrologist for the National Weather Service office in Pendleton, the snowpack in the Central Oregon Cascades is between 130 to 140 percent of normal at elevations above 5,000 feet.

The long-range forecasters get high marks for correctly predicting our cool, wet winter. One wonders, though, what might have happened had La Niña not wandered off during the dead of winter, perhaps in search of a warmer clime.

Original article link at NuggetNews.com