Scott Hammers / The Bulletin

Despite a pleasant last day, April 2011 will go into the books as one of the coldest Aprils in Central Oregon and around the Northwest in years.

National Weather Service stations around the region put the average temperature during April significantly below the historical average (between 1971 and 2000, the 30-year period on which that figure is based). In Redmond, April was actually colder than March, with average temperatures 6.3 degrees below normal.

Steve Pierce, vice president of the Oregon Chapter of the American Meteorology Society, said April in Portland was the coldest in 36 years, and while he hasn’t gone through all the data for Central Oregon, he expects it will look much the same.

“Six degrees below normal at Redmond — I would be shocked if there was another April that cold probably since the mid-’70s,” Pierce said. “1974, ’75 (there) was a very strong La Niña, and that was the last time a lot of cities in the Northwest set record lows.”

The La Niña phenomenon, which alternates with El Niño every five years or so, is driven by water temperatures in the eastern Pacific Ocean. During La Niña years, cold waters in this region push the jet stream north, bringing lower temperatures and increased moisture to the Northwest.

During El Niño years, warmer waters pull the jet stream south, bringing higher-than-normal precipitation to California.

This year’s La Niña, the primary driver of weather conditions in the Pacific Northwest, was the strongest since the winter of 1955-56, Pierce said.

At the Redmond Airport, which provides the most comprehensive records of any Central Oregon weather station, April saw just one record low — 15 degrees on April 22, tying the previous record set in 1972. But on 10 different days, the average temperature was at least 10 degrees below the historical normal.

What’s ahead?

The good news, Pierce said, is the current La Niña cycle appears to be coming to a close, suggesting a warmer spring next year.

“There have only been a few La Niñas or El Niños that have carried over more than two seasons,” he said. “We’re already at season two now, and it’s starting to decay now. It’s unlikely we would see the return of another La Niña returning next winter.”

Rob Brooks, forecaster with the National Weather Service office in Pendleton, said the steady flow of small storm systems throughout April also contributed to the cool temperatures. Successive days of clear and sunny conditions were few, Brooks said, preventing the region from warming up faster than clear nights between storms could cool it off.

Brooks said today’s forecast anticipates temperatures in the low to mid-60s in most of Central Oregon. Conditions should continue to improve through most of the week, with temperatures in the 70s in Bend for the first time this year Thursday, Brooks said — followed by a chance of showers Friday.

For a full weekly weather report, see Page B6.