'Bubble of warm air' deflects precipitation north into Canada

By MATEUSZ PERKOWSKI

Capital Press

Snowpack levels across the Northwest are well below average at the end of 2011, but the outlook for irrigators seems likely to improve in the new year.

Experts say the weather pattern that kept the region largely free of storms in late autumn probably won't hold up in wintertime.

"The good news is our storm track is starting to change," said Jon Lea, snow survey supervisor for the USDA's Natural Resources Conservation Service.

A ridge of high pressure -- basically a "bubble of warm air" -- had deflected storms northward into Canada, said Pete Parsons, meteorologist at the Oregon Department of Forestry.

As a result, by late December snowpack levels were more than 50 percent below average in Oregon, more than 40 percent below average in Idaho and about 20 percent below average in Washington, according to NRCS.

"It just won't let those storms come in," said Scott Pattee, NRCS water supply specialist for Washington.

The Pacific Ocean is currently subject to La Niña, a recurrent climate cycle typically associated with lower temperatures in the Northwest.

High pressure ridges can form during such cycles but typically don't last, giving the region's overall snowpack time to build up, Parsons said.

With wind and rain returning in late December, it appears the ridge is shifting eastward, opening the way for more precipitation across the region, Lea said.

"We're hoping La Niña finally comes in and blows away this ridge of high pressure that's been blocking storms," said Ron Abramovich, NRCS water supply specialist for Idaho.

Statistics also provide some reason for optimism.

Historically, roughly 60 percent of the region's snowpack accumulates after Jan. 1, Lea said. "We certainly can catch up."

Leftover water supplies are promising as well. NRCS data indicate that reservoir levels in Oregon, Washington and Idaho were generally average or higher at the end of the 2011 irrigation season.

The cool, wet spring resulted in late snow melt and reduced the demand for irrigation, Abramovich said. "They didn't use very much reservoir water last year."

Water levels in key California reservoirs are also average or above, but the state's snowpack and rainfall was 30 percent below average as of early December, according to the state's Department of Water Resources.

Newer data aren't yet available, but the snowpack and precipitation levels are probably even more below average right now, said Frank Gehrke, chief of California's Cooperative Snow Surveys Program.

Gehrke said he's hopeful the snowpack outlook will improve by late winter.

"It's still very early in the season," he said.

Unlike the Northwest, California's snowpack levels traditionally do not benefit from La Niña cycles -- the trend is toward drier winter weather in that state, Parsons said.

"There's definitely a disconnect there," he said.

That may have significance in the long term.

Since 2007, it appears the Pacific Ocean has been experiencing a longer-term shift known as Pacific Decadal Oscillation, in which La Niña climate cycles become more common over 30 years or so while El Niño climate cycles become less common, Parsons said.

Typically, El Niño is associated with milder winters in the Northwest and La Niña with harsher ones, with the opposite effect in California, he said.

Parsons' long range forecast for the Northwest in early 2012 calls for below normal temperatures and above average precipitation through March, with cooler conditions persisting into spring.

Though cool springs are more likely under the current phase of Pacific Decadal Oscillation, he said it's unlikely the region will experience a spring quite as cold and wet as in 2011.

"That was an extreme anomaly," Parsons said. "Odds are that's not going to happen every spring. It's not the new norm."