El Niño should mean drier Seattle winter, but maybe not

The odds are Seattle won't get snowed in again this winter.

But when it comes to El Niño, it's not always smart to play the odds, say weather experts gathering in Seattle today to discuss the upcoming season.

In moderate El Niño years, as this is shaping up to be, Washington's winters usually are slightly warmer and drier, said Ed O'Lenic, of the National Weather Service's Climate Predication Center in Maryland.

Severe floods are less likely, as is the lowland snow that paralyzed the Puget Sound region last December.

But average trends aren't guarantees — as anyone who remembers the winter of 2006 will testify.

That was also an El Niño year, but Seattle set rainfall records and floods swept away an entire campground in Mount Rainier National Park.

"We have had El Niño years with more snow, more precipitation," said University of Washington meteorologist Cliff Mass. "It's not a perfect correlation at all."

Reduced rainfall would be welcome along the Green River this year, where residents face possible floods from a leaking dam upstream. The status of the Howard Hanson Dam also is on the agenda for today's annual Northwest Winter Weather Outlook workshop at the National Weather Service's Seattle headquarters.

Storms last year damaged an abutment on the earthen dam southeast of Seattle in the Cascade foothills, and the U.S. Army Corps of Engineers has warned it may be forced to release more water than usual this winter to reduce stress on the structure. That could mean flooding in parts of the Green River Valley, including Kent, Auburn and Tukwila.

Because there is so much uncertainty, the El Niño forecast doesn't really change that picture, said Corps spokeswoman Casondra Brewster.

"We can't count on Mother Nature to be calm," she said. "We need to be prepared for any contingency."

El Niños develop when the Pacific Ocean becomes unusually warm along the equator, said O'Lenic, who traveled to Seattle to help lead today's workshop. Right now, the warmth is slight. Computer models predict it will strengthen to moderate levels this winter.

The unusually warm seawater heats the atmosphere and causes changes in circulation patterns that reach around the globe. The result can be drought, floods and even ice storms, such as the one that knocked out power to much of the East Coast during the winter of 1997-98.

The pattern recurs every two to seven years and can persist as long as two years.

During an El Niño, the jet stream that normally shuttles rain to the Northwest often splits, with the storms being funneled north to Alaska and south to California.

On average, winter temperatures in the Northwest are a degree or two warmer during an El Niño. Overall precipitation is about 10 percent less than normal.

The biggest impact is to snow, Mass said.

In the Puget Sound lowlands, a degree or two can mean the difference between snow and rain. And warmer temperatures in the mountains mean less overall snowpack.

"The most dramatic aspect will be less snowpack in the mountains and much less chance of lowland snow than last year," Mass said.

The strongest regional El Niño effects usually don't kick in until January or later.

Ski resorts are wary of the forecast. The El Niño winter of 2004-2005 was one of the worst on record for snowfall.

But other El Niño seasons haven't been bad, said Tiana Enger, marketing director for Crystal Mountain ski area. In the El Niño year of 2006, the resort received nearly 100 inches more snow than usual.

"The ball could bounce in many different directions at this point," she said.

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com