Winter could be a whopper, forecasters say

The most intense La Niña conditions since 1955 are brewing near the equator, raising the odds of a wild winter in the Pacific Northwest.

Meteorologists say more rain, colder temperatures and bigger snowstorms are likely.

"There's the potential for whoppers — but no guarantees," Washington state Climatologist Nick Bond said Thursday at a National Weather Service briefing.

The snow that paralyzed much of the Puget Sound region during the winter of 2008-2009 is one example of what a La Niña pattern can produce.

"La Niña winters are snowy winters," said Brad Colman, National Weather Service meteorologist-in-charge for Seattle. "Skiers and departments of transportation should be paying attention."

The flip side of the more-famous El Niño pattern, La Niñas occur when the ocean near the equator becomes colder than usual. Current temperatures are the coldest for this time of year since the Eisenhower administration.

Computer models predict the pattern will continue, and possibly strengthen, throughout the season, Colman said.

Ocean temperatures affect air circulation. Those patterns in turn tweak the strength and location of the jet stream that brings the Northwest much of its weather.

Generally, La Niña winters start out wet, with fairly average temperatures, said University of Washington meteorologist Cliff Mass. By January, temperatures plunge and snowfall increases.

"We could end up with a snow-free Thanksgiving, but no one should think that means anything," Mass said. "We tend to have the big snow buildup after the new year, both in the mountains and the lowlands."

Floods are possible, particularly early in the season.

The La Niña year of 2007 brought drenching rains to southwest Washington, closing Interstate 5 for several days. But though wetter overall, the majority of La Niña years are not marked by major floods, said National Weather Service hydrologist Brent Bower. That's because the powerful storms that trigger most Northwest floods are more common during so-called neutral years — when there's neither a La Niña nor El Niño.

The risk of flooding in the Green River basin is much lower this year, thanks to improvements at Howard Hanson dam, said Larry Schick, meteorologist for the U.S. Army Corps of Engineers. Seepage from the dam raised fears last winter, but installation of a grout curtain within the weakened embankment has bolstered the structure. Improvements to a drainage tunnel that are expected to be finished by next fall should lower the risk even more, Schick said.

"We're not out of the woods completely ... , but the situation has improved."

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