La Niña can be 'tough nut to crack'

Lowland snow in November?

Skiing before Thanksgiving?

With one of the strongest La Niña patterns in more than 50 years in place, Northwesterners may be wondering if this will be a winter for the record books.

There's no way to answer that question now, but the region's weather history offers some perspective and a few hints.

The last time a La Niña got off to such an intense, early start was 1955. Single-digit temperatures in November shattered records. By the end of the year, Sea-Tac airport had logged more than 10 inches of snow.

"We keep hearing 1955 come up," said Kirby Cook, science officer for the National Weather Service in Seattle.

But the winter of 1955-56 was neither the coldest nor the snowiest. It wasn't even close.

Which shows that the intensity of a La Niña pattern isn't a perfect predictor of a harsh season, Cook pointed out.

"You can have a wishy-washy impact here, even with a very strong La Niña," he said. Some of the region's most bitter winters have come during middling Las Niñas.

"La Niña is a bit of a tough nut to crack in terms of how it's going to pan out here," Cook said.

What is clear is that, on average, La Niña years tend to be wetter, snowier and colder in the Pacific Northwest. The most frigid temperatures generally kick in after the first of the year.

This season is following the playbook so far.

"The pattern we have now is very La Niña-esque," said University of Washington meteorologist Cliff Mass.

La Niña occur when stronger-than-normal trade winds shove aside warm, equatorial water, allowing cooler water to surge to the surface. The pattern is reversed during an El Niño, when water along the equator is warmer than usual.

Scientists gauge the strength of a La Niña by measuring water temperatures. Right now, the eastern Pacific is about 3.6 degrees F cooler than normal. "That's pretty strong," said state climatologist Nick Bond. "It's a solid bet that there's going to be a La Niña persisting through the winter."

Changes in ocean temperatures lead to changes in air circulation that can affect weather thousands of miles away. During La Niña, high-pressure systems often form off the coast of Alaska, diverting a branch of the jet stream north and funneling cold air from Canada into Washington. That's what turned rain to snow this week.

But despite the winter's impressive start, there's no guarantee this La Niña will have a doozy of an impact on the Pacific Northwest.

The La Niña winter of 1949-50 was the coldest and snowiest of the 20th century in Washington and Oregon. The low at Sea-Tac airport hit zero degree F on Jan. 31. More than 20 inches of snow fell in a single day in Seattle.

But Seattle's all-time snowfall record was set in a non-La Niña winter: 67.5 inches in 1968-69.

In the late 1800s, the Columbia River sometimes froze for weeks at a time. In 1861, Seattle's Lake Union was covered with up to 6 inches of ice, according to Mass' book "The Weather of the Pacific Northwest."

La Niña or not, winters like that seem to be a thing of the past, Bond said.

Whether as a result of man-made climate change, natural shifts or a combination of both, severe cold spells are less common and less intense.

That doesn't mean a brutal winter is impossible — just less likely than in past decades.

"The baseline has changed," Bond said.

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