Weakening La Niña Likely Means Stormy Northwest Weather Next Winter | KNKX

Weakening La Niña Likely Means Stormy Northwest Weather Next Winter

Apr 6, 2018

‘April showers bring May flowers,’ or so the saying goes. Like Groundhog Day or dozens of sayings you can find cataloged in a Farmer’s Almanac, it’s part of the weather-related folklore that’s been passed down for generations. They hold some truth.

But most meteorologists prefer more scientific observations. For long-term forecasting, the ENSO - or "El Niño Southern Oscillation" - provides some of the most reliable guidance on what kind of weather to expect in winter.

It’s based on the sea surface temperatures measured in the central and eastern tropical Pacific Ocean. El Niño years are when those temperatures are warmer than average. La Niña is when they’re below average and La Nada, or neutral, is when they are close to average.

The ENSO predictions for the following winter become most reliable in the summer months. But by springtime, the guidance starts shaping up.

Right now, we’re in the midst of a La Niña pattern.

‘Classic La Niña in 2017-18

“This winter was a classic La Niña year. After the New Year, we started seeing more precipitation, colder temperatures. The snow pack here in the northwest zoomed up – in northern California as well,” says KNKX weather expert Cliff Mass. “Classic La Niña.”

But Mass, who is a professor of atmospheric sciences at the University of Washington in Seattle, says it looks like that is beginning the shift.

“The latest predictions by the climate prediction center models [a NOAA entity] is that we’re going to see La Niña weakening out during the next few months,” he said. That means the temperatures in the tropical Pacific will warm up to their normal or nearly average level, leading to neutral or La Nada conditions.

“So, by the time we get to June, I think La Niña is going to be toast,” Mass said.”

La Nada = Stormier Weather In The Northwest

While La Niña years bring some of the wettest and coldest winter weather, Mass says neutral years are the ones in which the strongest atmospheric rivers form and the most extreme weather takes place.

“Now it’s not a perfect predictor, but we tend to have the most extreme storms: the biggest snowstorms, the biggest floods, the biggest windstorms,” said Mass.  

“They’re kind of rare, but when they do happen, they do have a tendency to happen in the neutral years. And so we have to be ready for next winter.”

ENSO Causes Changes In Storm Track

Mass says the relationship between the sea surface temperatures and the weather that hits the northwest is not just an observable correlation. There’s also an underlying cause.

“It turns out that when you have unusual temperatures in the tropical pacific, that changes where thunderstorms are in the tropic,” Mass says. “And that in turn changes how energy propagates from the tropics into the mid-latitudes. It changes the circulation in the mid-latitudes.”

“And so when we have an El Niño year, which we had two years ago, then we have storms tending to go into California, we tend to be a bit drier,” Mass says, referring to the Northwest.

We’re also usually a bit warmer, and there is less snow.  

“So the storm track is changed,” said Mass.

And when the temperatures in the tropical Pacific are cooler than normal, the storm track hits us more.

“During La Niña years, we tend to get this ridge of high pressure off shore, that tends to bring cooler air into the Northwest and therefore more snow,” Mass says. “So, it’s both a correlation and it’s causative as well.”

A Relatively New Tool – With UW Connection

Long-term weather prediction is still an imperfect science; truly reliable forecasts rarely extend beyond a week or so. But the ENSO does give valuable general guidance for the next season’s weather that is widely used. And it has only been part of the common vernacular since the mid-1980s.

“There was a strong El Niño in 1982 that really surprised people. We had all this rain in California and people didn’t understand the connection,” he said.

“But during the 80s there was a lot of work done – a lot of it at the University of Washington by Mike Wallace and his associates – that actually figured out this El Niño La Niña business and how it correlates with mid-latitude weather.”

Mass says since then, meteorologists and government agencies and others have used the ENSO to get an idea of what the next season is going to be like.

“It’s an imperfect tool – maybe it explains like 30 percent of the variability. But it’s the only tool we have for looking into the future more than a few weeks.”

To hear the full conversation, you can click on the 'play' icon at the top of this post. 

Weather with Cliff Mass airs at 9:02 a.m. Friday, right after BirdNote, and twice on Friday afternoons during All Things Considered. The feature is hosted by KNKX environment reporter Bellamy Pailthorp. Cliff Mass is a University of Washington professor of atmospheric sciences, a renowned Seattle weather prognosticator, and a popular weather blogger. You can also subscribe to podcasts of Weather with Cliff Mass shows, via iTunes or Google Play.