BELLINGHAM — We’ve known for a while that the upcoming winter was expected to see the effects of La Niña. But as the calendar turns to December, the weather pattern still hasn’t emerged.
“We’re expecting a La Niña to emerge in the Tropical Pacific,” Karin Bumbaco, a University of Washington climate scientist who serves as the state’s deputy climatologist, said in a phone call with McClatchy. “It is a little later than usual. Usually by this point in the late fall, if we’re expecting a La Niña or an El Niño, then it should already be in place. So it’s a little slower than usual, which does give me pause.”
La Niña, the term used to describe the cyclical cooling of the Tropical Pacific, is associated with certain weather patterns across the world.
“So we’re looking at that region, sea surface, temperature anomalous, and we’re trying to figure out if it’s going to hit a certain threshold or not,” National Atmospheric and Oceanic Institute scientist Michelle L’Heureux told McClatchy in late October.
L’Heureux leads NOAA’s team on the The El Niño-Southern Oscillation, the catch-all name for the ocean temperature cycle in the Tropical Pacific that causes to La Niña and its warmer counterpart, El Niño.
“The El Niño-Southern Oscillation dynamics rest on these ocean waves traveling back and forth across the entire Tropical Pacific. It just takes a long time to do that,” L’Heureux said.
La Niña events typically occur every three to five years and can affect weather across the globe. According to L’Heureux, the wide-ranging effects are a result of the size of the Pacific Ocean as much as anything else.
“It’s because the Tropical Pacific Ocean is really big,” L’Heureux said. “It’s such an anti-climatic answer. But seriously, even though these little warming and coolings exist in other basins, like the Indian Ocean and the Atlantic Ocean, they don’t have these huge knock-on impacts, in part because they’re just not big enough to persist. The reason the Pacific is special and these temperature and precipitation anomalies can persist so long is because the ocean moves slow, and the basin is big.”
- How will La Niña affect WA this winter?
While Washington is already known for its gray winters, La Niña years tend to bring more rain and cooler temperatures than usual to the state, according to L’Heureux.
“So when we look at our impacts on our Washington weather, we look at averages over seasons,” L’Heureux said. “On average, our winters tend to be a little bit cooler, so we expect below normal temperatures this winter. They tend to be a little bit on the wet side, so a little wetter than our usual wet…I would say about 60% to 80% of the time, you get temperature and precipitation of that sign.”
NOAA breaks its forecasts into three categories for each area: above average, around average and below average. NOAA’s nationwide winter forecast favors those outcomes over the other options, although that doesn’t necessarily mean they’re more likely to occur than not.
“In the temperature category, it’s about 40% to 50% chance of below,” L’Heureux said. “And then for [above average] precipitation, it’s about 33% to 40%.”
Bumbaco said that Washington is expected to see more snow than usual this winter as well.
“What I’m excited about for this year is the chance of having an above normal snow pack by the end of our snow accumulation season in the mountains. The other piece that would affect people more is that we do tend to have a higher likelihood of getting lowland snow during La Niña years,” Bumbaco said.
- What are the chances La Niña emerges?
According to a recent NOAA report, surface temperatures at Niño 3.4, the area of the Pacific used to measure El Niño and La Niña events, were 0.3 degrees Celsius below average in mid-November. In order to meet the criteria for La Niña, monthly surface temperatures have to be 0.5 degrees below typical levels, with those conditions expected to persist for five consecutive three-month rolling averages as well. The August-September-October average temperature was 0.22 degrees Celsius below normal.
That’s a stark drop from the 0.11 degrees below normal temperatures measures across July, August and September, suggesting that the three month average ending in November could be even lower when it’s released.
Currently, Bumbaco says there’s a 75% chance that La Niña emerges this winter, even with the delay.
“There’s still about a one in four chance that we could remain neutral,” Bumbaco said. “But obviously that means there’s three in four chances that it will be La Niña.”
A mid-November NOAA report put the odds that it emerges by December at 57%. If it does emerge, its effects aren’t expected to be as strong or long-lasting as normal.
- But what happens if La Niña doesn’t emerge?
According to L’Heureux, it wouldn’t make too much of a difference.
“Despite that, the model forecasts are surprisingly La Niña-like around the globe, which is interesting to us, it’s almost suggesting an offset between the confidence and the impacts and whether we’re actually going to hit those thresholds or not,” L’Heureux said.
While surface temperatures are how NOAA measures La Niña events, L’Heureux said that there are plenty of other factors at play that could cause La Niña-like effects if the temperature criteria are met.
“What’s interesting about the Tropical Pacific is that even though we have these thresholds for sea surface temperatures, it really is the overall pattern across the Tropical Pacific that we’re interested in, and whether those sea surface temperatures cause changes in pressure and rainfall, winds and all these things happening in the South Pacific… So we may still see some of these La Niña structures emerge, even if we don’t hit our requirements on the nose for La Niña,” L’Heureux said.
L’Heureux also pointed out that even if ocean temperatures reach La Niña levels, they have to do so for months at a time to be considered a La Niña event. As a result, we could see the effects of La Niña regardless of whether or not La Niña emerges.
“We also want to see five of these overlapping seasons. So we look at September-October-November, October-November-December, and we want five of these to string up. But you know, I do tell people, even if we don’t hit five — like, say we hit four — it doesn’t mean that we’re not finding the impacts,” L’Heureux said. “But with that said, we don’t want to hand weave our forecast. We do want to give our best shot here and have some stringent criteria.”
Bumbaco had a different take on the topic, citing the relatively low threshold for La Niña — surface temperatures of 0.5 degrees Celsius below the long-term average.
“If it doesn’t meet that threshold, I wouldn’t blame anything on La Niña. The threshold is actually pretty low,” Bumbaco said.
- When would La Niña hit Washington?
Had La Niña emerged at its usual time, Bumbaco said that Washington still wouldn’t be seeing any effects yet.
“Even if we were to see the La Niña develop by this time, we see the impacts on our weather really after January 1. It takes a little time for that kind of atmospheric pattern to shift and start to see the impact on our weather,” Bumbaco said.
NOAA’s forecasts also suggest that the first weather effects in Washington could be seen in January. According to L’Heureux, the first stretch expected to see La Niña-like three-month averages in Washington will be December, January and February.
“The first season that the below average temperature signal starts is in December-January-February,” L’Heureux said. “And I think the last season is March-April-May. For precipitation, it actually ends earlier. It’s more like February-March-April.”
