It’s looking increasingly likely the U.S. will see a La Nina winter this year. Is that good news or bad news? Well, it’s complicated news.
As of Thursday, NOAA’s Climate Prediction Center says there’s a 70% chance of La Nina returning between November 2021 and January 2022. (We actually had a La Nina winter last year, too. Right now we’re in a “normal state.”)
So what does that mean for winter weather?
“It really depends where you are in the United States. It will vary by location,” explains NOAA climate scientist Michelle D’Heureux. “The southern tier during a La Nina is often drier than average during the winter, and that often extends into spring.”
That southern tier includes Southern California, the southwestern states, Texas, and the Gulf Coast states through to Florida. For those areas, there’s a 40% chance of below-average precipitation, D’Heureux said.
“You might think 40% doesn’t seem that large, but remember we have three possible outcomes: below average, near average and above average. It’s not a coin flip. There are three potential outcomes. That means randomly you can get 33%, so a 40% chance really does mean we favor below-average conditions.”
If you got lost in the probability there – and you couldn’t be blamed if you did – D’Heureux is basically saying the most likely outcome for the drought-plagued southwestern states is more drought this winter.
“La Nina doesn’t mean anything absolutely,” she said. “Just saying there’s going to be a La Nina doesn’t mean it’s going to be dry and drought. We have certainly had La Nina winters where, lo and behold, there’s more precipitation than expected. That’s just less common.”
So La Nina could mean bad news for the southwest, but the opposite is actually true for Northern California and the Pacific Northwest, where La Nina winters tend to bring more precipitation, not less.
If that happens, there could be consequences for the Midwest and parts of the South later in the winter, specifically Ohio and the Tennessee Valley.
D’Heureux said to think of the atmospheric river as a body of water, but in the sky.
“It’s like throwing a rock into a pond – it skips and has a ripple effect. The atmosphere behaves similarly. There’s waves in the atmosphere. That’s one of the reasons Ohio and Tennessee Valley have these ripples later in the winter.”
How La Nina affects the northeast is the biggest mystery, D’Heureux said. It’s pretty much equal chances of a wet winter and a dry winter. Only time will tell exactly how La Nina plays out — something that makes D’Heureux’s job challenging.
“Climate is not as predictable as the weather,” she lamented.