Our latest string of storms during mid-late September made me more excited than ever for the beginning of storm season. There’s nothing I love more than a good Pacific Northwest lowland snowstorm, and windstorms, mountain snow, and heavy rain aren’t far behind. So when I caught wind on September 14th that the Climate Prediction Center had issued a La Niña watch, I was overjoyed at the news. La Niñas tend to bring us cooler and wetter-than-normal winters, with significantly more snow in the mountains and a greater-than-normal chance of snow in the lowlands.
According to the Climate Prediction Center’s latest El Niño/Southern Oscillation (ENSO) Diagnostic Discussion, sea-surface temperatures in the central and eastern Equatorial Pacific have decreased from slightly above-average to slightly below-average over the past couple months. Additionally, both low-level and upper-level trade winds are above average (albeit in different places), convection is enhanced over Indonesia and suppressed over the central/western tropical Pacific, and ocean temperatures have decreased below the surface. These are all things we’d expect as we transition into a La Niña in the Tropical Pacific.
Models are split on whether our current pattern (which, while showing La Niña characteristics, is not strong enough to be considered a La Niña) will transition into a full-fledged La Niña over the next several months. But because the atmosphere is currently in and continues to trend toward a La Niña pattern, the forecasters at the Climate Prediction Center give a 55-65% chance of La Niña forming during the winter compared to a 35-45% chance for Neutral conditions. If I had to guess, I’d say our most likely outcome for this winter is a weak La Niña developing in October/November and persisting through February.
La Niña’s propensity for cooler and wetter-than-average Pacific Northwest winters is due to their tendency to create a persistent ridge of high pressure in the NE Pacific, pushing the jet stream north into Alaska where it can tap into a cooler air mass before coming back over the ocean and sending storms into our area.
Typical wintertime pattern during a La Niña year, Credit: Climate Prediction Center
That being said, there is more year-to-year variability during weak La Niña (and El Niño) years than strong ones. So while the chances are in our favor for a snowier-than-average winter in the mountains and lowlands, I wouldn’t go placing any bets on it. One thing is for sure: any La Niña year is far better than an El Niño year, which squashes the hopes and dreams of Pacific Northwest schoolchildren and skiers alike!
Sizable (though not record-breaking) flooding is also more frequent during La Niña years, as are strong windstorms. Our very strongest windstorms have all occurred during Neutral years, however.
It’s worth mentioning that last year was also a weak La Niña, and last winter was pretty darn epic (particularly in the snow and forecast bust departments). If this winter is even half as exciting as last winter, consider me satisfied. But there’s nothing I’d love more than for the winter of 2017-2018 to put last year’s winter to shame.
Charlie Phillips a Madrona resident, received his B.S. in atmospheric sciences from the University of Washington and works in Portland as a meteorologist forecasting wind energy along the Columbia River Gorge. Check out his weather website at weathertogether.net.