Well, it was nice while it lasted. But all good things must come to an end, and this year’s weak La Niña was no exception. NOAA officially announced that La Niña ended early last February, meaning that the La Niña of 2016-2017 only lasted four months. Though this was one of the weakest and shortest La Niñas on record, it still managed to have quite an impact on our weather this winter.
La Niña is name for the periodic cooling of water temperatures in the central and eastern Tropical Pacific due to an increase in the strength of the trade winds and a resulting increase in upwelling off the coast of Peru, which is where cool, acidic, nutrient-rich water from the deep rises to the surface. La Niña has a variety of effects on weather around the world, but here in the Pacific Northwest, it is known for giving cooler-than-average winter temperatures and slightly above average winter precipitation, resulting in heavier-than-average snowfalls for both the mountains and the lowlands. This is because during La Niñas, a ridge of high pressure often forms in the eastern Pacific with a trough over our region, giving us cool and moist northwesterly flow.
The wintertime pattern with this year’s La Niña was similar to what we would expect, with a large ridge in the Pacific and a trough over our area. However, this trough was much deeper than usual, sending the jet stream further south to California and leaving us cold and relatively dry here in the Pacific Northwest.
La Niñas tend to make California drier than normal, but this one was a real drought-buster for them. The Sierras have been blessed with over 200% of their normal snowpack for this time of year, Mammoth Mountain plans to stay open until the 4th of July after recording an incredible 20 feet of snow in January alone, the North Sierra is on track for their wettest season EVER, and extended forecasts from the Climate Prediction Center keep the West Coast cooler and wetter than normal for the next month. Nearly every reservoir in California is above their average level for this time of the year, and these levels will only increase this spring as the snow begins to melt in the Sierras. This was the type of winter California needed to end their multi-year drought.
But it isn’t just California that has seen gobs of snow – the vast majority of the Western U.S. has well-above-average snowpack for this time of the year. Utah, Colorado, and Wyoming are all doing extremely well, Oregon is well-above normal, and even Washington is sitting near average despite the fact that we’ve had a relatively dry winter with the exception of a few warm and wet “Pineapple Express” events this past February.
Most models keep us in neutral conditions through autumn, though some develop a weak El Niño later in 2017. A return to La Niña conditions next winter looks unlikely, but the Climate Prediction Center still gives us a 15% chance of returning to a La Niña by next September.
~ Charlie Phillips
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.us.