La Nina Is Dead

Well, it was nice while it lasted. But all good things must come to an end, and this year’s weak La Niña is no exception. Brie Hawkins of Little Bear Creek Weather mentioned to me this morning that NOAA announced that La Niña was officially over, so I thought I’d write a blog reviewing its impacts 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 the Cascades and Olympics. 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 aloft and lots of snow in the mountains.

The wintertime pattern with year’s La Niña resembled the chart above, with a large ridge in the Pacific and a trough over our area. However, this trough was much sharper than usual, sending the jet stream further south to California and leaving us colder and drier with lowland snow while California got heaps of snow in the mountains. La Niñas tend to make California drier than normal, but this one was a real drought-buster for them.

A look at the current snow-water-equivalent map throughout the Western U.S. shows that the Sierras have been blessed this year by 200% of their normal snowpack. Utah and Colorado have well-above-normal snowpacks as well, and Wyoming is also doing extremely well with the exception of the NE corner of the state which is near 85% of their total snowpack. Oregon is well-above-normal, and Washington is sitting near average even though we have had a relatively dry winter.

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.

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 stills gives us a 15% chance of returning to a La Niña by next September.

Enjoy the sunny weather this week! Today through Tuesday will be the nicest days we’ve seen so far this year if you can break out of the fog at lower elevations. Highs should be in the low 50s all three days for the Portland area, with morning fog and afternoon sunshine.

Thanks for reading,
Charlie