La Nina Bids Farewell

Within a few weeks, La “Nina” will be replaced by her cousin, La “Nada,” which is the moniker we’ve lovingly given to ENSO-Neutral Conditions in the Tropical Pacific, where we are neither in a La Nina or an El Nino state. “ENSO” simply refers to “El Nino Southern Oscillation” and is the broad term that describes the oscillation between El Nino, Neutral, and La Nina conditions in the Tropical Pacific.

While La Nina’s effects are most pronounced after January 1, La Ninas exert some influence on large-scale atmospheric patterns earlier in the winter, and December and the first half of January featured uncharacteristically calm weather for a La Nina winter over our area as large, persistent ridges of high pressure repeatedly formed over the Pacific Northwest. La Ninas are typically characterized by an active, northwesterly storm track that keeps us cooler and wetter than normal, delivering copious amount of snow to the mountains.

Though these ridges were interspersed by wet systems that brought moderate amounts of snow to the Washington Cascades/Olympics, these systems dropped very little snowfall to areas further to the south. I went skiing on Mt. Hood during late January and encountered all sorts of early-season hazards on many of the runs. The Southern Oregon Cascades were in even worse shape, with many spots barely having 20% of their normal snowpack by mid-January. The Sierra Nevada were worse still, but  is common for them to receive poor snow totals during La Nina years due to the tendency for the storm track to be slightly further north throughout the course of the winter.

However, the 2017-2018 La Nina made up for lost time later in January and February, and though March was relatively calm, the first half of April was extremely cold and wet, with copious amounts of snow in the mountains. As of mid-April, every single region in Washington and much of Northern Oregon had above-average snowpack.

This year’s La Nina was never too strong in the first place, but the most recent SST measurements from the Tropical Pacific show how much La Nina has weakened over the past 12 weeks. Back in early February, we had a well-defined line of cooler-than-normal sea-surface temperatures along the equator, but we now have a much less-defined, smaller, and less anomalous region of cooler SST to the south of the equator. SSTs are currently just above the threshold for an official “La Nina” designation, and I expect we’ll officially transition into Neutral conditions by mid-May at the very latest, possibly even in the next week or two.

Longer-range ENSO forecasts are typically very poor at this time of the year because the low-level winds that drive the changes in ocean circulation that lead to the development of El Nino and La Nina events are weakest at this time. Most models show us staying in a neutral pattern or transitioning to a weak El Nino by the end of 2018, but I wouldn’t put too much stock into these models just yet. Interestingly, last year’s models at this time had a similar forecast, yet we ended up going against the model consensus into a La Nina. We’ll have a much better idea of what state the Tropical Pacific will be in for the following winter by the time schoolchildren everywhere put away the books and break out the baseball bats for summer vacation.

In the much shorter-term, expect mostly sunny skies today and partly cloudy skies tomorrow with highs in the 60s both days. We may see a stray shower Saturday morning as a weak system passes by to our north, but a massive ridge of high pressure will fill in behind it, giving us blazing sunshine and our warmest weather of the year next week. Highs should reach the mid-70s on Monday and stay near 70 for much of next week, though with how cool it’s been recently, I’m sure it will feel far warmer than that.

Have a great Thursday, and as always, thanks for reading!
Charlie