Weather Blog

Recent near-'double-dip' La Ninas have snowy legacy for Seattle

Recent near-'double-dip' La Ninas have snowy legacy for Seattle
Pavement finally shows through snow in Renton. (YouNews photo courtesy of 'Gnus4u')

Even though the weather had been relatively tame for October and early November, La Nina is definitely here... again.

And while it's not as strong as it was at this point last year, some of the latest forecasting models indicate that La Nina will quickly strengthen this autumn, to where by the winter time, we could be on par -- to perhaps even a touch stronger - than the La Nina of 2010-2011.

Peter Wickersham has been intensely following the forecasts and to help give an idea of this trend, made an animation of the forecast model's output, taken roughly every day or every other day.

The horizontal line is 0 degrees anomaly of the ocean temperatures in the region where we look for La Nina/El Nino. Anything below -0.5C difference is considered La Nina, while above 0.5C is an El Nino. In between, it's neutral.

The solid black curve is the observed anomalies and the colored lines are where individual models are predicting it will go from here on out. The black, dashed-line is the mean of the model outputs.

Over the summer, the black dashed line was in the neutral area, but has been drifting more and more La Nina-esque as the year has progressed. In late October, the model had moved to predict a very strong La Nina on the order of -2.0C -- considered a strong La Nina, although the past week has seen the model moderate a touch back to about -1.5, which is on par with last year's La Nina episode.

Jason Phelps, who also tracks the weather quite closely around here, recently sent me a list of three somewhat-recent sorta-kinda Double Dip La Nina winters.

"I looked up all the past winters that had a winter where it was right on the cusp between La Nina and Neutral just after coming out of a La Nina," he said. "One of the criteria I looked at was that it went all the way to neutral (0.0 degrees or very close deviation from normal sea surface temperature during the summer) just like we have done this year, and the following are recent winters that had a similar pattern..."

These are the three winter he came up with, although technically for official classification purposes, the second part of the "Double Dip" came up juuuust shy of an official La Nina designation.

1) The winter of 2008-09. Don't need to remind too many people about how December went in 2008. Let's just say there still might be some snow tucked in corners of the city that haven't melted yet. It was the first widespread White Christmas since 1990.

2) Before that, was the winter of 1996 to 1997, which had a big pre-Thanksgiving snowstorm of 6-12" in several areas, then the big snowstorm of Dec 26-29.

3) The winter of 1985-86. That had a long snow event that lasted from Nov. 17 into Dec. 2 with the big Thanksgiving snow, and highs were only in the teens on some of those dates. It also snowed again on Valentine's Day in 1986.

Again, technically those years were neutral following a La Nina winter, but they had negative temperature anomalies that ended up just short of qualifying as La Nina, especially 1996 and 2008.

Now, all that said, it doesn't mean this fall and winter will follow those scripts. There are lots of other factors in play. But long range models for the winter continue to suggest it'll be cooler and wetter -- just like most La Ninas are.