Sunday, May 1, 2016

La Nina Expected for Winter 2016-2017

A La Nina is expected to develop this summer, and should reach maximum intensity during the winter of 2016-2017.

A look at sea surface temperature anomalies across the central Pacific reveals a change in progress. Over the last several months, we have been in a notable El Nino event, which is characterized by above to well-above average temperature anomalies in the waters along the Equator, west of Ecuador. However, in the last few weeks, we've started seeing those warmer water anomalies wane, and even be replaced by cooler than normal anomalies. This switch is occurring mainly in the waters immediately offshore Ecuador, as well as a little farther out to sea, between the 120W and 110W longitude lines.

We can trace this cooling to an upwelling event ongoing across the Equatorial Pacific. Shown is a cross-section diagram of the ocean at the Equator, from a depth of the surface to 450 meters. Ever since February, note how we've been seeing well-below normal water anomalies pushing towards the surface, and recently breaching the surface near Ecuador. This is the root cause behind those cooling anomalies on the surface image discussed earlier in this post, and so long as this upwelling of cooler waters continue, the rest of the Equatorial Pacific should continue to cool down from this El Nino.

But what about the future... specifically the winter ahead?

A chart showing the forecasted sea surface temperature anomalies in a portion of the Equatorial Pacific known as Nino region 3,4 shows our decline from an El Nino into 'ENSO-Neutral' conditions around May and June. You may recall that Neutral conditions are when the waters are too warm for a La Nina, but too cool for an El Nino, and this is defined when water temperatures in Nino region 3.4 are between +0.5 degrees anomaly and -0.5 degrees anomaly. We begin to enter a La Nina around August or September, when the average of model guidance dips below -0.5 degrees anomaly, the threshold for a La Nina. You'll also notice it reaches maximum intensity as we head into the winter of 2016-2017.

Now the big question: what does this mean for winter 2016-2017?
The big answer: we don't exactly know. But we do have an estimate of what things will look like if this forecast holds.

The above graphic shows typical weather conditions in a La Nina pattern. In the U.S., La Nina's tend to bring cooler than normal conditions to the Pacific Northwest, northern Plains, and Midwest. Wetter/snowier than normal conditions also tend to evolve in the Midwest, Ohio Valley, and portions of the Great Lakes. The Southern U.S., however, generally can expect warm and drier than normal winters out of La Nina's. This isn't a set-in-stone interpretation of what the coming winter will be like, but we could see something similar to this graphic set up.

To Summarize:

- A La Nina is expected to develop this summer, and intensity into the coming winter.
- As is typical in La Nina winters, the Ohio Valley may be wetter/snowier than normal, while cooler weather overspreads the North U.S. The South may remain warm and dry.

For more weather updates, follow me on Twitter: @akracki and @TheWxCentre.