El Nino, La Nina, and the Atmospheric River

In California, around this time of year, we often hear the local weather forecasters discussing El Nino, La Nina, and how the long term forecast for the winter suggests this or that.

I've always been rather baffled by the discussion, because in the short time available to them, the forecasters rarely have enough time to really explain what they're observing, why it matters, and how they're deriving their conclusions.

But I've been reading the blog of a Seattle-area forecaster, Cliff Mass, and he's written several great posts this fall explaining how La Nina affects the weather of the West Coast of the USA.

Short summary: the changed ocean temperature affects the air patterns, and the "Atmospheric River", often called the "Pineapple Express" by Bay Area forecasters when it causes warm, wet air from Hawaii to come streaming right at Northern California, shifts just a couple degrees in direction and instead of being pointed at California, becomes pointed at Oregon/Washington instead. The result: very very wet weather in Oregon and Washington, rather dryer-than-usual weather in California.

Here's some of Mass's recent essays on the subject:

• The First Atmospheric River of the Season


• The Long Term Forecast


• Atmospheric River on the Way

And a bonus link to a short Science article on the subject: Rivers in the Sky are Flooding the World with Tropical Waters.

If you're looking for something a bit different to read, you could do much worse than tuning into Cliff Mass's blog from time to time. I particularly enjoy how he illustrates his articles with the charts and graphs of various forecasting tools, showing how these tools are used, and how forecasters continue to improve their technology in order to further their understanding of the world's weather.