Portland area weather geeks from the public, private and academic sectors got together Friday for the annual "What Will The Winter Weather Be Like?" meeting at OMSI, sponsored by the Oregon Chapter of the American Meteorological Society. Amazingly, most agreed we're probably in for a stormy winter. The reason is La Nina, which is the name of a pattern of colder than average sea surface temperatures in the tropical Pacific.

What could ocean temperatures in the tropical Pacific possibly have to do with Northwest weather? The tropical Pacific is the largest earth-based source of heat and moisture on the planet. It's the 900-pound gorilla of climate forcing. When it gets a cold, we all feel the fever.

But enough of the meteorological metaphors. The graphic below from the Oregon Climate Service describes how colder than average water in the eastern tropical Pacific, and warmer than average water inthe western tropical Pacific impact our winter weather patterns.

Anecdotally, what I've noticed in La Nina years such as this is the winter tends to start slowly, but really picks up after New Years. This is when we're most likely to see valley snow, wind storms, or flooding rains. We tend to get a lot of wild swings in our weather patterns in La Nina years because the La Nina pattern is almost opposite of what is considered our "normal" weather pattern, and the atmosphere is frequently re-adjusting itself. So we can end up with cold weather and a lot of low elevation snow, followed by a pineapple express and lots of high elevation rain. A perfect recipe for flooding, of course. So storm-watchers can delight in the prospects for an active winter, everyone else, get ready to hunker down.

Matt Zaffino
KGW Chief Meteorologist

P.S. I've picked January 8th as the day for Portland's first snowflakes.

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