It’s cloudy about 226 days of the year in Seattle, Washington. Portland, Oregon is right behind it with 222 overcast days a year. That’s over three-fifths of the year that those of us who live west of the Cascades are bereft of sunlight. We pay for it in vitamin D deficiency, too.
The frequent thick cloud cover does not necessarily translate to rain, however. Seattle and Portland get an average of around 38 and 42 inches per year respectively. Compare that to around 36 inches for Chicago, 56 for Miami, or 42 for Boston and you see that the total precipitation in the major cities of the northwest isn’t far from what most Americans experience. The major difference is that the rainy season is much longer, drawn out over nine months of the year, with little relief from grey skies even while rain isn’t falling.
Washingtonians and Oregonians will likely have to break out the umbrellas soon, however, because this winter is going to be exceptionally wet in the Pacific Northwest. This place that normally has long, cool, rainy winters is about to experience an even longer, cooler, rainier one, the reason for which is easy to say, but harder to explain: La Niña.
La Niña is the climactic counterpart to El Niño. Most people are probably used to hearing one or both of those names invoked to explain the weather, but forecasters rarely explain what these phenomena are and how they affect regional weather patterns.
El Niño and La Niña are sea-surface temperature anomalies that occur in the equatorial pacific. El Niño is what we call the warm anomaly while La Niña is the cool phase. They’re actually one half of a coupled phenomenon called “ENSO” an acronym for El Niño / Southern Oscillation. The Southern Oscillation half of the phenomenon is a measurement of the change in atmospheric pressure between the island of Tahiti and the city of Darwin, Australia. These two measurements, sea-surface temperature and atmospheric pressure, move together. During El Niño events, sea-surface temps are high over the eastern pacific and, correspondingly, the atmospheric pressure in that region is low. The opposite is true during La Niña; sea-surface temperatures are low from the central pacific to the coast of South America and that area experiences relatively high atmospheric pressure.
So, how does an event happening way down at the equator affect weather in the Pacific Northwest? It does so by forcing changes in the flow of the Pacific Jet Stream, a west-to-east flowing swift air current. The changes in atmospheric pressure associated with La Niña push the jet stream further north than its customary track, bringing storms along with it.
To the left, the NOAA graphic shows the course of the Pacific Jet during El Niño (top) and La Niña (bottom). Niña brings the jet stream directly over the Pacific Northwest.
As the Oregon map above indicates, this winter we can expect an increase in precipitation of three inches or more over the regions shaded in dark green. Temperatures are likely to be slightly lower to boot, so don’t be surprised if we have one or more significant snow or ice events in the valley regions this year. (We all know how well that turns out around here.) Snow pack will be higher, and snow at lower elevations is likely, increasing the opportunities for rain-on-snow events that are frequent causes of flooding.
ENSO is a local phenomenon that has global ramifications. The map below gives a general impression of the dramatic effects that people around the world feel from this climate signal.
If you’re a Portland resident and are interested in learning more about what to expect this winter, I suggest you attend this event: The 18th Annual “What Will Winter Be Like?” Meeting. Saturday, October 16th from 10:00 am to noon at OMSI. It’s free, and I hear tell there’ll be prizes.
Photo credit: Lightning – Crater Lake, OR by delriophoto.com on Flickr.