I said in yesterday's post that El Nino brings warmer and drier than normal weather to the Pacific Northwest. But I realized that I did not explain why. You see, what El Nino does is amplify the jet stream. You would think that this would give us more rain, since a stronger jet stream coming into a region gives the storms more potential energy and power. However, under that El Nino pattern, a semi-permanent low-pressure system usually sits way off in the Pacific. Further east, a high pressure system develops right to our west. This gives rain to the north and especially to the south of us, but leaves us dry. Between two jet streams, we are protected from arctic invasions and stormy weather, and typically have benign winters with above average temperatures and below average precipitation. In La Nina, the jet stream is pointed right at us, and often has a fairly northerly component to it, giving us below average temperatures and above average precipitation, and TONS of snowfall (2007-2008 was a La Nina year, and Alpental got 50 feet of snow that year!).
This is the current jet stream over the Pacific and western North America. Notice how there is a large circle in the middle of the screen, above where the jet stream winds turn purple. That is a low pressure system. Then, look by the Pacific Northwest. High pressure is dominating the area, forcing the jet stream, now considerably weakened, to split into two and go north and south. Even though California and Canada seem to be spared of active weather as well with this particular scenario, they often get hammered while the Pacific Northwest stays dry. It's that type of weather that makes me resentful and jealous, and the only good thing about El Ninos is that they reduce upwelling off of our coast, bringing in tuna that I can fish for in the summer. But I dislike everything else about it.