El Nino is caused when the temperatures are warmer than normal. La Nina is when they're cooler than normal.
Scientists are still trying to find out what causes these phenomenon and their apparent effects on the climate around the world.
However, during El Nino events, the winters around the Pacific Northwest tend to be warmer and drier than normal, as the jet stream tends to point further south into central and southern California.
La Nina events tend to lead to cooler and wetter-than-normal, as the jet stream tends to park over the Pacific Northwest.
"Neutral years" -- winters where there's neither of those two events going on -- tend to be more variable, as the jet stream tends to move around more often. So we could have a stretch where it's warm and dry, followed by a stretch of cold and snowy. In essence, all bets are off, and we're open to anything.
Neither of the events appear to have much of an effect on summer weather, but there's still plenty of research going on in the field.
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