El Nino and La Nina are both part of the El Nino Southern Oscilliation (ENSO) pattern in the eastern tropical Pacific ocean. This oscillation, or switching, refers to the temperature of the water in that region. In a La Nina phase, the water temperatures are cooler than normal, and in an El Nino phase, the water temperatures are warmer than normal. This is because in a La Nina phase, there is more upwelling in the eastern tropical Pacific, which brings cool, nutrient rich water up from the depths to the surface, and in an El Nino phase, there is decreased upwelling, so the water is more sterile and warmer. El Nino and La Nina both have different effects around the world, but since there are so many, I'm just going to talk about their effects in our location, the Pacific Northwest. Generally, El Nino will make us warmer and drier, while La Nina will make us cooler and wetter. Unfortunately for skiers, it looks like this year will be an El Nino year, with warmer and drier conditions prevailing during the winter. Still, we live in the Pacific Northwest, so we should get a good amount of snow anyway. And if there is no snow here this year, you can always head south to California during an El Nino year because the storm track is centered over them in El Nino years as opposed to us. In a La Nina year, the mountains get tons of snow because we are both wetter and cooler than normal. Especially in La Nina years, upper-level trofts from the Gulf of Alaska slide down south and direct cool and unstable air into our state. The Seattle metropolitan area usually is shadowed by these events, but the Cascades, who are perpendicular to this flow, commonly pick up foot after foot of snow. That is what happened during the 2007-2008 ski season, when Alpental, my local ski area, picked up 50 feet of snow. One interesting thing about El Nino and La Nina is that we generally do not have windstorms on these years. However, we can still get "Pineapple Express" flood events with El Nino and La Nina years.
Here's a great website to check out the tropical Pacific and see what it is doing. http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_update/sstanim.shtml
Somebody asked me about the probability of Pineapple Expresses in El Nino years as opposed to others, and here was my response.
Pineapple express events can happen in the El Nino, La Nina, and neutral phases of the ENSO, but they are most common in weak El Nino or neutral phases. Most Pineapple Express events are first set off by the Madden-Julian Oscillation. MJOs are pretty complex, but what they ultimately end up doing for the West Coast is bringing copious amounts of rain and warm temperatures via a long, "training" (staying in one spot, stalled) subtropical plume of moisture originating from the Hawaiian Islands or beyond (hence the name "Pineapple Express").
The thing is, in El Nino winters, a "blocking" ridge of high pressure usually sits off the Pacific Northwest, pushing the jet stream to the north and south. So while Southern Alaska and California (especially California) see enhanced precipitation, the Northwest is left dry. This means that even if an MJO produces a Pineapple Express event, it may not be pointed directly at the Pacific Northwest, which is the location most Pineapple Expresses over the past two years have been pointed. So while there may be more Pineapple Expresses, they may not be directly pointed at us.
Do you remember the huge landslides that occurred in Southern California several years back? I know for sure that they were associated with El Nino; they might have been associated with a Pineapple Express. The chance of Southern California, normally dry in the winter, getting extremely heavy rain will be much greater this year due to El Nino than other years. So if we aren't getting flooded, somebody else will be.