Last year was classified as having the worst storms ever recorded in history of the United States. In 2011, La Ni�a disappeared in early January and was not replaced by El Ni�o, resulting in an untamed jet stream that wreaked havoc on the United States. The spring was plagued with severe storms, tornadoes, hail and lightening that killed hundreds of people.

Climatological conditions known as El Ni�o and La Ni�a develop when abnormally warm or cool sea surface temperatures (SST) in the eastern Pacific interact with the atmosphere. An El Ni�o occurs when the SST rise above normal (3 month average >0.5 Celsius) following months of faltering trade winds. The warm ocean surface water adds moisture to the air above it, forming low-pressure systems. A La Ni�a occurs when the trade winds push the warmer surface waters away from the equator. The warm water is replaced by cooler, nutrient-rich water rising from deeper in the ocean, a process called upwelling. The cooler water impedes the formation of rain-producing clouds, leading to dry conditions near the equator in the Pacific Ocean. These two phenomenon provide structure to the climate system for the entire winter allowing climatologists to make fairly decent long range forecast.  

During an El Ni�o, weather conditions between October and March tend to be wetter than normal in the southern United States. The opposite is observed in the northern United States, with drier winters resulting in reduced rain and snowfall (Figure 1). The climatic effect of a La Ni�a in these regions is almost the opposite compared to El Ni�o. The southern region experiences drier than normal conditions while the northern region experiences wetter than normal conditions (Figure 2).

Although El Ni�o generally brings more rainfall to the southern region, there are some exceptions. La Ni�a appears to be the more reliable of the two, almost always bringing dry conditions to the southern region. Climate effects in the northern region, on the other hand, are not as reliable and harder to predict.   While La Ni�a generally brings wetter conditions, the effects of El Ni�o appear to be less predictable in this region.