There have been several excellent and very informative posts by Joe Bastardi and Joe D’ Aleo at WeatherBell on the current weather pattern and its conduciveness to severe thunderstorm outbreaks including tornado family outbreaks. These weblog posts include their latest
We have looked at the issue of the relationship of La Niña to family outbreaks of tornadoes in our report
Knowles, J.B., and R.A. Pielke Sr., 2005: The Southern Oscillation and its effect on tornadic activity in the United States. Atmospheric Science Paper No. 755, Colorado State University, Fort Collins, CO 80523, 15 pp. (Originally prepared in 1993, published as a Atmospheric Science Paper in March 2005).
Our abstract reads
The Southern Oscillation has been shown in previous research to cause changes in the weather patterns over the continental United States. These changes, caused by either the warm El Niño or cold La Niña, could potentially affect numbers, locations, and strengths of tornadoes in the United States.
Using a variation of the Southern Oscillation Index, the seven strongest El Niño and five strongest La Niña events during the period 1953-1989 were examined to see what effect, if any, that they would have on: 1) Total tornado numbers, 2) Violent tornado track length, 3) Violent tornado numbers, and 4) >40 tornado outbreaks.
Little difference was found in total tornado numbers between El Niño and La Niña events. However, significant differences were found in the number of violent tornadoes, and in large number tornado outbreaks. La Niña event years were found to have longer than average track lengths, more violent tornadoes, and a good probability of having an outbreak of 40 or more tornadoes. El Niño event years were found to have shorter than average track lengths, less violent tornadoes, and only a slim possibility of having an outbreak.
Possible reasons for the above conclusions include: 1) Warmer than normal temperatures in the western U.S./Canada along with cooler than normal temperatures in the southern U.S. during El Niño years; and 2) Colder than normal temperatures in the western U.S./Canada along with warmer than normal temperatures in the southern U.S. during La Niña years. This would act to weaken/strengthen the interactions between warm and cold air in the midwest U.S. during El Niño/La Niña event years and decrease/increase the numbers and lengths of violent tornadoes.
The current Spring 2011 certainly fits this pattern.