The record number of tornadoes in April, multiple tornado outbreaks this Spring, and high number of violent tornadoes may be a result of La Nina. Scientists a lot smarter than me will be researching this for years to come, and no doubt that we'll need more data to feel confident about the connection, but it appears there is a connection.
The photo above is from my trip to Tuscaloosa, Alabama the day after a tragic and devastating tornado hit the town. In the picture I am standing in what was once a Chinese restaurant and looking out on a very wide path of damage. That tornado was rated EF4 at it's worst and had a width of 1.5 miles (5 times wider than the North County tornado in St. Louis!). I've never seen tornado damage that widespread and that intense (mostly EF3-EF4). While that type of destruction is unfortunately not uncommon in the US, the fact that so many other cities and people were dealing similar damage on the same day is extraordinary. The April 26-28th, 2011 tornado outbreak produced the second deadliest day of tornadoes in US history. And further, the latest preliminary tally of 875 tornadoes this April is a record for any month and is almost equivalent to an average year's worth of tornadoes in one month. And La Nina may play a large role.
This past Winter we had a particularly strong La Nina, and it turns out we see a pattern where many (not all) tornado outbreaks in the US have occurred during or transitioning out of a La Nina.
La Nina is when cooler than normal temperatures occur in the Equatorial Pacific (It is the opposite of El Nino when warmer than normal temperatures occur). Our oceans interact with our atmosphere, and when vast areas of ocean cool or warm, it has impacts on our weather patterns. More specifically to the U.S., La Nina is thought to bring a pattern that is favorable for tornadic storms including the clash of warm and cold air coupled with a jet stream position and strength that can cause rotating storms. This doesn't mean every La Nina produces a crazy amount of Spring tornadoes. And it doesn't mean that all tornado outbreaks occur during La Nina's. However, there certainly appears to be a connection where there is an increased risk for numerous and violent tornadoes during a La Nina.
This scientific paper (click here) supports past research that shows the connection between the top 15 tornado outbreaks (40 or more tornadoes) and La Nina using data through 1990.
6 Occurred during La Nina (including 2 of the top 3)
8 Occurred during neutral conditions (No El Nino or La Nina)
1 Occurred during an El Nino
And furthermore, during La Nina years tornado outbreaks tend to have stronger tornadoes than neutral or El Nino years. La Nina years had about 16 F4-F5 tornadoes while Neutral and El Nino years had roughly 8 F4-F5 tornadoes.
No doubt there will be more research on this connection. But it's a connection I will always keep in mind after such numerous and violent tornadoes this Spring, which by the way followed the 6th snowiest season in St. Louis history.