Cloudy and Cool

By Paul Yeager, author of Weather Whys: Facts, Myths, and Oddities

Over 45o people have been killed in the United States by tornadoes so far this year–a much higher-than-average number–so many of us are asking why there have been so many deadly storms this year. The questions is understandable, especially in light of the Joplin, Missouri, tornado of yesterday, which may have been the deadliest single tornado in more than 50 years.

Short Answer: La Nina

The problem for meteorologists like myself is that only simple answers are wanted. No one wants to hear about all of the complexitites of the atmosphere, how difficult forecasting tornadoes is, etc.

With that in mind, I will say that the one meteorological factor most influencing the ongoing extreme weather is La Nina, which is something that I highlighted in a March 1 article on AOL News (More Dangerous Tornado Season Expected). Please read the article for details about how La Nina influences the  overall pattern.

Placement of Storms

Another factor in the deadly nature of the tornadoes this year is where they’ve most frequently occurred, which is farther to the south and east than the less populus Tornado Alley. With frequent tornadoes in the Midwest, Missouri Valley, Tennessee Valley, Deep South, and Carolinas, the tornadoes are much more likely to hit high population regions. (Image below courtesy of the Storm Prediction Center.)

This shift in tornado activity also tends to be a feature of La Nina.

2011 tornado reports

Global Warming

I would not directly related the increased tornado frequency of the season to date to global warming. Just as global warming cannot be judged by a single hot summer or cold winter, it’s the same for tornadoes. Climate changes associated with global warming need to be judged on decades of data, not months.

Forecast for the Rest of the Season

As the La Nina continues to weaken (and lose its influence on the overall weather pattern), it seems logical that the remainder of the tornado season would be more normal (as opposed to above normal as it has been). We’ve already seen a decrease in the number of tornadoes in May compared to April. The season peaks in May and June.