WSET.com - ABC13On Tornadoes, Let the Buyer Beware by Sean Sublette

It is very easy to get carried away.

Tornado videos are scary. The eyewitness accounts are harrowing. The damage is heartbreaking. And when you see it repeatedly over several weeks, it is easy to believe something is wrong with the weather. Too often, someone wants to offer a 10-second explanation of why it is so bad. Don't believe it.

Tornadoes do not just drop out of the sky. They are born of thunderstorms which often come from disturbances racing through the jet stream. And there have been an unusually large number of strong disturbances this spring.

The strongest disturbances tend to generate more spin in the atmosphere, and this spin (or shear) is a necessary ingredient in the development of supercell thunderstorms. Supercells are those thunderstorms that produce tornadoes, namely, the largest and most destructive ones.

So why the stronger disturbances this year? That is a tougher nut to crack. The going idea right now is based on two large scale fluctuations working in concert.

First, we are coming out of a La Niña pattern. La Niña is an abnormal cooling of the water along the equator in the central and eastern Pacific Ocean. This alters the amount of heat energy transferred from the ocean to the atmosphere, which it turn, affects the pattern of steering winds downstream.

In a La Niña pattern, atmospheric disturbances that race into the Pacific Northwest often dive southeastward across the Rockies, then turn more to the east, or even northeast, once they move away from the Rockies. This turning enhances the environmental spin needed to generate supercells.

Second, there is a longer term fluctuation of water temperatures in the northeastern Pacific Ocean (Pacific Decadal Oscillation, or PDO) which is has returned to its cold phase. This cold phase was observed through most of the 1950s, ‘60s, and ‘70s, but then shifted into a warm phase (Figure 1). Since the late 1990s, it has returned to that cold phase (except 2003-06). Like La Niña, the PDO also affects steering winds and the strength of disturbances racing into the Pacific Northwest.

One other caution: Do not blindly blame climate change. Newsweek recently published an alarmist article that attempts to make such a connection. The evidence to make such a link is not there. You can pump all the water into the atmosphere you want for thunderstorm fuel, but if you don't have any ambient spin in place, you won't get a tornado.

True, tornadoes have hit more populated areas this year. But remember, the population of the United States has nearly doubled since 1950. Consider how many people are living in areas that, as little as 30 years ago, were either open fields or uninhabited forests. It stands to reason that more people are going to be affected.

Plus, in the early 21st Century, it is much easier to document tornadoes. There were no camera phones 20 years ago, much less the ability to share tornado video with the world in a matter of seconds.

There is also an analogy to be made with the 2005 Atlantic Hurricane Season (the year of Katrina): a fear that something had suddenly gone wrong and every hurricane season would be that way from then on. It obviously has not. 

The tornado count will be down in 2012. Take some comfort knowing the long term trend in tornado deaths is down substantially since 1950 (Figure 2), even with the increased population. Better housing, more warning time.

It may not be pleasant, and we may not like it, but the atmosphere is just doing what it does. Unfortunately, it sometimes does it in very violent ways.