What's going on?

What's going on?
It's been an incredibly dangerous and deadly year for tornadoes. On April 14 - 16, we had the largest tornado outbreak in world history, with 162 tornadoes hitting the Southeast U.S. That record lasted just two weeks, when the unbelievable April 25 – 28 Super Outbreak hit. Unofficially, that outbreak had 327 tornadoes, more than double the previous record. The legendary April 3 – 4 1974 Super Outbreak has now fallen to third place, with 148 tornadoes. Damage from the April 25 – 28, 2011 outbreak was estimated to be as high as $5 billion, making it the most expensive tornado outbreak in history; the Tuscaloosa-Birmingham tornado of April 27 may end up being the most expensive tornado of all-time—until the damage from Sunday's Joplin tornado is tabulated. Officially, 875 tornadoes hit the U.S. In April 2011, making it the busiest tornado month in history. The previous record was 542 tornadoes, set in May 2003. The previous April tornado record was 267, set in 1974, and April has averaged just 161 tornadoes over the past decade.

So what's going on? Why are there so many tornadoes, and so many people getting killed? Well, the high death toll this year is partly just bad luck. Violent EF-4 and EF-5 tornadoes usually miss heavily populated areas, and we've had the misfortune of having two such tornadoes track over cities with more than 50,000 people (the Joplin tornado, and the Tuscaloosa-Birmingham EF-4 tornado in Alabama, which killed 61 people on April 27.) This sort of bad luck occurred in both 1953, when F-5 tornadoes hit Flint, Worcester, and Waco, and in 1936, when F-5s hit Tupelo and Gainesville. However, this year's death toll is more remarkable than the 1953 or 1936 death tolls, since in 2011 we have Doppler radar and a modern tornado warning system that is very good at providing an average of twelve minutes of warning time. The warning time for the Joplin tornado was 24 minutes. The first tornado warning wasn't issued until 1948, and virtually all tornadoes from the 1950s and earlier hit with no warning. On average, tornado deaths in the United States decreased from 8 per 1 million people in 1925 to 0.12 per 1 million people in 2000. Had this year's tornadoes occurred 50 years ago, I expect the death toll would have exceeded three thousand.



Figure 3. Number of strong to violent EF-3, EF-4 and EF-5 tornadoes from 1950 to 2011. There are no obvious trends in the numbers of these most dangerous of tornadoes. Image credit: NOAA/National Climatic Data Center (updated using stats for 2008 – 2011 from Wikipedia.)

Tornadoes require two main ingredients for formation—instability and wind shear. Instability is at a maximum when there is record warm air with plenty of moisture at low levels, and cold dry air aloft. April 2011 sea surface temperature in the Gulf of Mexico were at their third highest levels of the past 100 years, so there was plenty of warm, moist air available to create high instability, whenever approaching storm systems pulled the Gulf air northwards into Tornado Alley, and brought cold, dry air south from Canada. The La Niña event in the Eastern Pacific, in part, caused this spring's jet stream to have very strong winds that changed speed and direction with height. This sort of shearing force (wind shear) was ideal for putting a twist on thunderstorm updrafts, allowing more numerous and more intense tornadoes than usual to occur. Was this year's heightened wind shear and instability the result of climate change? We don't know. Over the past 30 years, there have not been any noticeable trends wind shear and instability over the Lower Mississippi Valley, according to the NOAA Climate Scene Investigations team. Furthermore, there have been no upward trend in the number of violent EF-4 and EF-5 tornadoes over the past 60 years, or in the number of EF-3 and stronger tornadoes (Figure 3.) However, this year's remarkable violent tornado activity—17 such tornadoes, with tornado season a little more than half over—brings our two-year total for the decade of 2010 – 2019 to 30. At this rate, we'll have more than 150 violent tornadoes by decade's end, beating the record of 108 set in the 1950s. In summary, this year's incredibly violent tornado season is not part of a trend. It is either a fluke, the start of a new trend, or an early warning symptom that the climate is growing unstable and is transitioning to a new, higher energy state with the potential to create unprecedented weather and climate events. All are reasonable explanations, but we don't have a long enough history of good tornado data to judge which is most likely to be correct.

http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=1808