None — Severe Weather Team 2 meteorologist David Chandley breaks down this year's tornado outbreak using data and information from the Storm Prediction Center on climate and on tornadoes. Some information also came from the blog of Dr. Jeff Masters.
The Joplin, Missouri tornado has claimed 116 lives, making it the deadliest single tornado since 1936 when tornadoes hit Tupelo, Miss. (216 killed) and Gainesville, Ga. (203 killed) The Storm Prediction Center rates the Joplin tornado as the 9th deadliest of all time.
This year's tornado death toll now stands at 482, making it the deadliest year for tornadoes in the U.S. since 1953, when 519 people died. That year, three heavily populated cities received direct hits by violent tornadoes. Waco, Texas (114 killed), Flint, Michigan (115 killed), and Worcester, Massachusetts (89 – 94 killed) all were hit by violent F-4 or F-5 tornadoes. A similar bad tornado year occurred in 1936, when violent tornadoes hit Tupelo Mississippi (216 killed), and Gainesville, Georgia (203 killed.) 2011 has been a year for twisters. 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.
Unofficially, 660 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.
As the graphic depicts, we have recorded a high number of EF-4's and EF-5's this year, but it is nowhere near the record set in 1974. In fact there have been other years with more powerful storms.
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 bad timing and storm locations. 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 was also the case in 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 ever NWS 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 the death toll would have likely been in the thousands.
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 Nina 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.
More to worry about; the tornado season is still happening (especially in the Midwest and Northern Plains) and then there is the threat of tornadoes from land falling tropical systems.
2011 could be one for the record books by year's end.