April tornadoes are no surprise. Tornado season, which varies by state, dictates that strong storm systems can develop as large air masses meet in the Midwest and South. Warm, moist air from the Gulf of Mexico, in conjuntion with seasonal changes in the jet stream, leads to the possibility of severe storm development. In particular, the development of supercell thunderstorms is most likely during the peak of tornado season. These supercell storms cause the most damaging tornadoes to form.
Statistics are courtesy of the National Weather Service.
1. Death Toll
Tornadic events have been documented since the late 1600's. The statistical data is somewhat slim until the 20th century, but still an indicator of the frequency of tornadoes. The events of April 27-28, 2011 were grave. More than 350 people have been confirmed dead. Thousands more were injured. Alabama was the hardest hit although only 23 tornadic storms occur on average per year.
Only one tornado has a higher death toll. The Tri-State tornado on March 18, 1925 still ranks as the deadliest tornado. In a list of the top 10 deadliest tornadoes from the National Climatic Data Center, the next highest death tolls were 332 deaths on March 21, 1932, and 317 deaths on May 6, 1940 in the Natchez break out.
2. Number of Tornadoes in One Event
The events of April 2011 were statistically significant because of the total number of tornadoes confirmed. Reports indicated the number of tornadoes to be greater than 800, but the confirmed number so far is 288. The data is still preliminary as the National Weather Service works to confirm the total numbers.
The number is still nearly double the previous record holder for the number of tornadoes in a single event. The April 3-4, 1974 super outbreak held the record for 36 years before being broken by this storm system. In 1974, the total number of confirmed tornadoes was 148.
3. Average Number of Tornadoes in April
April 2011 was an active month for tornadoes. That understatement is best explained by looking at the average number of tornadoes that typically occur in April. During April, the average number of tornadoes is 160. April 2011 had more than 600 tornadoes. The death toll from the 2011 outbreak is lower, but the number of tornadoes for the April 2011 storms were higher than the 1974 storms. Better warning systems, higher lead time, imagery via weather satellites, and advanced forecasting tools are reasons the death toll remained lower.
4. Total Tornadoes in April
Similar to the average of tornadoes in April, sometimes storm events can be way above and way below average. The 1974 tornado outbreak was such an event. During April 1974, the total number of tornadoes reported was 267. The number of tornadoes in April 2011, based on National Weather Service preliminary data, is over 600 tornadoes. That makes April 2011 another record-breaking storm.
5. Total Tornadoes for Any Month
The 600+ tornadoes in April 2011 broke the record for all months in the year. During the peak of tornado season, May 2003 set a record for a total number of tornadoes in one month at 542 reported events. The large number of tornadoes in April 2011 were significantly larger. The United States is also the hardest hit location for tornadoes.
6. Year to Date Number of Tornadoes
Prior to 2011, the number of tornadoes, on average per year, is 1000 to 1200. In 2004, a large number of tornadoes occurred. 2004 had a total of 1,817 tornadoes. Taking into account the number of tornadoes up to and including April 2011, 835 tornadoes were confirmed so far. With the most active month for tornadoes still ahead, the 2011 tornado season is on the right track to break yet another record. May is typically the most active month for tornadoes.