Tornado Season in the USA

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  • Image of a tornado at Canton Lake, May 2011. Photo: Helen Rossington, MeteoGroup.

    Map to illustrate positioning of the jet stream causing a trough over the dryline.

    Map to illustrate air masses meeting on 24th February 2012.

  • Tornado Season in the USA
    25.02.2012 15:40

    On Friday 24th February, NOAA issued a tornado warning for the Appalachians in the USA for what is believed to be the beginning of the tornado season.

    Tornadoes are most commonly associated with the United States of America, with an average of 1,000 tornadoes being recorded a year. Tornadoes can be defined as rapidly rotating vertical columns of air that drop from cumulonimbus clouds and make contact with the ground.

    They are a violent phenomena and are capable of reaching speeds of 200mph, although they do not usually touch the ground above 150mph. Tornadoes usually occur in the months of April, May and June, known as “Tornado Season.” However, these violent weather systems appear to be occurring earlier and with more intensity. 

    2011 was a remarkable and unusual year for the USA, with 1,897 tornadoes being recorded. It marked the fourth deadliest year on record, with 550 people being killed by tornadoes. In comparison the deadliest year was 1925 when 794 people were killed. As mentioned previously tornadoes usually occur in the months of April, May and June, but 2011 was different with the first destructive tornado forming on New Year’s Day. It is believed the reason for these records was because of the strong La Nina. 

    Tornadoes usually occur in “Tornado Alley” which runs from Texas and Louisiana, through Oklahoma and Kansas to North Dakota. This area is prone to tornadoes due to the dryline that develops in the spring months. A dryline is the boundary between maritime air which is moist and warm, and the dry continental air.

    Tornado alley marks the meeting of these air masses. There is great instability and thunderstorms, which leads to a spinning affect as the warm air updrafts and the cold downdrafts. 2011 was different because there was a strong La Nina.

    The La Nina caused the jet stream to move north through the Pacific Northwest and the Great Lakes, which in turn lead to the usual dry line moving east across Oklahoma and Kansas, where the most destructive tornadoes occurred. For example Texas on average experiences 135 tornadoes a year, but in 2011 only 100 were recorded. Whereas in Alabama 146 were recorded. 

    Even though the La Nina is weakening this year, it is expected that there will be a higher than average number of tornadoes in 2012, but these should occur mostly in Tornado Alley. However, the tornado season already seems to be starting, with 83 confirmed tornadoes already occurring in January and February.

    The 24th of February marked a clear dryline with NOAA’s Storm Prediction Centre issuing a tornado watch for the Appalachians. A number of small tornadoes were recorded, but none as destructive as those expected to come.

    By: Sally-Jean Webb