USA tornado season 2013 - a quiet start

  • Fig. 1 - Overshooting cloud tops associated with a potential storm outbreak on 9th April 2013 (NOAA)

    Fig. 2 - A classic supercell tornado (NOAA)

    Fig. 3 - A graph to show the development of the 2013 hurricane season in comparison to the previous 8 years (NOAA)

  • USA tornado season 2013 - a quiet start
    20.04.2013 14:39

    The USA tornado season of 2013 is well and truly underway. However, it has been a distinctly quiet year so far with just 213 reports of tornadoes. Compared to almost 1,000 by the end of April in 2011, does this set the scene for what’s to come or could this be the calm before the storm?

    Tornado activity varies from year to year with a prime example being the years 2011 and 2012. During 2011, a total of 1,690 tornadoes were recorded with fatalities estimated at 553. This is the highest number of tornado fatalities recorded in a single year since records began. In contrast, 2012 was a significantly quieter year with just 939 tornadoes and 70 fatalities. To put this into some perspective, there have only been two other years since the mid 1980s when fewer than 1,000 tornadoes occurred (1989 and 2002) and when the weakest (EF0) tornadoes are subtracted, 2012 was among the least active years in the last 60 years. One factor that is thought by some to have suppressed the frequency and intensity of tornadoes in 2012 is that many of the states in ‘Tornado Alley’ were in the clutches of severe drought. This lack of moisture is thought to have had a knock-on effect on the atmosphere leading to insufficient instability to produce tornadoes.

    Some tornadoes are formed when a strong thunderstorm develops a rotating mesocyclone in the atmosphere. As the storm develops, it produces an area of descending air known as the rear flank downdraft, which accelerates as it nears the ground, thereby dragging the rotating mesocyclone towards the ground with it. As the mesocyclone lowers beneath the cloud base, it takes in moist air from the downdraft column and its convergence with warm air in the updraft leads to a rotating column of air. This column of air becomes a tornado when it comes into contact with the ground and these can have a devastating impact on relatively small areas of land.

    While it has been a somewhat inactive start to the 2013 season, there have already been reports of 213 tornadoes, many of which have resulted in structural damage with a few leading to loss of life. Relatively early in the season, dozens of tornadoes were reported on 30th January across Alabama and Georgia. One of the tornadoes occurred in Bartow, Georgia, killing a man in a mobile home and injuring 14 others. The storm overturned approximately 100 cars in the local area. Another EF3 tornado with sustained winds of 145mph occurred in Kemper, Mississippi on 11th April killing one and seriously injuring nine others.

    No two tornado seasons are the same and as it’s been made clear from the years of 2011 and 2012, it would be foolish to use the previous year when attempting to forecast the coming season. It has been a relatively quiet start to the 2013 tornado season, possibly due to a number of factors. The eastern two thirds of the USA have had a similarly delayed spring to that of the UK, whereby cold wintry conditions persisted for much longer than usual. This has inevitably reduced the heat and moisture required to produce powerful storms. On the other hand, severe drought continues across the Plains, potentially limiting tornadogenesis for yet another year. However, as the graph shows in Figure 3, the year of 2011 began on a similarly quiet note, only to pick up dramatically from mid-April to become one of the most active years on record.

    By: John Lee