Deadly Texan Tornadoes - 15th May 2013

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  • Radar reflectivity image of a well defined supercell and 'hook echo' giving away the location of a strong tornado approaching Cleburne, on evening of 15th May 2013. Picture by: WFAA-TV facebook page.

    Storm Prediction Center cumulative trend of local storm report (LSR) tornadoes since 2005, showing how quiet the 2013 season has been so far (to 15th May 2013). From: Storm Prediction Center (SPC) website.

    Complete destruction sustained to a property in Granbury, Texas on evening of 15th May 2013. Picture by: Fort Worth National Weather Service via Facebook.

    4 inch diameter hailstone that was discovered in Granbury, Texas on 15th May 2013 after the supercell and tornado had passed. Picture via: user earthless_orchid on reddit.com.

  • Deadly Texan Tornadoes - 15th May 2013
    16.05.2013 18:07

    May is typically the month when the Great Plains and Midwest of the USA experience their most severe weather. Giant, rotating thunderstorms known as supercells develop and tornadoes occasionally spawn from their bases. Baseball-sized hailstones, torrential rain and intense lightning are also par for the course when caught beneath these supercells, the most powerful thunderstorms on the planet. When compared with the size of the entire thunderstorm, the violent and often damaging winds that a tornado produces only affect a very small area of the earth’s surface. This ensures that predicting the development, movement and intensity of a tornado more than a few minutes in advance is one of the most challenging aspects of weather forecasting. However, due to the high impact that tornadoes can have on infrastructure. the economy, and indeed human life, intensive meteorological research continues into trying to improve forecasting techniques and warning systems.

    For severe thunderstorms and large tornadoes to break out across the US plains, several meteorological ingredients are required. A persistent southerly or south-easterly wind is needed to transport very warm and moist air from the Gulf of Mexico northwards across the Plains. As much colder and drier air from the Rocky Mountains overspreads this at higher levels in the atmosphere, then huge instability is created. This gives storm clouds the ability to grow to great heights and also to develop very rapidly. Combined with situations where the wind in the atmosphere strongly increases in speed and also and changes direction with height, then severe storms and tornadoes are possible. The 2013 severe storm season has been unusually quiet until recently. This can be attributed to a rather persistent synoptic weather pattern across the US during most of April and the first half of May, with frequent west or north-westerly winds and fairly cool and dry conditions across the Plains.

    Aside from an outbreak of tornadoes across the Mississippi Valley states on 29th January and then several smaller scale outbreaks in mid-April, the season has been notable for almost no tornadoes occurring since then. However, during the evening of Wednesday 15th May, a small-scale weather disturbance across Texas has broken this trend. At least 6 people were killed and many more injured after a large tornado tore through the town of Granbury, Texas, approximately 25 miles south-west of Fort Worth. In the nearby town of Cleburne, storm chasers reported a tornado that was approximately 1 mile wide! Atmospheric conditions across the US Plains during the next few days remain favourable for further rounds of severe thunderstorms and probably further tornadoes.

    By: Matthew Dobson