A severe weather event on 29 December 1997: Synoptic and mesoscale perspectives

Abstract

On 29 December 1997 a severe hailstorm occurred over the eastern parts of the Free State. Hailstones with a diameter of 10 to 25 mm were reported. In this paper the authors try to establish the nature of the storm by looking at the synoptic and mesoscal e analyses of the weather features at that time. Due to a lack of sufficient data, the South African version of the Eta model’s 3 and 6 h forecast fields were used to establish the storm environment. Most of the model parameters indicated that severe weather could have been anticipated in the area of interest. Mesoscale analyses showed that steep temperature, pressure and dew point temperature gradients were evident over the area and that this contrast in air masses served as a good trigger mechanism for the development of thunderstorms. Analyses with the upper-air data showed that the air was very unstable at Bloemfontein earlier that afternoon, while Bethlehem was under the influence of an inversion (subsident motion in the upper layers of the atmosphere). The latter was removed during the early evening due to orographic forcing.

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