In January months there are, on average, double the number of days with rainfall of more than 50 mm. Even in 2015, when rainfall over the Highveld was below normal, severe weather occurred in the form of localised heavy rainfall and hail.ĭyson (2009) indicated that on average there are 3.5 days per November with at least 1 station over Gauteng measuring rainfall greater than 50 mm in 24 h. Climate summaries of the South African Weather Service (SAWS) for November months make interesting reading and Table 1 lists a few severe events from 2011 to 2015 over the Highveld. A supercell is characterised by a deep mesocyclone and is an important small-scale weather system (from 2-20 km) as it is often associated with disastrous weather (Moller et al., 1994). Rae (2015) identified 4 supercells in November months over the Highveld between 20. More recently, in 20, large hail fell over Gauteng resulting in a dramatic increase in hail-related insurance claims (Piennaar and Dyson, 2015). One of the worst documented hailstorms in Pretoria occurred on 1 November 1985 (Terblanche, 1985). Severe weather events over the Highveld are not uncommon in November months. The situation was exacerbated as heat waves occurred over the Highveld during late October 2016 with the mean October 2016 maximum temperatures being between 2-4☌ warmer than the climate over the Highveld (SAWS, 2016a). By 7 November, the Vaal Dam had reached close to 25% capacity and water was released from the Sterkfontein Dam to replenish the water in the Vaal Dam (DWA, 2017a). The large metropolitan areas in Gauteng all instated water restrictions, as early as September 2016 in the City of Johannesburg but followed soon after by the other metros (City of Johannesburg, 2017). The very dry conditions continued into October 2016 with only isolated reports of good rainfall over the eastern half of the country.
BIG WEATHER EVENTS 2016 FULL
The media was full of reports of water drives where ordinary people helped to collect water and distribute it to drought-stricken areas (News24, 2017). Over the Highveld during this period, rainfall was around 75% of the norm.
BIG WEATHER EVENTS 2016 FREE
The Northern Cape, North West and Free State provinces received less than 50% of their average rainfall. The summer rainfall areas of South Africa were in the grip of a severe drought in the 12 months from July 2015 to June 2016 only a fraction of South Africa received normal yearly rainfall (SAWS, 2017). This was in sharp contrast to the very dry conditions which had prevailed over large parts of South Africa since 2015. November 2016 (henceforth Nov16) saw several severe weather events over the Highveld of South Africa. It is recommended that an operational meteorologist incorporates upper air sounding data into the forecasting process and not to rely on numerical prediction models exclusively. The consideration of the daily variables derived from sounding data were good precursors for the prediction of severe thunderstorm development over the Highveld during November 2016. The favourable circulation patterns and moisture gave rise to an atmosphere in which severe weather was a possibility, and the awareness of such factors is used as one of many tools when considering the severe weather forecast. The combination of these factors and weak steering winds resulted in flash flooding on the 9 th while favourable wind shear conditions caused the development of a tornado on 15 November.
Low-level moisture originated over the warm Mozambique Channel and the 500 hPa temperature trough was located favourably over the Highveld further east than normal. The surface temperatures and dewpoint temperatures were found to be higher than normal resulting in increased conditional instability over the Highveld. Furthermore, a few of the severe weather events are described in detail. The aim of this paper is to compare the synoptic circulation of November 2016 with the long-term mean November circulation and to investigate some sounding derived parameters. Very heavy rainfall, large hail and tornadoes were reported. November 2016 was no exception and a large number of severe events occurred. November months are notorious for severe weather over the Highveld of South Africa. IIDepartment of Geography Geoinformatics and Meteorology, University of Pretoria, Pretoria ISouth African Weather Service, 442 Rigel Avenue, Erasmusrand, Pretoria, 0181
Severe weather over the Highveld of South Africa during November 2016
0 kommentar(er)
