An extreme sub-daily precipitation event produced about 300 mm of rainfall in less than 4 h overnight from 13–14 February 2019 resulting in high floods in Thohoyandou, a small town northeast of South Africa. We employed station, radar, satellite and reanalysis datasets to investigate the rainfall, circulation and thermodynamic fields and understand the meteorological structure of the extreme event via a multiscale analysis. The large-scale synoptic environment was characterized by a mid-tropospheric tropical-temperate trough and attendant cloud band coupled to a surface high ridging over the southeast coast of the country. We found that whilst heavy rainfall (>50 mm/24 h) was widespread ahead of the upper trough, extreme amounts (∼100 mm/h) were localized due to a cloudburst. A small perturbation to the favorable large scale mid-tropospheric environment also contributed to localized heavy rainfall. The south-north pressure gradient was steepened by a surface low over southern Mozambique resulting in enhanced moisture fluxes deriving from the southwest Indian Ocean. The interaction of prevailing surface winds and a low-level jet with the steep topography of the adjacent Soutpansberg Mountain Range enhanced low-level convergence and lifting in the area. We also show that the highest rainfalls were uphill of the location of flooding which was contained in a poorly drained valley. Whereas the Unified Model forecasts appeared accurate for the large-scale pattern of heavy rainfall in the area, the rainfall peak was generally underestimated, whilst the timing of extreme rainfall was delayed in the 18Z simulation, which is used by forecasters operationally. Our findings contribute to understanding the occurrence of extreme weather events over northeastern South Africa and also how models treat them, towards natural disaster risk reduction.

从2019年2月13日至14日，在不到4小时的时间内，次日降水量极端极端事件在不到4小时的时间内产生了约300毫米的降雨，导致南非东北部小镇Thohoyandou发生高洪灾。我们利用站，雷达，卫星和再分析数据集来调查降雨，环流和热力学场，并通过多尺度分析来了解极端事件的气象结构。大尺度天气环境的特征是对流层中温带热带低谷和随之而来的云带，加上该国东南沿海地表高起的褶皱。我们发现，虽然高降雨（> 50毫米/ 24小时）在上谷之前广泛分布，但由于暴雨而出现了极端降雨（〜100毫米/小时）。对有利的大规模对流层中层环境的小扰动也造成了局部大雨。南北压力梯度由于莫桑比克南部的地表低而陡峭，导致来自印度洋西南部的水分通量增加。盛行的表面风和低空急流与邻近的索潘斯贝格山脉的陡峭地形的相互作用增强了该地区的低空交汇和抬升。我们还表明，最高的降雨量是在排水不畅的山谷中所处的洪灾发生地点的上坡。尽管统一模型的预测对于该地区的强降雨的大范围模式似乎是准确的，但在18Z模拟中，降雨峰值通常被低估了，而极端降雨的时机却被延迟了，预报员在操作上会使用它。我们的发现有助于理解南非东北部极端天气事件的发生，以及模型如何处理这些事件，以减少自然灾害风险。