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  • Heavy rain at our doorstep

  • Tuesday, 12th July 2016

It is nearly my habit to glance at the radar imagery every morning before leaving home. The situation I saw this morning (12 July 2016) is worth sharing. There was rain, some rather heavy, in many places around Hong Kong, yet Hong Kong was almost rain-free (Figure 1). Why did this happen?

Figure 1

Figure 1      Radar imagery at 8.36 a.m. on 12 July.

Actually, an active southwesterly airstream affected the coast of Guangdong today. The airstream came from the ocean and brought with it abundant moisture. Together with the effect of an upper-air disturbance, the atmosphere was relatively unstable as a whole. These conditions favoured precipitation, and according to past experience, rain could be heavier in the morning under such active southwesterlies. However, even though the large-scale weather pattern favoured the occurrence of heavy rain, the weather was not bad everywhere. Along the coast of Guangdong, rain was rather heavy in some areas, but there was almost no rain in places such as Hong Kong. Rainfall carries a degree of randomness (please note that luck may not visit the same place every time). Moreover, the upper-air disturbance which affected Hong Kong today was moving relatively slow. This caused not much change to the overall distribution of rain around Hong Kong. Those areas of heavy rain in our vicinity did not come and affect us. As shown in Figure 2, the rainfall (as estimated using radar data) over Hong Kong from midday yesterday to midday today was significantly less (areas in blue and green) when compared with our adjacent areas.

Figure 2

Figure 2      The total rainfall over a 24-hour period ending noon 12 July, as estimated using radar data.

Given the above situation, the state-of-the-art "numerical weather prediction models" on computers are still unable to forecast perfectly where and when heavy rain will occur. One of the reasons is that observation data are relatively sparse over the seas south of Hong Kong. In view of this, weather forecasters will adopt nowcasting techniques to analyse the changes in precipitation in our vicinity and the movement of rain areas on the radar, in order to assess the change of weather in Hong Kong in the next few hours and to update the weather forecast accordingly. As such, on a day with changeable weather, it is advisable to keep monitoring the latest weather forecasts, warnings, Special Weather Tips and radar imageries provided by the Observatory.

LEE Lap-shun

Last revision date: <04 Aug 2016>