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  • Are you ready for the rain season?

  • Friday, 29th April 2016

The rain season in Hong Kong this year has begun. Showers and thunderstorms occurred every now and then, sometimes affecting only isolated area and sometimes more intense and widespread. This to some extent affected the daily life of the public. Very often, forecasting heavy rain is a major challenge faced by the Observatory forecasters.

Nowadays, when forecasting the weather for the next few days, the meteorological community mainly relies on numerical weather prediction models run on computers, which simulate the evolution of atmosphere based on physical laws. Despite the advancement in science and technology, computer model predictions are still not perfectly accurate. It is especially challenging to predict weather phenomena such as thundery showers that change rapidly and affect relatively small areas. Generally speaking, the shorter the forecast lead time, the more accurate the computer model predictions will be. Therefore, for the weather forecast for a particular day, forecasters have to continuously assess the latest results of computer models as the subject day gets closer and closer, and consider whether or not to adjust the forecast.

We are usually able to predict environmental factors that favour the occurrence of heavy rain a few days in advance, such as whether the atmosphere will be unstable, and whether a trough of low pressure or an upper-air disturbance will come close to Hong Kong. Yet there is still a degree of randomness as to which hours of the day and at where the heavy rain will eventually occur, or whether the heavy rain will just narrowly miss Hong Kong. As such, for the forecast for the current day, forecasters still need to make reference to a vast amount of actual observation data, including those from automatic weather stations, weather radar and satellite to complement the forecast from the computer models; and if deemed necessary, immediately issue updates to provide a more accurate and detailed forecast to the public. This happens more often during spring and summer. Besides, another reason to update the forecast during the course of the day is that as the day progresses, the period covered by the forecast will shorten from 24 hours at the beginning to a few hours. The weather situation in the weather forecasts will naturally be different and therefore requires updates appropriately with time.

Heavy rain in spring and summer evolves rapidly. The rainfall distribution is also highly uneven. Coupled with the small geographical size of Hong Kong, any small deviations in the location and movement of rain areas can lead to drastically different local weather. As shown in Figure 1, there were two episodes of heavy rain in April 2016, which fell on our neighboring Shenzhen and the waters south of Hong Kong respectively, yet no heavy rain or just localized rain in Hong Kong. These speak for themselves on the difficulty in forecasting heavy rain.

Figure 1

Figure 1      Radar imageries showing heavy rain over Shenzhen and waters south of Hong Kong (areas in yellow and red) but hardly any over Hong Kong.

My advice to all of you before going out in the rain season is: not to judge the weather condition elsewhere simply based on how heavy the rain is at where you are. Instead, please keep track of the weather forecasts of the Observatory, check the location and development of rain areas using radar imageries, and pay attention to the latest 'Special Weather Tips' issued by the Observatory. In doing so, you can grasp the latest situation and get better prepared.

Finally, I would also like to take this opportunity to remind everyone that thunderstorms could bring not only heavy rain but also severe gust, squall line[1], hail[2] and even tornado[3]. Given the rain and typhoon season has begun, those who work in sectors including marine, logistics, transport and construction should take precautionary measures to avoid objects being blown down by strong winds in heavy rain. Members of the public should also pay attention to the relevant weather warnings and 'Special Weather Tips' issued by the Observatory to ensure their own safety.

LEE Lap-shun


[1] Thunderstorms and squalls

[2] Severe Weather in Hong Kong

[3] How is a tornado rated?

Last revision date: <22 Jul 2016>