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  • Heavy rain on 10 May 2016

  • Tuesday, 10th May 2016

Two episodes of intense rain and thunderstorms associated with a trough of low pressure affected Hong Kong today (10 May 2016) prompting the issuance of two Red Rainstorm Warnings at 7:35 a.m. and at 11:20 a.m. respectively.

The Observatory has forecasted heavy rain for today several days ahead. As an area of intense rain and thunderstorms started to develop over the Pearl River Estuary in the early hours today, the Observatory issued Special Weather Tips at 4:00 a.m., indicating that this area of intense rain and thunderstorms would affect Hong Kong.

As the area of intense rain and thunderstorms edged closer to Hong Kong, the Observatory issued the Amber Rainstorm Warning at 6:00 a.m. The heavy rain subsequently affected the northwestern part of the New Territories as it moved southeastwards. The Observatory kept monitoring the heavy rain situation and assessing the need of the Red Rainstorm Warning, and communicated closely with the Education Bureau throughout the entire period. The Observatory issued the Special Announcement on Flooding in the northern New Territories at 7:05 a.m.

Initially at around 7 a.m., the area of heavy rain was fast moving and was confined to Yuen Long and Tuen Mun (Figure 1). Objective methods, which include the Observatory’s Short-range Warning of Intense Rainstorms in Localized Systems – SWIRLS, did not forecast a high chance of Red Rainstorm situation. As such, the Red Rainstorm Warning was not issued at that time. Thereafter, the area of rainstorm slowed down (Figure 2) and further developed, probably owing to the effect of terrain as it moved across the hills of Kowloon. In view of this change, and expecting the heavy rain situation to persist, the Observatory issued the Red Rainstorm Warning at 7:35 am. During the first episode of intense rain, over 50 millimeters of rainfall in an hour were recorded over many regions of the New Territories, including Tsuen Wan, Tai Po, Sha Tin and Sai Kung between 7:00 a.m. and 8:30 a.m. (Figures 3 and 4). Rainfall also exceeded 30 millimeters in an hour over Sham Shui Po and Wong Tai Sin. When the rain over Hong Kong weakened, the Observatory replaced the Red Rainstorm Warning by the Amber Rainstorm Warning at 9:35 a.m..


Figure 1
Figure 1 Rainfall distribution map for the hour ending 7:00 a.m.

Figure 2
Figure 2 Track of the most intense rain area during the first rain episode. Rain area slowed down between 7 a.m. and 8 a.m..

Figure 3
Figure 3 Rainfall distribution map for the hour ending 7:30 a.m.

Figure 4
Figure 4 Rainfall distribution map for the hour ending 8:00 a.m.

As another rain area developed over the Pearl River Estuary and intensified over Hong Kong, the Observatory re-issued the Red Rainstorm Warning at 11:20 am. During the second episode of intense rain, more than 50 millimeters of rainfall in an hour were recorded over Kowloon, the southern New Territories, Southern district, Central and Western districts, and the Islands between 11:00 am and 1:00 pm (Figure 5 and 6). As this second area of rain moved to the south of Hong Kong, the Observatory replaced the Red Rainstorm Warning by the Amber Rainstorm Warning at 1:45 p.m.. The Amber Rainstorm Warning was subsequently cancelled at 2:45 p.m..


Figure 5
Figure 5 Rainfall distribution map for the hour ending 12:00 noon
Figure 6
Figure 6 Rainfall distribution map for the hour ending 1:00 p.m.

Both rainfall episodes today met the criterion of the Red Rainstorm Warning.

Rainstorms in summer usually develop rather rapidly and there is still a considerable degree of randomness in their evolution. Therefore forecasting rainstorms remains a big challenge despite state-of-the-art forecasting systems. As the heavy rain occurred during the rush hours in the morning, this inevitably brought inconvenience to the members of the public. The Observatory will continue to strive to provide weather forecast and warning based on the latest objective methods with the aim to ensuring the safety of the public.