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  • The Observatory issued the first Red Rainstorm Warning Signal this year

  • Saturday, 20th April 2019

Between spring and summer, the development of rainstorm over southern China can be somewhat random and often changes rapidly and is a great challenge for weather forecasting. In the past few days, the local weather has been unsettled with occasional heavy showers, which necessitated the issuance of rainstorm warning signals by the Observatory for three consecutive days.

At around 9am today (April 20), a band of thundery showers developed at around 200 kilometres to the northwest of Hong Kong (Figure 1). While moving southeastwards, it became more organised. At 11.45am, when the band of thundery showers moved to the vicinity of Guangzhou (Figure 2), the Observatory issued a Special Weather Tip reminding the public that the thundery showers might affect Hong Kong in the afternoon and advised them to pay attention to weather changes. Then at 12.45pm, the Observatory issued the Thunderstorm Warning to alert the public of the occurrence of squally thunderstorms. The Amber Rainstorm Warning Signal was issued at 1.40pm.

As showers became heavier, the Observatory issued the Red Rainstorm Warning Signal at 2.20pm, the first for this year. The band of thundery showers moved across Hong Kong rapidly. The Observatory issued the Amber Rainstorm Warning Signal at 3.20pm to replace the Red Rainstorm Warning Signal, and cancelled all rainstorm warning signals at 4.45pm.

During the rainstorm episode, more than 40 millimetres of rainfall were generally recorded over the territory. Showers were heavier over the western and northern parts of the New Territories and Lantau Island where rainfall exceeded 60 millimetres (Figure 3). Heavy showers were also accompanied by squalls. Gusts exceeding 100 kilometres per hour were recorded at Green Island and North Point.

Figure 1 Radar imagery at 9 am, April 20, when the band of thundery showers were located at around 200 kilometres northwest of Hong Kong
Figure 2 Radar imagery at 11.48am, April 20, the band of thundery showers moved to the vicinity of Guangzhou
Figure 3 Rainfall distribution map of Hong Kong on April 20 (up to 6 pm)