Explainer | Hong Kong bombarded with scores of rainstorm warnings over past few days. Why?

Hong Kong has been battered by heavy rainstorms, with the Observatory issuing the black rainstorm signal, the highest in a three-tier warning system, twice over two days. The second black signal lasted for more than 11 hours on Tuesday as downpours triggered significant flooding in areas such as Tseung Kwan O, Central and Western district, and Tai Po. Floodwaters reached knee height at one point on Tuesday, with muddy water flowing like waterfalls in some locations. The Post looks at how such weather can impact the city and why some rainstorm warnings can last a whole day. 1. What happened? The Observatory issued a black warning at 5.50am on Tuesday, the second time in six hours at 5.40am, as heavy showers brought the city to a standstill and triggered the suspension of schools and public services. The Drainage Services Department said that, as of 3pm, it had confirmed 29 cases of flooding across the city, spanning areas in Sha Tin, Tseung Kwan O, Sai Kung, Tai Po and Central and Western district. Floodwaters submerged vehicles parked at King Yin Lane in Tseung Kwan O, with authorities forced to deploy a pumping robot to clear the area. Queen Mary Hospital in Pok Fu Lam at one point suspended its accident and emergency services for more than two hours on Tuesday morning and had to divert patients to other public hospitals after the area outside its main block became seriously flooded. As of 2.30pm, the Geotechnical Engineering Office received 15 landslide reports across the city, including in Sai Kung, Tai Po, Kwai Chung, Lantau Island, Wan Chai and Southern district. 2. Why so many warnings? Monday also saw the Observatory issue four amber warnings in one day, with the last one issued at 9.45pm. It was later upgraded to red and then black over the next two hours. The black signal lasted for nearly 2½ hours and was downgraded to red and then amber for about three hours before it was issued again at 5.50am and stayed up for more than 11 hours. According to the Observatory, the three-tier system depends on whether heavy rain has fallen or is expected to fall generally over Hong Kong in an hour, with the threshold at 30mm for amber, 50mm for red and 70mm for black. Choy Chun-wing, a senior scientific officer with the forecaster, said the city’s continuous rainfall was due to the impact of an active southwest monsoon and upper-air disturbances. He noted that the associated intense thundery showers had prompted the issuance of the first black warning, with the rainstorm weakening for several hours before strengthening again and triggering the second black signal. The Hong Kong Observatory said that the continuous heavy rainfalls over the past few days and the city’s second-longest black rainstorm signal in history were driven by several factors. In an article published late on Tuesday, the forecaster said that the southwest monsoon usually brought in warm, moist air and could lead to heavy rain sometimes. “As the southwest monsoon gradually strengthened last Saturday, it delivered a continuous supply of moisture to the coast of southern China,” it said. “Together with the approach of an upper-air disturbance in the last few days and good upper-air divergence, conditions were created to allow low-level air to rise more easily to the top of the atmosphere. “This was highly favourable for the development of heavy rain.” An upper-air disturbance refers to a region in the upper atmosphere where the airflow direction undergoes significant changes. It also could indicate the possibility of unsettled weather nearby. Former Observatory assistant director Leung Wing-mo said Hong Kong had been affected by a strong rainband moving from west to east, resulting in the city being battered by heavy rain for a prolonged period of time. He also said the issuance of a rainstorm warning involved not only meteorology but an emphasis on avoiding any public confusion. Leung noted that as schools were normally suspended when such signals were issued, people normally did not need to travel back to their workplace. 3. Why are some areas hit harder? The Observatory’s Choy said rainfall was a stochastic process, meaning its patterns were random. Some areas of the city, such as Lei Yue Mun along the coast, generally experienced flooding during typhoons rather than rainstorms, as storm surges brought seawater to the inland. Leung explained that low-lying areas and catchment areas that could gather water flowing downhill had a higher risk of flooding, adding that it also depended on whether drainage was blocked. The Drainage Services Department said on Tuesday that it had deployed about 180 emergency teams to handle flooding cases and repeatedly inspected around 240 spots that were prone to flooding. The city issued a landslip warning at 2.15am that day and urged the public to stay away from steep slopes and retaining walls. According to the Civil Engineering and Development Department, landslides are a common natural hazard under the influence of seasonal rainstorms, due to Hong Kong’s hilly terrain and large number of steep man-made slopes. Raymond Chan Kin-shek, former head of the Geotechnical Engineering Office, said the chances of landslides had increased as the continuous rainfall in the past few days had increased water pressure in the soil and could have damaged slope structures. 4. What precautions can I take? The Drainage Services Department urged the public to pay attention to the weather forecast and have stop-boards and sandbags ready at the entrances of their homes. It also advised residents to leave basements immediately when flooding occurred. Leung warned that flash floods could occur during and after the rain, and even under fine weather, as it could take time for water to gather and flow downhill. He said the public should always pay attention to the weather forecast before hiking and refrain from trekking near streams in the summer. The Civil Engineering and Development Department said residents living near slopes should find safety on higher floors, while pedestrians should stay away from slopes and motorists should use alternative routes. It also said the public should watch out for signs of landslides, such as new large cracks or subsidence in slopes and water flowing from slopes suddenly changing from clear to muddy.

Explainer | Hong Kong bombarded with scores of rainstorm warnings over past few days. Why?

The Observatory issued its second-longest black signal on Tuesday, after warnings sent out at all hours in recent days

1. What happened?