The exceptionally warm weather in the first quarter of 2019 continued in April 2019, mainly attributing to the warmer than normal sea surface temperature and stronger than usual southerly flow in the lower atmosphere over the northern part of the South China Sea. The mean minimum temperature of 22.9 degrees and mean temperature of 24.7 degrees were both 2.1 degrees above the corresponding normal and respectively one of the highest and second highest on record for April. The mean maximum temperature of 27.2 degrees was 2.2 degrees above the normal and the fifth highest on record for April. The monthly rainfall was 185.8 millimetres, about 6 percent above the normal of 174.7 millimetres. The accumulated rainfall recorded in the first four months of the year was 445.7 millimetres, about 33 percent above the normal figure of 336.1 millimetres for the same period.

Under the influence of a fresh to strong northeast monsoon, it was rather windy with a mixture of sunny periods and a few rain patches on the first four days of April. The northeast monsoon also brought the minimum temperature at the Hong Kong Observatory down to 18.9 degrees on 2 April, the lowest of the month. With the moderation of the northeast monsoon, local weather became generally fine and warm apart from some haze under light wind conditions on 5 April. Affected by a southerly airstream and an anticyclone aloft, it was generally fine and hot on 6 - 10 April.

With a trough of low pressure over Guangdong moving across the coastal areas on the night of 11 April, local weather became cloudy in the afternoon and there were heavy showers and squally thunderstorms that night. More than 30 millimetres of rainfall were recorded over the northern part of the New Territories, and the rainfall even exceeded 70 millimetres over parts of the area. Together with the setting in of a strong easterly airstream, it was cloudy and windy with a few showers and thunderstorms in Hong Kong on 12 April. The weather remained showery with a few squally thunderstorms in the next four days under the influence of the trough of low pressure lingering over the coastal areas of Guangdong and the northern part of the South China Sea. As the unstable weather moved to the south of Hong Kong, there were sunny periods on 17 April.

With the onset of an active southerly airstream over the coast of Guangdong, the weather of Hong Kong deteriorated with outbreaks of heavy showers and squally thunderstorms on the night of 18 April. The weather became even more unsettled in the next two days. There were heavy showers on 19 April with 75.8 millimetres of rainfall recorded at the Observatory, making it the wettest Good Friday on record. Rainfall even exceeded 100 millimetres over parts of the New Territories. On 20 April, a band of intense thunderstorm packed frequent lightning, heavy rain and severe squalls swept across Hong Kong in the afternoon and necessitated the issuance of the first Red Rainstorm Warning in the year. More than 40 millimetres of rainfall were generally recorded over the territory, and the rainfall even exceeded 70 millimetres over parts of the New Territories. The severe squalls also induced gusts exceeding 100 kilometres per hour in some places of the Victoria Harbour. The maximum gust recorded at King’s Park was 117 kilometers per hour. Excluding tropical cyclone cases, this is the highest record since the operation of the automatic weather station in King’s Park in 1992. Moreover, 9094 cloud-to-ground lightning strokes were detected in Hong Kong on that day, the highest on record for April since the launch of the lightning location system in 2005. During the inclement weather in that afternoon, a person died after being struck by lightning while hiking in Tai O and two persons were tragically drowned when a sampan capsized in seas off Tung Lung Chau. There were also reports of tree failure and scaffolding collapsing in different parts of the territory.

Affected by a strengthening anticyclone aloft, apart from a few showers, sunny skies emerged gradually on 21 and 22 April. The weather improved further and became fine and hot on 23 - 26 April. The daily mean temperature of 28.5 degrees on 25 April was one of the highest on record for April and the maximum temperature at the Observatory soared to 31.5 degrees on 26 April, the highest of the month. Under the influence of a strong easterly airstream and an upper-air disturbance, there were heavy showers and squally thunderstorms in Hong Kong on the afternoon of 27 April. As the easterlies abated gradually, it was mainly cloudy with the showers easing off during the day on 28 April. With the easterly airstream replacing by a southerly airstream, there were sunny periods on April 29. Under the influence of a trough of low pressure, local weather became cloudy with occasional showers and squally thunderstorms on the last day of the month.

There was no tropical cyclone over the South China Sea and the western North Pacific in the month.

Details of issuance and cancellation of various warnings/signals in the month are summarized in Tables 1.1 to 1.5. Monthly meteorological figures and departures from normal for April are tabulated in Table 2.

Remarks :

Extremely high: above 95th percentile Above normal: between 75th and 95th percentile Normal: between 25th and 75th percentile Below normal: between 5th and 25th percentile Extremely low: below 5th percentile Percentile and 5-day running average values are computed based on the data from 1981 to 2010

Extract of Meteorological Observations in Hong Kong for April 2019