Gregorian/Lunar Calendar
Traditional Chinese Simplified Chinese
  • The Weather of September 2018

  • 3 October 2018

   September 2018 was marked by the ferocious strike of severe typhoon Mangkhut which necessitated the issuance of the No. 10 Hurricane Signal in Hong Kong for 10 hours on 16 September. This is the second longest duration of No. 10 Hurricane Signal in Hong Kong since 1946, just next to the record of 11 hours set by Typhoon York in 1999. Mainly attributing to the rainfall brought by Mangkhut, the month was wetter than usual. The monthly rainfall was 383.3 millimetres, about 17 percent above the normal of 327.6 millimetres in September. The accumulated rainfall recorded in the first nine months of the year was 1973.3 millimetres, a deficit of 12 percent compared to the normal of 2233.1 millimetres for the same period. The month was also hotter than usual with a mean temperature of 28.0 degrees, 0.3 degrees above normal of 27.7 degrees.

   Under the influence of a trough of low pressure, the weather in Hong Kong was mainly cloudy with occasional heavy showers and thunderstorms on 1 September. More than 30 millimetres of rainfall were recorded over most parts of the territory, and rainfall even exceeded 70 millimetres over Sai Kung and Cheung Chau. With the trough of low pressure weakening gradually, there were sunny periods and a few showers on 2 - 3 September. An anticyclone aloft southern China brought generally fine weather to the territory on 4 September.

   Under light wind conditions, it was very hot with sunny periods on 5 - 7 September. High temperatures also triggered thundery showers in the afternoon over Sai Kung and parts of the New Territories in these few days. As a cold front moved across southern China on the night of 7 September, local weather became mainly cloudy with some showers on 8 September. More than 20 millimetres of rainfall were recorded over parts of the urban areas, and rainfall even exceeded 40 millimetres over Happy Valley. The northeast monsoon associated with the cold front brought a few showers and slightly cooler weather to Hong Kong on 8 - 10 September.

   Meanwhile, an area of low pressure over the Luzon Strait intensified into a tropical storm and was named as Barijat on 11 September. It moved westwards across the northern part of the South China Sea on the next day. Barijat then moved across Leizhou Peninsula and weakened gradually on 13 September, and dissipated over inland Guangxi afterwards. Under the influence of the outer subsiding air of Barijat, the weather of Hong Kong was generally fine on 11 September. Local weather became showery and windier on 12 September when Barijat skirted past to the south of Hong Kong. With winds subsiding gradually, apart from a few showers at first, there were sunny periods on 13 September.

   Meanwhile over the western North Pacific, Super Typhoon Mangkhut tracked northwestwards on 14 September and made landfall over Luzon in the small hours of 15 September. After crossing the northern part of Luzon, Mangkhut continued to track northwestwards quickly across the northern part of the South China Sea on 15 September, edging towards the coast of Guangdong. Mangkhut weakened into a severe typhoon on the morning of 16 September and skirted about 100 km south-southwest of Hong Kong in the afternoon. It made landfall over the vicinity of Taishan of Guangdong before dusk and moved into western part of Guangdong. Mangkhut degenerated into an area of low pressure over Guangxi the next night.

   Locally, the outer subsiding air of Mangkhut brought generally fine weather to Hong Kong on 14 – 15 September. It was also very hot on 15 September with the temperature at the Hong Kong Observatory soaring to 35.1 degrees, the highest of the month and the second highest on record for September. With the approach of Mangkhut, local winds strengthened on the night of 15 September. The weather in Hong Kong deteriorated rapidly during the passage of Mangkhut on 16 September. The destructive storm to hurricane force winds, severe storm surge and squally heavy rain associated with Mangkhut ravaged the territory and caused extensive damages to Hong Kong on that day, including serious flooding in many coastal and low-lying areas, substantial damages of coastal structures and buildings, huge amount of fallen trees, many reports of smashed windows or glass curtain walls, and interruptions of water and power supply in some places. Over 450 people were also injured during the stormy weather. Traffic and transportation services were also seriously affected on 16 – 17 September. The maximum 60-minute mean wind speeds recorded at Waglan Island and Cheung Chau were 161 km/h and 157 km/h respectively. Both are the second highest record at the corresponding stations. The storm surge induced by Mangkhut resulted in unusually high water level in many parts of Hong Kong. The water levels at Quarry Bay of the Victoria Harbour rose to a maximum of 3.88 metres above Chart Datum on the afternoon of 16 September 2018, the second highest since 1954 and only lower than the record high of 3.96 metres above Chart Datum set by Super Typhoon Wanda in 1962. Moreover, the maximum storm surge (above astronomical tide) induced by Mangkhut at Quarry Bay was 2.35 metres which was the highest on record, breaking the previous record of 1.77 metres kept by Wanda in 1962. More than 100 millimetres of rainfall were generally recorded over Hong Kong, and rainfall even exceeded 200 millimetres over parts of the territory on that day. During the downpour, the temperature at the Hong Kong Observatory fell to a minimum of 23.6 degrees, the lowest in the month. With Mangkhut departing from Hong Kong, local winds subsided gradually on 17 September, but the outer rainbands associated with Mangkhut continued to bring squally showers to Hong Kong on that day.

   With the subtropical ridge extending westwards, apart from a few morning showers, local weather became fine during the day on 18 September. The weather over Hong Kong remained generally fine and hot on 19 – 22 September. Under light wind situation, isolated thunderstorms triggered by high temperatures also brought more than 10 millimetres of rainfall to parts of the New Territories on the afternoon 23 September.

   With the setting in of an easterly airstream, local weather became slightly cooler and mainly cloudy with occasional showers and thunderstorms on 24 - 25 September. Showers were heavy on the morning of 24 September with more than 30 millimetres of rainfall generally recorded over the territory and rainfall even exceeding 70 millimetres over Tai Po, Kwai Tsing and Kowloon. As the easterly airstream moderated gradually, local weather was marked by a mixture of sunshine and showers on 26 - 27 September. Apart from one or two morning showers on 28 and 29 September, the weather in Hong Kong became generally fine and dry towards the end of the month as affected by the northeast monsoon.

Radar imagery of severe typhoon Mangkhut at 10:00 a.m. on 16 September 2018
Radar imagery of severe typhoon Mangkhut at 10:00 a.m. on 16 September 2018



Storm surge and huge waves induced by Mangkhut at Heng Fa Chuen
Storm surge and huge waves induced by Mangkhut at Heng Fa Chuen



Bamboo scaffolding at Tsim Sha Tsui collapsed during the strike of Mangkhut
Bamboo scaffolding at Tsim Sha Tsui collapsed during the strike of Mangkhut



Glass curtain walls at Hung Hom were severely damaged during the passage of Mangkhut
Glass curtain walls at Hung Hom were severely damaged during the passage of Mangkhut



Fallen trees at Kwun Tong during the strike of Mangkhut
Fallen trees at Kwun Tong during the strike of Mangkhut



A yacht was washed ashore by the powerful waves at Sai Kung during the passage of Mangkhut
A yacht was washed ashore by the powerful waves at Sai Kung during the passage of Mangkhut


     Five tropical cyclones occurred 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 1.7.  Monthly meteorological figures and departures from normal for September are tabulated in Table 2.
 

Warnings and Signals issued in September 2018

Table 1.1   Tropical Cyclone Warning Signals
Name of
Tropical Cyclone
Signal
Number
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
BARIJAT 1 11 / 9 1040 12 / 9 1220
3 12 / 9 1220 13 / 9 0410
1 13 / 9 0410 13 / 9 0740
MANGKHUT 1 14 / 9 2220 15 / 9 1620
3 15 / 9 1620 16 / 9 0110
8 NE 16 / 9 0110 16 / 9 0740
9 16 / 9 0740 16 / 9 0940
10 16 / 9 0940 16 / 9 1940
8 SE 16 / 9 1940 17 / 9 0520
3 17 / 9 0520 17 / 9 1440
1 17 / 9 1440 17 / 9 1910


Table 1.2   Rainstorm Warning Signals
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Amber 1 / 9 1255 1 / 9 1415
Amber 2 / 9 0740 2 / 9 0945
Amber 16 / 9 0910 16 / 9 1055
Red 16 / 9 1055 16 / 9 1850
Amber 16 / 9 1850 16 / 9 2230
Amber 24 / 9 1010 24 / 9 1200


Table 1.3   Landslip Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
16 / 9 1420 17 / 9 1120


Table 1.4   Thunderstorm Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
1 / 9 0755 1 / 9 1900
2 / 9 0645 2 / 9 1100
3 / 9 1055 3 / 9 1215
5 / 9 1655 5 / 9 1945
6 / 9 1240 6 / 9 1430
7 / 9 0520 7 / 9 0800
7 / 9 1155 7 / 9 1330
7 / 9 2225 8 / 9 0715
8 / 9 1340 8 / 9 1500
14 / 9 1720 14 / 9 1830
17 / 9 0430 17 / 9 0630
23 / 9 1545 23 / 9 1845
24 / 9 0705 24 / 9 1330
24 / 9 2005 24 / 9 2340


Table 1.5   Fire Danger Warnings
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Yellow 25 / 9 1000 25 / 9 1800
Red 29 / 9 0600 30 / 9 2045


Table 1.6   Very Hot Weather Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
5 / 9 1215 5 / 9 1700
6 / 9 0940 6 / 9 1800
14 / 9 0645 15 / 9 2315
21 / 9 1145 21 / 9 1800
22 / 9 1325 22 / 9 1645


Table 1.7   Special Announcement on Flooding
in the Northern New Territories
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
16 / 9 1125 16 / 9 2330
24 / 9 1110 24 / 9 1345


Table 2   Figures and Departures from Normal - September 2018
Meteorological Element Figure of the Month Departure from Normal*
Mean Daily Maximum Air Temperature 31.0 degrees C 0.9 degree above normal
Mean Air Temperature 28.0 degrees C 0.3 degree above normal
Mean Daily Minimum Air Temperature 26.0 degrees C 0.2 degree above normal
Mean Dew Point Temperature 23.7 degrees C 0.3 degree above normal
Mean Relative Humidity 78 % normal
Mean Cloud Amount 68 % 2 % above normal
Total Rainfall 383.3 mm 55.7 mm above normal
Number of hours of Reduced VisibilityΔ 5 hours 73.5 hours below normal§
Total Bright Sunshine Duration 183.3 hours 11.0 hours above normal
Mean Daily Global Solar Radiation 15.65 Megajoule / square metre 1.04 Megajoule above normal
Total Evaporation 101.5 mm 24.4 mm below normal


  Remarks : All measurements were made at the Hong Kong Observatory except sunshine, solar radiation and evaporation which were recorded at King's Park Meteorological Station and visibility which was observed at the Hong Kong International Airport.

  Δ

The visibility readings at the Hong Kong International Airport are based on hourly observations by professional meteorological observers in 2004 and before, and average readings over the 10-minute period before the clock hour of the visibility meter near the middle of the south runway from 2005 onwards. The change of the data source in 2005 is an improvement of the visibility assessment using instrumented observations following the international trend.
Before 10 October 2007, the number of hours of reduced visibility at the Hong Kong International Airport in 2005 and thereafter displayed in this web page was based on hourly visibility observations by professional meteorological observers. Since 10 October 2007, the data have been revised using the average visibility readings over the 10-minute period before the clock hour, as recorded by the visibility meter near the middle of the south runway.


  *   Departure from 1981 - 2010 climatological normal, except for number of hours of reduced visibility

  §   Departure from mean value between 1997 and 2017

daily values of selected meteorological elements for HK for September 2018

The percentile map of mean temperature of September 2018

  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



Rainfall Map For September 2018 (isohyets are in millimetres)