A warm 2017 with frequent tropical cyclone activity

Hong Kong (HKSAR) -      According to the World Meteorological Organization's preliminary assessment, 2017 is very likely to be one of the world's three warmest years on record. The sea-ice extent over the Arctic was well below average throughout 2017 and reached record-low levels for the first four months of the year. Various extreme weather events wreaked havoc in many parts of the world in 2017 including cold spells in parts of Argentina, southeastern Australia and the Gulf region in the Middle East; heatwaves in Chile, Argentina, eastern Australia, Turbat in Pakistan, Iran, Oman, Shanghai in China, California in the United States and the Mediterranean region; and droughts in Somalia, Kenya, Ethiopia, South Sudan, Uganda, Italy, Spain, Portugal and the Korean Peninsula. High temperatures and drought also contributed to destructive wildfires in Chile, eastern Australia, New Zealand, South Africa, central Portugal, the United States, Canada and the Mediterranean region. Extreme rainfall triggered severe flooding and landslides in Sierra Leone, southern Colombia, the Indian sub-continent, Sri Lanka, many parts of Peru, the Yangtze River basin in southern China and the western United States. High winds, storm surges and torrential rain induced by tropical cyclones brought severe damage and heavy casualties to the state of Texas in the United States, the Caribbean islands, Mozambique, Zimbabwe, Madagascar, Australia, New Zealand, Japan, the Philippines, Macao and the Pearl River Delta region in southern China. 

     Over the central and eastern equatorial Pacific, sea surface temperatures were slightly warmer in the first half of 2017. However, a cooling trend was observed in the second half of the year with the sea surface temperatures of the region falling below the La Niña threshold in November and December.

     The weather in Hong Kong was warmer than usual in 2017, with an annual mean temperature of 23.9 degrees, 0.6 degrees above the 1981-2010 normal (Note 1) (or 0.9 degrees above the 1961-1990 normal) and among the third warmest years since records began in 1884. In particular, the monthly mean temperatures of 18.5 degrees for January and 29.0 degrees for September both ranked the highest for their respective months since records began in 1884.

The daily maximum temperature of 36.6 degrees recorded at the Hong Kong Observatory on August 22 was the highest on record since records began in 1884. There were 41 Hot Nights (Note 2) and 29 Very Hot Days (Note 3) in Hong Kong in 2017, ranking the highest and the sixth highest on record respectively.
     There were nine Cold Days (Note 4) in the year - eight days fewer than the 1981-2010 normal. The lowest temperature recorded at the Observatory in the year was 9.8 degrees on December 18.

     The year 2017 brought more rain than normal to Hong Kong. The annual total rainfall was 2572.1 millimetres, a surplus of about 7 per cent compared to the 1981-2010 normal of 2398.5 millimetres (or about 16 per cent above the 1961-1990 normal). The number of days with thunderstorms reported in Hong Kong was 48, nine days more than the 1981-2010 normal. Affected by a trough of low pressure, there were torrential rain and squally thunderstorms on May 24 in Hong Kong, which led to the issuance of the only Black Rainstorm Warning Signal in the year.

     A total of 32 tropical cyclones occurred over the western North Pacific and the South China Sea in 2017, more than the long-term (1961-2010) average of around 30. Twelve tropical cyclones reached typhoon intensity (Note 5) or above during the year, less than the long-term average of about 15, and four of them reached super typhoon intensity (maximum 10-minute wind speed of 185 km/h or above near the centre). Seven tropical cyclones necessitated the issuance of tropical cyclone warning signals, slightly higher than the long-term average of about six in a year.

The Hurricane Signal No. 10 was issued during the passage of Hato in August, while the No. 8 Gale or Storm Signal was issued five times for the passages of Merbok in June, Roke in July, Hato and Pakhar in August, and Khanun in October, tying with the records of 1964 and 1999.
     A detailed description of the weather for individual months is available on the Monthly Weather Summary webpage:
http://www.weather.gov.hk/wxinfo/pastwx/mws.htm" target="_blank">www.weather.gov.hk/wxinfo/pastwx/mws.htm.

A detailed version of the Year's Weather for 2017 with some significant weather events in Hong Kong is available at: http://www.weather.gov.hk/wxinfo/pastwx/ywx.htm" target="_blank">www.weather.gov.hk/wxinfo/pastwx/ywx.htm.
Note 1: Climatological normals for the reference period of 1961-1990, 1971-2000 and 1981-2010 are available at: http://www.weather.gov.hk/cis/normal_e.htm" target="_blank">www.weather.gov.hk/cis/normal_e.htm. Climatological normals of 1981-2010 are referenced in the text unless otherwise stated.

Note 2: "Hot Night" refers to the condition with the daily minimum temperature equal to or higher than 28.0 degrees.

Note 3: "Very Hot Day" refers to the condition with the daily maximum temperature equal to or higher than 33.0 degrees.

Note 4: "Cold Day" refers to the condition with the daily minimum temperature equal to or lower than 12.0 degrees.

Note 5: Information on the classification of Tropical Cyclones is available at: http://www.weather.gov.hk/informtc/class.htm" target="_blank">www.weather.gov.hk/informtc/class.htm.

Published on: 2018-01-08

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