Observation and assessment (latter part of August 2017)
1. Up to 31 August 2017, the rainfall recorded at the Hong Kong Observatory this year is 2249 mm, about 18% above the normal value for the same period. Meanwhile, five tropical cyclones entered the 500-km range of Hong Kong, namely Merbok, Roke, Haitang, Hato and Pakhar. Apart from Haitong, the others all necessitated the issuance of tropical cyclone warning signals of No.8 or higher.
2. Latest climate model predictions indicate that rainfall in Hong Kong is likely to be normal to below normal for the rest of this year while the number of tropical cyclones entering the 500-km range of Hong Kong is likely to be near normal. Taking into account the actual observations in January-August, the overall annual rainfall and the annual number of tropical cyclones entering the 500-km range of Hong Kong are likely to be normal to above normal.
The yearly numbers of tropical cyclones entering 500 km of and affecting Hong Kong are not necessarily the same. For example, even when a tropical cyclone comes within 500 km of Hong Kong, its weakening, change in movement or making landfall would not necessitate the issuance of warning signal. A tropical cyclone outside the 500-km range but bearing a large circulation or interacting with the northeast monsoon over southern China could affect Hong Kong and necessitate the issuance of warning signal. Since the long-term averages of the number of tropical cyclones entering 500 km of and affecting Hong Kong are roughly the same, the forecast of the former is, to some extent, indicative of the latter.