Observation and assessment (Early September 2019)
1. Up to 9 September 2019, the rainfall recorded at the Hong Kong Observatory so far this year is 2189.1 mm, about 180 mm above the normal value for the same period. Meanwhile, four tropical cyclones entered the 500-km range of Hong Kong and necessitated the issuance of tropical cyclone warning signals, namely Mun, Wipha, Bailu and Kajiki*.
2. In the past month or so, the cooling trend of the central and eastern equatorial Pacific continued. Overall speaking, sea surface temperatures of the region returned to normal in July 2019, indicating that the El Niño had come to an end. Based on the latest oceanic observations as well as forecasts by a number of climate models around the world, sea surface temperatures of the central and eastern equatorial Pacific are expected to remain normal this autumn (September-November).
3. Based on the statistical analysis of rainfall and tropical cyclone activities against different ENSO states, latest climate model predictions and forecasts based on statistical methods, rainfall in Hong Kong is expected to be normal to below normal for September-December this year while the number of tropical cyclones entering the 500-km range of Hong Kong is expected to be near normal. Taking into account the actual observations so far this year, the overall annual rainfall is expected to be normal to above normal and the annual number of tropical cyclones entering the 500-km range of Hong Kong is expected to be near normal.
*While Podul was more than 500 km away from Hong Kong, it also necessitated the issuance of tropical cyclone warning signal by the Hong Kong Observatory.
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.