The first El Nino in more than four years could distort Hong Kong's weather, pushing back the arrival of typhoons and increasing rainfall later in the year, forecasters say.
"Sea surface temperatures are already getting warmer," Lee Sai-ming, Observatory senior scientific officer, said. "An El Nino could happen soon."
The Australian Bureau of Meteorology has predicted a more than 70 per cent chance of an El Nino this year - the first since 2010 - and scientists say it could be the strongest in decades.
Judging from previous occurrences, the weather anomaly - caused by a band of warm ocean water that periodically develops off South America's Pacific coast - could last a year or longer.
Australian meteorologists said sub-surface sea temperatures had already warmed by as much as six degrees in recent months, with the warm water rapidly travelling eastwards.
"I think this is very characteristic of a strong El Nino," Dr Wenju Cai, a climate expert at Australia's Commonwealth Scientific and Industrial Research Organisation, said.
While floods and droughts are expected elsewhere, an El Nino is expected to delay Hong Kong's typhoon season and increase rainfall in winter and spring.
While storms in the northwestern Pacific are likely to be stronger, there is no indication that those hitting Hong Kong will be more intense, Lee said, adding, however, that there was still a lot of uncertainty about the strength of the El Nino.
El Ninos have become increasingly unpredictable and associated with more extreme weather as industrialisation has gathered pace since the 1880s, according to research by Dr Li Jinbao, of the University of Hong Kong's geography department.
Li's work examining the width of tree rings from areas sensitive to the phenomenon showed that El Ninos had appeared more frequently and with greater ferocity in the past few decades.
Li's research is based on the fact that during an El Nino event, South America receives more rain, so trees in the region tend to grow faster and produce wider tree rings.
Meanwhile, Southeast Asia is typically hit by drought, so trees there have narrower rings.
One of the worst El Nino events happened between 1997 and 1998, when 24,000 people were killed in weather-related disasters. Economic losses amounted to more than US$100 billion.
Early last month, Hong Kong experienced its heaviest hourly rainfall for March since the Observatory's records began in 1884, leading to widespread flooding and large hailstones.
Additional reporting by Patrick Boehler and Reuters