A warmer and wetter Christmas is expected as the city is facing the effects of global warming and a strong El Nino.
The Observatory forecast for winter - December 2015 to February 2016 - shows the city is expecting temperatures and rainfall levels that are higher than normal.
Scientific Officer for the Observatory, Tong Hang-wai, told Young Post El Nino - the climate cycle of warming waters in the Pacific Ocean that affects weather patterns - is part of the reason for the unusual climate this winter.
Hong Kong's summer this year is also the hottest since the Observatory began keeping records in 1884. On August 8, the temperature was a record 36.3 degrees Celsius. The average temperature was a toasty 29.5 degrees. Autumn, too, is likely to go down as the hottest on record.
Average temperatures between January and October were 24.7 degrees. That means it was as hot as 2002 in that period, which was the hottest year ever. Every month this year the weather was warmer than normal, with May and June more than 1.5 degrees above average. The Observatory said this year may very likely be the warmest since historical records began.
It looks like the average temperature in November will also be at its highest since 1998.
The World Meteorological Organisation has forecast that 2015 will be the hottest on record globally.
Today, world leaders are meeting in Paris, France, to talk about climate change. They want to stop the rise in global temperatures and avoid the worst effects of global warming.
Climate change isn't just bad for the planet, it's bad for our health, too. Hotter temperatures and higher levels of air pollution are dangerous, said Professor Amos Tai Pui-kuen, of Chinese University.
"Vulnerable groups such as the elderly would be at higher risk of heat stroke," he said.
"And some researchers expect cases of mosquito-borne diseases like dengue fever in the city."
Temperatures in Hong Kong will drop gradually this week.