In December even HK can be cool, overcast , windy and damp feels cold even at 68F (mid to high teens C if the wind is from the north) and I've experienced day temperatures getting no higher than 8°C last winter there and believe me that feels cold as hell in a strong to gale force constant wind with drizzle...
From the south it could be in the 80's F or mid to high 20's C and be very nice. I've experienced both during winter there, within a few hours, the weather can change 20C in a matter of hours due to wind direction. Don't count on it being warm is the main message despite how far south you are.