The Observatory issued the Standby Signal Number 1 on Sunday evening.
The signal was issued at 9.20pm, indicating that a tropical cyclone has come within 800km of Hong Kong. It will remain in force before 5am.
Forecasters said Tropical Cyclone Nesat will edge closer to the coast of Guangdong on Monday and be around 400km from Hong Kong.
They predicted the storm and the combined effect of a northeast monsoon will bring showers and winds to the SAR, with temperatures dipping to a low of 19 degree Celsius on Wednesday.