The Observatory is set to issue the Standby Signal No 1 at 2.20pm on Wednesday, with Hong Kong expected to experience rain and thunderstorms.

"The area of low pressure over the central and northern parts of the South China Sea has intensified into a tropical depression," the weather forecaster said.

"Under the influence of the tropical depression, there will be a few showers [on Wednesday night] and more showers with squally thunderstorms [on Thursday]."

The tropical depression is expected to move northwest in the general direction of the vicinity of Leizhou Peninsula to the eastern part of Hainan, the forecaster added.