The observatory says it could issue a higher typhoon warning signal between 11pm on Tuesday and 3am on Wednesday as Super Typhoon Ragasa gets closer to Hong Kong.

The number eight signal was issued at 2.20pm on Tuesday.

Forecasters said a higher alert would depend on the storm's distance from the city and the change in local wind conditions.

They said the territory was already seeing some gale force winds on high ground.

Ragasa is expected to be at its closest to the SAR on Wednesday morning.

"The weather is expected to deteriorate rapidly overnight, with winds strengthening quickly. The weather will be persistently adverse tomorrow. Gale to storm force winds will prevail locally, and winds will reach hurricane force offshore and on high ground at first," the observatory warned.

"Seas will be phenomenal with swells. There will be overtopping waves over the shoreline, which will be particularly significant along the eastern and southern coasts."