The Observatory has warned that Super Typhoon Ragasa is not expected to weaken before coming closest to Hong Kong on Wednesday morning, hitting the city with "very strong" prevailing winds.

The path that Ragasa takes is expected to be different from Mangkhut, which wreaked havoc in the SAR in 2018.

Forecasters will issue Standby Signal No. 1 at 12.20pm on Monday and consider upgrading it to a No. 3 warning between 8pm and 10pm.

"Before Super Typhoon Mangkhut hit Hong Kong, it was moving in a westward direction east of the Philippines," said senior scientific officer Choy Chun-wing.

"At that time, Mangkhut made landfall before moving towards the South China Sea. And when a typhoon makes landfall, it will weaken to some extent.

"But according to the present forecast, Ragasa will not reach any significant landmass... so there's a considerable threat to the coast of Guangdong."

The typhoon – packing maximum sustained winds of about 230 kilometres per hour near its centre – will bring "very strong" southeasterly winds blowing directly from the ocean onto land as it edges close to the territory, Choy said.

He said Hong Kong is vulnerable to very strong winds this time around.

"Winds from the east or southeast will be blowing from the surface of the ocean, so there will not be much barriers and winds will be very strong," Choy said.

Ragasa will bring heavy rain and thunderstorms, along with a significant surge that may bring sea levels similar to those seen in Mangkhut as well as Typhoon Hato in 2017.