The observatory on Monday said the Standby Signal No 1 will be issued at 10.20pm as Tropical Cyclone Toraji moves towards Hong Kong.

The storm has passed over the Philippine island of Luzon and will come within 800 kilometres of the SAR overnight, forecasters said.

"Toraji will move towards the vicinity of the western coast of Guangdong to the Pearl River Estuary in the next couple of days," the observatory said.

"Under the combined effect of Toraji and the northeast monsoon, winds over the region will gradually strengthen with squally showers."

The observatory said it will consider issuing the Strong Wind Signal No 3 later on Tuesday or early Wednesday.