At 12:40pm (0440 GMT), the Hong Kong Observatory issued a strong wind advisory, with speeds of 41kph to 62kph (25.5mph to 38.5mph) expected. The storm, about 440km (273 miles) southeast of Hong Kong, will lash the city with heavy rains later Wednesday and into Thursday morning.

The Maritime Bureau of the Hong Kong-Zhuhai-Macau Bridge issued a typhoon-level II emergency response, closing it to ships.

The ninth cyclone of the season, Ma-on - also called Ma'an - will produce strong winds, heavy rain and waves of five to eight metres across the northern part of the South China Sea, according to local media. Storm surges are expected to affect several cities in Guangdong province, including Shenzhen, Chaozhou, Huizhou and Yangjiang.

The Shenzhen Meteorological Station upgraded the city's blue typhoon warning signal to yellow, and Zhongshan issued a yellow signal, suspending primary, secondary schools and child care education institutions classes throughout the city.

Forecasters warned coastal governments and relevant departments to make emergency preparations for storm surges.

China has a four-tier, color-coded weather-warning system, with red representing the most severe warning, followed by orange, yellow and blue.

Ma-on is expected to gradually weaken after landfall, weather forecasters predicted, but the winds and rain could bring much-needed relief to southern areas assaulted by days of scorching heat, which has pressured China's national electricity grid, damaged crops and caused wild fires.