SANSHA -- Typhoon Trami is strengthening its intensity, and is expected to enter the South China Sea Thursday evening, according to the meteorological observatory in China's southernmost island province of Hainan.

The observatory forecast heavy rainfall and strong gales on the typhoon's path through Xisha and Zhongsha Islands, which will affect the city of Sansha.

Fishing boats and other vessels have returned to the harbor for shelter, and sandbags were used to reinforce the buildings in Sansha.

China's National Meteorological Center on Thursday morning renewed a blue typhoon warning to Trami, which has been listed as the 20th typhoon of this year.

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