The Observatory raised the Number 3 signal at 1:40 p.m. Monday as tropical cyclone Sarika gathered strength.
It warned that Hong Kong may see more severe weather by the end of this week with another typhoon heading this way.
Senior scientific officer Song Man-kuen said Haima, now a severe typhoon in western north Pacific, will bring unsettled weather over Hong Kong and nearby areas over the weekend.
She said Sarika, now south of Hong Kong, is forecast to move west-northwest at about 20 kilometers per hour and intensify gradually.
Its outer rainbands are expected to move close to Guangdong province and affect its coast in the afternoon.
The Observatory raised the Number One signal on Monday morning and the government canceled all kindergarten classes and schools for physically and mentally-disabled children.
In Vietnam, at least 24 people were killed after Sarika drenched the central parts of the country, the Associated Press reported.
Heavy rains of up to 90 centimeters on Friday and Saturday submerged 125,000 homes in the region, temporarily disrupted the North-South Highway and damaged infrastructure, crops and fish farms.
In the Philippines, the storm left two people dead and more than 150,000 displaced, the report said.
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