Typhoon Nesat -- which strengthened into a moderate storm Friday -- is already bringing crashing waves and strong winds to the east coast, where it is due to make landfall Saturday night.
At 0215 GMT, Nesat was 210 kilometres southeast of Hualien county, packing gusts of up to 137 kilometres an hour.
Taiwan's weather bureau warned of torrential rain in the eastern and southern counties, with the tropical storm expected to dump as much as 900 millimetres of water in some areas.
Schools and work were cancelled for two outlying islands, while several eastern counties were expected to follow suit in the afternoon.
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More than 36,000 soldiers are on standby to help with disaster relief.
Although Nesat is Taiwan's first typhoon this year, the island was pounded by heavy rains last month.
At least one person died in June floods in the north of the country, after 600 millimetres of rain fell in under 11 hours in some areas.
Taiwan's government has stepped up preventive measures against tropical storms since typhoon Morakot left more than 400 people dead in its wake in 2009.