TAIPEI, Aug 6 (Reuters) - Taiwan's weather bureau issued land and sea warnings on Thursday for a mid-strength typhoon, the most severe this year so far, with heavy rain and wind gusts up to 173 kph (107 mph) expected overnight before the storm heads to China.

Typhoon Morakot, centred 660 km (410 miles) southeast of Taiwan at 0030 GMT, was expected to reach northern Taiwan by early Friday with sustained winds up to 137 kph (85 mph), the Central Weather Bureau said on its website (www.cwb.gov.tw).

Morakot will grow to a category 4 typhoon, on a 1-5 scale, early on Friday, passing directly over the island's capital Taipei before reaching southeast China by Saturday at a weaker category 2, said forecasting website Tropical Storm Risk (www.tropicalstormrisk.com).

Typhoons regularly hit China, Taiwan, the Philippines and Japan in the second half of the year, gathering strength from the warm waters of the Pacific Ocean or South China Sea before weakening over land.

(Reporting by Ralph Jennings; Editing by Nick Macfie)

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