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Hong Kong was lashed by heavy downpours as the local forecaster upgraded its rainstorm warning to red on Tuesday afternoon, with Lantau and other islands bearing the brunt of the unstable weather.

The signal was issued at 2.50pm and means heavy rain has fallen or is expected to fall generally in the city, exceeding 50mm in an hour and is likely to continue, according to the Hong Kong Observatory.

Over on Lantau Island, more than 90mm of rainfall per hour was recorded at Cheung Sha as of 11.45am, with the forecaster expecting the volume to increase amid a heightened risk of flooding.

More than 90mm of rainfall was also recorded on Lamma Island as of 3.20pm.

The Observatory earlier sent out an amber warning at 10am and told the public to brace for heavy showers.

The signal, along with a thunderstorm warning issued at 9.30am, was slated to end at noon. Both were extended to 3.30pm before the red rainstorm warning was issued.

The forecaster said the weather was the result of a trough of low pressure in a northern portion of the South China Sea and along the coast of Guangdong.

The weather was expected to weaken slightly on Wednesday, but would remain unsettled over the southern coast for the next couple of days, it added.

The city is expected to grapple with unstable weather until early next week as a result of another round of incoming low pressure.