This screenshot from the website of Hong Kong Observatory shows the forecast track of tropical cyclone Rai.

HONG KONG – The Hong Kong Observatory issued the typhoon signal number 1 on Monday in its first December typhoon signal since 1974.

The HKO issued the Standby Signal No 1 at 11:30 am, adding that the last time a tropical cyclone signal was issued this late in the year was in 1946.

The T1 signal was raised as tropical cyclone Rai came within 800 meters of Hong Kong and is expected to remain in force for the most of the day, said HKO Acting Senior Scientific Officer Yeung Kwok-chung.

Tropical cyclone Rai will edge closer to the coastal area of Guangdong today but will continue to weaken. It will be closest to Hong Kong around noon tomorrow, said HKO Acting Senior Scientific Officer Yeung Kwok-chung

Rai will edge closer to the coastal area of Guangdong today but will continue to weaken. It will be closest to Hong Kong around noon tomorrow. Under the combined the effect of Rai and the northeastern monsoon, winds will be strong offshore and on high ground, Yeung said.

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According to the HKO forecast, it will be cool and cloudy with rain tomorrow. There will still be one or two rain patches and temperatures will rise gradually midweek.

An intense winter monsoon is expected to reach the southern coast of the Chinese mainland on Christmas Day. It will be appreciably cooler over the weekend, with the mercury dropping to as low as 12 C next Monday.

The only other time that Hong Kong hoisted a No 1 typhoon signal in December was in 1974, when Super Typhoon Irma struck.

READ MORE: HKO: Weekend temperatures to drop as tropical cyclone nears

While extreme weather events have become more frequent due to climate change, its links to December typhoons are unclear and need further research, Yeung said.