If Usagi passes over or just south of Hong Kong, storm surge could be a massive threat – somewhat caused by dome of water "sucked up" by low pressure, but chiefly with water piled inshore by the powerful winds.
Long time since a mighty storm surge in Hong Kong, but historically not rare.
From account of Typhoon Wanda, in 1962:
Destructive tidal surges were reported in Tolo Harbour in 1874, 1906, 1923 and 1937 but only the latter is considered here. The Typhoon of 1937 occurred only one day later in the year on September 2. The centre passed about 7 miles south of the Royal Observatory at 4.30 a.m. compared with "WANDA" which passed 10 miles south of the Royal Observatory at 10.45 a.m. Tides in the harbour in 1937 were estimated to be 6 ft. above normal and the tidal wave in Tolo Harbour was reported to be about 30 ft. high. However there were no tide gauges in Tolo Harbour and the stationmaster at Sha Tin said that there was practically no difference between the two typhoons in the maximum heights reached at the Railway Station. It has been(i) estimated that the high water mark in Tai Po left by the 1937 typhoon was about 20 ft. 6 inches above chart datum.
The main difference between the two typhoons is that the 1937 typhoon occurred in darkness and the flooding was totally unexpected. Fatalities were estimated at 11,000 compared with 127 in "WANDA".
August 27 to September 2, 1962