Tam said the high temperatures boosted convection, creating unstable air in the upper atmosphere, fuelling the thunderstorm.
Tom Grundy, editor of the independent Hong Kong Free Press, tweeted the exact point when the storm overtook 10,00 strikes, posting dramatic photographs of the lightning show sent in by members of the public.
By today the tally since the start of the storm was above 1140 strikes, said the Observatory, which is still maintaining a thunderstorm warning.
Film-maker Martin Williams posted a clip to YouTube on Sunday, showing the storm from Cheung Chau, with Hong Kong's lights in the distance.
Grundy later tweeted another batch of dramatic shots of the lightning show.
Most strikes hit the New Territories and Lantau Island, continuing well into Sunday.
Hong Kong PR manager Greg Cole posted a picture on Twitter showing the storm lighting up the city centre.