Thunder and lightning
I may have missed these amongst the good replies above.
On the central highland plateau of Southern Africa we in one of the lightning hot-spots of the world, that is, one of the great Flash/Bang play grounds. A biker is not protected by a Faraday Cage like a motorist is so get into safe protection if at all possible. If you can hear the thunder you are at risk, both before and after the storm. If there is no protection get off the bike and squat down with your feet together. If you have to ride enjoy the fireworks and be aware of sudden gusts of killer wind.
A huge problem and danger here is the fast breeding pothole, invisible under a few millimetres of water and up to a metre deep. Enjoy the flight as you go over the bars, but at least the road will be slippery when you hit it.
Hail can be lethal and fall during a storm without warning. It can drift in the same way that snow does and make ice cakes without any traction. ( I have ridden in snow only once so I shall not comment on that ).
I find that the main problem is not the rain, but the drivers here go totally mad when it rains and all thoughts of safe driving ( if there were any in the first place ) are lost and chaos is supreme.