Rain

There is always a 60% chance of rain here in Hong Kong. Especially if it is June, July, or August. It will probably rain every day for a week, then maybe only 3 days the next week, but don't be surprised if it rains every day the whole time you're here.
If there is a typhoon, go to 7-11, buy some Tsingtao or San Miguel, and drink. That's what everyone else does. Nothing will be open, don't even try. Even if it's not raining that much (you think) everyone else doesn't have to work so nothing will be open. When the storm goes back to Level 3, within two hours everyone has to be back at work. So, be prepared for the crazy traffic jam in the middle of the afternoon and wait, then eventually life will return to normal :)
Even if it is raining or likely to rain, don't be discouraged. Buy a good umbrella and carry it with you all the time. It usually rains in the morning and then the sun might even come out in the afternoon so just take advantage of the sun when you find it.