Yes, you are going to need mosquito repellent. They are vicious in August. So are the tiny stinging gnats referred to locally as "no see 'ums" as they are to tiny to notice until they've done their damage.
The rain will NOT cool things off, regardless of what you may be told. You'll need a poncho or something along those lines, but know that the whole time you have it on you will be melting from the heat. The real purpose of a poncho is to sit on once the rain has stopped and everything is covered in water. If you can find a flannel lined rain jacket, that would be the most comfortable as it keeps the plastic/vinyl from your skin. However, wherever the rain jacket ends, you'll be soaked. An umbrella is good as long as the wind doesn't get fierce. I use one because I wear glasses and HATE getting them all wet and blurry.
You'll want a small towel or facecloth with you too. They come in handy for wetting or wrapping around ice to use to cool off.
I carry a pretty precisely packed backpack for the summer months, just because of the heat, humidity, rain and stingin/biting bugs.
Be sure your footwear is something you don't mind getting wet, because you will definitely get your feet wet during the rain. I used to use Crocs, but don't anymore. I just carry a small pair of thongs and when the rain starts switch to them quickly to keep my shoes/trainers/socks dry.