There has been a lot of heavy rain lately, with some flooding in the seaside areas, you should be fine with a windbreaker and umbrella at most. It will vary day to day this time of year. You probably won't need a hat in September - when I go in winter I take one but if I use it I am usually the only one in sight wearing one.
Do you go out at night? If so you will notice the temps drop, a leather jacket is best for late night prowling.