Even if you check the weather report and it says sun, it's a good idea to bring cold or rainy weather outfits too. Our weather can shift and it can get breezy/chilly evenings during summer at times. Late Sept. gets less daylight and thus less daytime warmth vs. what we have right now.
We rarely get intense, heavy downpours of rain, in contrast to TV/movie stereotypes. What's more typical is gray/ overcast sky with drizzly rain off and on. Waterproof footwear or jackets are not usually a necessity. In fact, I really don't have 100% waterproof footwear and I've lived here my whole life.
Most locals don't use umbrellas (there are some areas that can get windy), but I use one.