Well, it's not always just cool here. It gets very hot and dry and most people wear shorts and light t-shirts or blouses. During Winter we all dress with heavier warm clothes like every one else. But this also depends on where you are talking about on the West coast as the weather here and in Washington state is quite different.
Well. It depends where you live...If you live further north like in organ. Washington and B.C.... It's usually rainy in the fall and winter months. A rain jacket and a good pair pants will do you well.
If you do to California. It's sunny all year round... I'm sure the cooler months (fall and winter) you can just wear a sweater or a light jacket and maybe even shorts and sweat pants.