Don't trust the weather.
About 2 years ago
I went to Paris and near Frankfurt, Germany over the summer. I was watching the weather pretty close before I left, and they stayed very close, and very warm, in temperature in the beginning on June (I went at the end of June). I didn't look too closely at what the weather was going to be while we were there, just stuck with what it was. I brought clothes ready for summer. In Paris the clothes worked, it was very warm the whole time, but the dresses worked. By the time we got to Germany though the temperature there had drastically dropped to the sixties in Fahrenheit (between fifteen and twenty Celsius). I had only brought one pair of jeans and a few pairs of leggings. I had to bundle up in layers and layers of clothing, which wasn't always the easiest, or most comfortable. Make it easy on yourself with not having to wear multiple tank tops to try to stay warm.