As an old bat, I tend to go by what I've experienced in the past. We all talk about climate change, but I haven't noticed that much either in the Rockies or in the Cascades during my lifetime. Yes, the glaciers are melting, but when the weather gets extreme, it isn't any different from what I've already experienced.

If I were going to a completely strange, I'd read up as much as I could. I'd look at the record highs and lows and, for the latter, consider maybe 5* F higher. The "normal" or "average" don't mean much; about half the time the temperature will be colder or hotter! I'd also look for as much info as I could about precipitation patterns. This is especially important in mountainous areas where you don't want to be caught on a high ridge in a thunderstorm.

I may be too pessimistic, but when I stop hiking, I get cold easily and don't enjoy it!