If you're not used to the altitude, it's a real thing to be taken seriously. An early trip to see what's it's like will help. It usually warms up great on a sunny day, so I wear a light weight, long sleeve shirt with hood, all kinds of outdoor brands and hunting brands make them. It can be super cold too, so layers are key. If the forecast is for sun, it's likely you'll have several layers on to keep warm early morning, then as you get moving you'll be stripping down and need to put them in a pack, because they'll likely go back on if a storm rolls in or when evening comes.