I come originally from Scotland. We say that there is no such thing as bad weather, only clothing unsuitable for the weather at that time.
The great trick is to stay dry. Wet can get in from the outside, but also from the inside, because if you are too warm you will sweat. Whatever it says on the label, no clothing will breathe enough to completely wick away all moisture.
Davebugg has much good advice from experience in serious walking. Search for his posts and learn, it will be less painful than trial and error.
BTW, there may be bad weather at any altitude, on any route. I remember Hôpital de Orbigo in late may as having heavy wet snow, which fell all day, and clung to everything it touched. Not very deep, as it melted more or less as it fell, but with a nasty cutting wind. That was a good day to stop asap.
My take on weather like that is to do a short day, then make sure everything is dry as possible for the next day. Maybe take a rest day, even if it's not an interesting place. Because sometimes you won't have the right clothing.