Build clothing around the specific regions, altitude, and day-night temperature change in your route. A light layer and weatherproof outer option are often more useful than extra heavy items.
Read the forecast close to departure
Seasonal averages help with broad planning, but the final week is when you should adjust footwear, rain protection, and day bag choices.
Pack for the activity you will actually do
A city-focused trip, a mountain route, and a train-heavy itinerary need different footwear and layers. Do not pack for an imagined version of the journey.