When in the mountains in Autumn the weather can be very variable, there can be all four seasons in a day which means being prepared for every weather condition is very important.
At this time of year I make sure all my spare clothes are kept dry in drybags and I always make sure I have plenty of layers including, A waterproof, an insulated jacket, a long sleave top, waterproof trousers, spare gloves, hat, waterproof over gloves, a first aid kit and a bivi bag.
It is also vital to make sure someone knows the route you are going to take. I make use of a Garmin watch so someone can track me at all times when out in the mountains.
My final piece of advice would be to know all your escape routes off the mountains and to know how to get back to your car once you have taken these escape routes. This is often missed now we rely on GPS watches for route finding rather than an old fashioned map and compass. In Snowdonia I like to know where the Sherpa buses run so I can plan a route back to the start of my run, it is also useful to know when the last bus runs :).
You can see my YouTube Video of this blog post where I spend sometime in Snowdonia in mid October 2024. Running in the Eryri (Snowdonia) Mountains when the seasons change. Make sure you are prepared.