We are doing a 2 1/2 week small group tour in October, traveling west from Bilbao, across to Santiago de Compostela, and then south as far as Porto. Will there be much variation in the weather from east to west or can we expect mild and damp the whole way?
It's a rainy area, but we also enjoy sunny weather. The truth is that you never know, our weather is extremely unreliable. Expect any kind of weather, today it´s been like rain, sun, rain, sun, wind, sun. And four different winds in just one day.
We have been to all the places you mentioned, but not in October.
The Atlantic coast of Portugal and Galacia province in Spain receive a bit of cooler weather than you find in the interior of those countries.
We did the Basque region in the Spring and the temps were a little warm, but nice. We did have some rain in San Sebastian that curtailed some of our touring.
By the way, we loved Porto and the Douro River Valley, which was exceptionally scenic. Recommend you take a cruise up the river, or in the alternative, take the train up river to Regua and Pinhao. The train runs along the north side of the river.
Ditto what Mikel said. Prior to the trip look up the weather in the places along the route. That’s about the best guess you’ll get until you’re actually there.
Case in point: Back in May 2017 we drove the north coast and had generally good weather. We were able to wear short sleeves most of the time and most days were sunny. Some days a light jacket was needed, but when we visited O’Cebreiro, that is in the mountains, it was drizzling and the temperature was 32 degrees.