In early October, 2013, starting my first Camino, the Pilgrim Office in SJPDP (where it was cloudy but dry), suggested that it was going to be foggy with light drizzle up on the Napoleon route. Before Orison, it started to rain, gently, and continued for a couple of hours. Then the fog rolled in and one had to watch the road closely. Eventually, the fog lifted a bit and then it started to rain very hard. The trail over the very top was 6-8" of muddy muck and the trail (not the road) down the mountain was very slick (mud and wet leaves). Made it safely to Roncevalles, but like weather everywhere, take forecasts with a grain of salt and be prepared for the worst. If in doubt, wait a day, or head up the Valcarlos route.
The only other rainy days during October that year were three days of pouring rain on the meseta and the last day into Santiago. It was a "grand trek" that set the tone for all the wonderful trips that have followed.
Buen Camino.