I walked May 18-22 Logrono to Burgos and every single day was misty rain the morning, a bit windy, and in the upper 40's deg F. At about 11am it will be up in the 50's deg F. The last 3 walking days I did not have to wear the poncho, but did wear gloves. I was not miserable because I was at the Camino, but it was miserable walking weather. A windbreaker, at a miniumum, was critical. That said, we watched the news of flooding at Roncesvalles while in Belorado where we had about a couple of hours of sunshine. It was not that it was cold in the morning, nor the misty rain or windy. It was that there was no relief.
I met a couple in Burgos that experienced snow and sleet SJPDP to Roncesvalles. She then showed me sunburned arms from the section Pamplona- Puente La Reina. May weather this year was erratic. I talked to many locals who told me they never saw a "crazier" Spring.
I also walked late May 2011. The weather was perfection. On May 2012, same as this past May.