I have walked the Camino in May 2011 and May 2012 and getting ready to return May 2013. I can honestly say that any attempt to predict with any level of certainty the weather conditions in May will be an exercise in futility. May is possibly the toughest month to predict weatherwise because it is that transition month where is not only markedly different one week from the next but also weather within the regions (Navarra, La Rioja) behaves temperamentally. My best advice is that you take a big poncho with long sleeves AND a windbreaker. Last year Mid-May I encountered torrential rain several kms before Estella. At some point a thunderstorm started. Many people weathered the episode under a highway tunnel, or pressed on to the next town. I found a horse barn and went in along with a few fellow pilgrims; the Camino flooded very quickly, but just as quickly the levels went down as the rain subsided. I was wearing waterproof hiking boots (Merrel); NO difference whatsoever; my feet were soaked. When we finally got back on the Camino we had to "daisy-chained" to go over a flooded bridge as we approached Estella. We could see bottom, so there was actually low risk, but I remembered a French pilgrim who was terrified of crossing. It took 2 American Pilgrims and 2 Japanese Pilgrims to escort her so that she could brave the crossing. I am telling you this story to make 3 points; 1) in challenging weather, there are safety in numbers, 2) study the route ahead of time; you may need that information quickly to ascertain whether you could pressed on or if you need to stay put, and 3) the Camino provides; this horse barn came out unexpectedly and it was the perfect shelter. Go figure.
