We cannot predict the weather nor the temperature in May. This is simply because the weather is unpredictable and especially on the west coast.
We can only rely on statistics: yr.no/place/…statistics.html
You may have warm and nice weather in beginning of May followed by rain and wind in mid May. We have experienced many miserable May 17th weather wise (our constitution day). This year was one of them, when appropriate clothing would have been wool underwear for base layer and a wind and waterproof top layer.
So the message is: be prepared for unpredictable weather and bring/buy clothing to stay warm and dry. This also mean dress in layers in order to adjust to the conditions if is warm and sunny.