The weather in the Baltic at that time of the year is about as unpredictable as our summer but in general if you dress as per our summer you will not go far wrong. I have seen temperatures in the high 20s even low 30s as well as storms when 14/15 was nearer the mark. Sorry, I can't be more precise than that regards....Ida Dow.
Indeed the weather can vary from very warm to cold.
Best to some light jumpers and perhaps a light fleece just in case it gets chilly or cold. You can then add layers of clothes or take off layers as the case might be.