San Sebastian effectively lies at the north west end of Pyrenees which rise steadily from the coast to a maximum height of over 3000m about 175Km inland. The Climate is Oceanic, being effected by the predominant westerly winds from the atlantic, resulting in mild winters and pleasant summers (by UK standards). These winds also bring a significant amount of cloud and rain (over 1600mm or 65 inches of rain per year on average).
Temperatures will be come significantly cooler and rainfall will increase as you move into the mountain area. Weather conditions will also become more localised.
Daily average temperatures for San Sebastian
Source: World Climate