Temperatures at Santander will be fairly typical of this part of the northern Spain coast, being about 2 or 3 degrees higher throughout the year than the coastline of south west England.
The height of the Cantabrian hills (upto 2000m) and the Picos de Europa (2500m) further west (a distance of around 100Km) will reduce temperatures and increase rainfall as you move inland. The average rainfall for Santander is 1095mm or 43 inches (Source: World Climate). Most of this rain falls in the winter and spring and much more will fall on the mountains.
The Picos de Europa themselves can see snow from October to April, with sudden changes in the weather possible at almost anytime. The mountains can be engulfed in thick mist, or may be in brilliant sunshine while there is mist below.
As the predominant winds are from the west or northwest, Santander city and the coastline are not normally affected by the mountains inland but a wind from the south or south west can bring colder air from the mountains when snow is present, or warmer air from the Central Massif in the summer.
Daily average temperatures for Santander
Source: World Climate