Portugal is beautiful all year round which makes it a great destination both in summer and winter. Summer is perfect for someone seeking high temperature and a lot of sunshine, together with a bit smaller, more forgiving waves. Winter is perfect for surfers that like to be in the line-up with no crowd and challenge themselves with bigger waves.
The high season in Portugal lasts from May to the end of September and offers very warm weather (average around 27 C) and smaller waves. Nevertheless, in the areas like Ericeira, situated close to the coast, it is never too hot and you can expect some wind, which can get a bit chilly in the evening. It is nice to bring sweaters, trousers, and sneakers to use in the evening and at night.
From October till March you still have a lot of sunny days, but this is the time when the wind is getting stronger and the waves bigger. You should expect some rain as well during these months. The average temperature would be around 15 C, but it can be higher up to 22 C in October and March and can drop a bit (10-12 C) in December-February.
Winter in Portugal is a very nice season to explore the sights and local restaurants since you have fewer tourists everywhere.
The temperature of the water in summer can be up to 20 C, but you still need to surf with a wetsuit. In winter it can drop to 12 C, but you still feel very comfortable if you are using a good wetsuit.
Here is a video capturing the beauty of the winter months in Ericeira: