Viana do Castelo, where it's located

The climate of Viana do Castelo is temperate oceanic, with mild, rainy winters and pleasantly warm, sunny summers.
The city is located in the extreme north-west of Portugal, on the coast.
The average temperature of the coldest month (January) is of 9.5 °C (49 °F), that of the warmest month (July) is of 20.6 °C (69 °F). The lowest record (of the 1971/2000 period) is -4 °C (25 °F), the highest is 38.6 °C (101 °F). Here are the average temperatures.
Viana do Castelo - Average temperatures
Min (°C)567911131515131186
Max (°C)141517182024262624211715
Min (°F)414345485255595955524643
Max (°F)575963646875797975706359

Precipitation amounts to 1,470 millimeters (57.9 inches) per year: it is therefore abundant. It ranges from 30 mm (1.2 in) in the driest months (July, August) to 230 mm (9.1 in) in the wettest (December). Here is the average precipitation.
Viana do Castelo - Average precipitation

On average, there are around 2,500 sunshine hours per year: therefore, the amount of sunshine is good. Here are the average sunshine hours per day.
Viana do Castelo - Sunshine

The sea temperature ranges between 14 °C (57 °F) and 19 °C (66 °F). So, it never becomes warm for swimming. Here are the average sea temperatures.
Viana do Castelo - Sea temperature
Temp (°C)141414141617181918181615
Temp (°F)575757576163646664646159

Viana do Castelo

Best Time

The best time to visit Viana do Castelo runs from June to September, since it is the warmest and the least rainy period of the year. July and August are the warmest months, but the heat is rarely excessive, and in addition, they are the driest. However, in September too, the rains are still quite rare, at least in the first half. In May, you can go, but cool and rainy days are more frequent.
Even in summer, it's better to bring a sweatshirt, and maybe a light jacket for the evening.