In late August & early September:
Er….slightly less predictable, but I can assure you that it certainly doesn’t rain all the time in Wales and when the sun is shining you will be glad to be in this glorious part of the world.
In the summer months (April to September) temperatures can range from 10 – 20 degrees Celsius.
As the saying goes, ‘there is no such thing as bad weather, only inappropriate clothing’, so be prepared.
See full weather reports at Wales climate and Mid Wales Weather and Climate