The Weather in Sitges is quite unique. Even though it is 30mins from Barcelona, the fact that it is surrounded by mountains gives Sitges its very own micro-climate. So on days when it is pouring rain in Barcelona, clear blue skies and temperatures of 30 degrees (July/Aug) are the norm for Sitges.
However, do a search for the weather in Sitges on any of the large sites, such as Weather.com and you will come up with nothing… or nada! Better to try the local tourism site, who have a great 3 day forecast which is amazingly accurate.