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Go to the website WEATHER UNDERGROUND and use the almanac section and this will give you all the info you need. Enjoy QM2 it is the best shipon the seas!
I live near Barcelona. It should be quite warm in September (low 20's C) but the climate seems to be changing so it could be unpredictable. Days could be very hot, evenings a pair of trousers and a sweater at the most. A light waterproof just in case you get some rain. It certainly won't be cold so no heavy clothin.
September is one of the best months, in my opinion, for crossing the Atlantic but you might still have some mid pond fog and occasionally the water can have some lumps in it. It should be pleasantly warm though. Once you are through into the Mediterranean the temperature will rise and in early September should give you days in the 80s'F. Later it cools down but not much until October. Still be prepared for inclement weather though, nothing at sea is for certain particularly of late ... regards Nei.
Have cruised this area several times in September and October and have always had pleasant hot weather Not as hot as August but much better for sightseeing. Short sleeve summer clothes. Take a light rain jacket just in case but even with rain it should be warm.