Hi there! If you're lucky you will still get some sunny days in Autumn in Barcelona. Some people say the climate is better than Madrid as the cold winter weather is for a shorter time on the coast. So far as I am aware you will miss winter, but you may see some rain. Of course it may feel colder if it has rained none-stop for 4 days. Even so, in September you should have days averaging around 28-30C in the afternoon.
If you're looking for shoes my advice is to hang on to January and then head to the sales in Madrid for real bargains!
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An English Bookseller in Madrid