Yes. If you are registered on the register of resident voters absent abroad (CERA) and you are temporarily in Catalonia during the election period, you can ask to vote by post from 4 August to 17 September, both inclusive. This will not mean you will be deregistered from the CERA.
To apply to vote by post you have to fill in the application form available from any post office, expressly stating that you are registered on the register of residents absent abroad, sign it and send it to the relevant provincial office of the Electoral Roll Office. You must state an address in Spain where you want to receive the documentation.
If the registration documents have already been sent because you had already applied for them from abroad, you will receive a certificate of registration marked as "copy".
Once you have received the electoral documentation at the address you indicated in your application, you must send your vote by registered post no later than 23 September. (Extension of the deadline: until 24 September. Exceptionally, up to 2 pm on 25 September in those places where 24 September is a holiday)