Sorry but you have to tell us more......When are you thinking of going? What route will the ship be taking? What ship is it? Tell us more and we'll tell you more otherwise we would have to write reams. ......Neil.
Silly me... I misread the question and you said it all..... Should be ok but the Atlantic is difficult to predict. You may get some winds and high seas but you might also get fog, if you get good sunny weather all the way it would be unusual, let's just hope you are going to the USA and get the extra hours aboard. Once more my apologies for my first reply. .... Neil.
We did New York to England at end of October and it was fine. Obviously everything depends on the weather. As you will know some years September is the best month for mild weather and others dreadful. Dont worry about it being rough you dont know your moving in these modern cruise ships, just go away thinking that you will enjoy yourselves, dont worry about the sea.
A few years ago we were out at sea in a hurrican. It was great fun one of the best cruises we had.
If I knew the answer to your question I could become a very rich person! There is NO way of knowing what the weather will be in the Atlantic in September - or in any other month of the year! However, I note you have indicated you are going on Cunard so I am HOPING that means you will be on the Queen Mary 2. If so, I wouldn't worry about the weather too much as the QM2 is a liner, rather than a normal cruise ship, and it is specifically designed to deal with heavier weather. I have done the crossing twice, both times experiencing a range of weather during the trip from calm to very rough. I cannot imagine how horrendous that trip would be on a regular top-heavy cruise ship. Stick to the QM2 for that crossing and you will be fine. The stabilization is excellent.