We do 2 transatlantics a year (April and sometime in the fall) and have had everything from gorgeous weather with glassy seas to last years April cruise on the Jewel of the Seas when we had 3 straight days of wind, rain, and seas that were always over 30 foot. The Brilliance, which was one day behind us had gorgeous weather. One problem with cruising from West to East is that if you get a big storm like we got in April, it stays with you all the way across the sea (our followed us all the way to Cobh, Ireland. Your odds of good weather are a little better if you are taking a more southern route such as from Florida to Rome. By the way, we just returned two weeks ago from a 15 day transatlantic that went from Barcelona to Miami (Brilliance of the Seas). You might expect cold weather in December, but we had perfect weather all the way across with temperatures in the low 70s to 80s, and calm seas (never over 3 foot with 6 foot swells). You just never know about the Atlantic.