What was your biggest hurricane story? Can there ever be a category 6 hurricane? Can a hurricane occur out of season? What causes major hurricanes? How often can major hurricanes make landfall?
Without doubt, Hurricane Katrina (2005) is the "biggest" story that I have covered as a weather scientist, followed by Hurricane Andrew (1992), then comes Hurricane Rita (2005).
A category 6? That's an interesting question, and one that is being discussed by the experts. The debate, however, is not about whether there could ever by a "super hurricane" so much larger than anything seen before. Most hurricane scientists agree that there is an upper bound (a maximum size) that even the most horrific storm can attain, and many say that Hurricane Gilbert (1988) and possibly Wilma (2005) came close to being the "biggest" possible for the Atlantic Basin.
Still, some scientists believe that the current Saffir-Simpson Hurricane Scale (Categories 1 through 5) doesn't do justice to the really big "Category 5" storms -- the real "monster" hurricanes (like Katrina, even Rita, Wilma, among others). One could argue, however, that a new category is not really all that helpful. If you want to know the details about a storm and it's intensity, you do not look just at the category. You must look into the details of peak wind speeds, lowest pressures, storm surges, the rainfall, and other aspects of the storm. These details define the true strength of a storm.
Remember, categories are just general groups ... it would be like saying that everyone in 9th grade is the same just because they are in 9th grade! However, when we look at each individual in 9th grade, we see that some are tall, some aren't. Some are females, some are males. Categories are grouping guidelines, but you must look at the individuals to get the details. Same with hurricanes. Some Category 5 storms are bigger than others. Same with categories 1, 2, 3 and 4. So, if you want to know the details, you've got to look closer than just category.
The hurricane season runs from June 1 through November 30, so the answer is ... yes! A storm CAN occur out of season! In fact, if we include tropical storms in your question, then there has been at least one tropical storm or hurricane to form in the Atlantic Basin in every month of the year over the last 100 years or so.
But out of season tropical storms are still uncommon, especially in months other than May and December, and hurricanes are very uncommon outside of the season.
However, in recent years we've seen several of out-of-season storms. In 2005, Hurricane Epsilon maintained hurricane strength through the first week of December, and Tropical Storm Zeta held on to tropical storm (T.S.) intensity through the first five days of January.
In 2004, Tropical Storm Otto still had T.S. strength for the first day of December before weakening. In 2003, we had two December tropical storms: Odette & Peter, with the 2003 season starting extra-early thanks to April's Tropical Storm Ana.
The term "major hurricane" applies to any hurricane ranked as a Category 3 or higher storm (Category 3: maximum sustained winds of 111 mph or greater). The cause of major storms is simply the fact that ocean and atmospheric conditions are sometimes working together in a way to allow a small hurricane to continue to grow in strength. Details about the process are simply too extensive for me to type here. If you need more information, we could talk by telephone.
How often hurricanes can occur is not an easy question to answer, since that appears to change over time. We can have a run of years (like 2004-2005) when anumber of major hurricanes are making landfall, then other years when the major storms are less threatening to land.
Plus we can have storms like Andrew ... that made landfall as a major storm in Florida, but was not a major storm when it hit Louisiana. And fortunately, some storms weaken just before landfall.
But ... according to the National Hurricane Center, "data indicate that an average of 3 major hurricanes every 5 years made landfall somewhere along the U.S. Gulf or Atlantic Coast." Keep in mind that this DOES NOT count landfalls in other countries or over the many Caribbean Islands. And this does not include hurricanes in other parts of the globe. Believe it or not, only about 15% of all hurricanes occur in the Atlantic Basin. The Pacific and Indian Oceans are much "busier" when it comes to tropical weather!