Hurricane Alex (developed from an area of low pressure which began in the Atlantic near Bahamas in January 2016, and developed subtropical and later tropical storm characteristics to the south of the Azores) clearly way outside the usual Jun-Nov hurricane season. How is it possible that such an energetic event forms during the North Atlantic winter? Clearly, this kind of hurricane is not common (previous one in 1938). What are the oceanic and atmospheric conditions necessary for the formation of Hurricane Alex?
Alex was an out of season freak event that bucked the trend and broke all the rules. As hurricanes go, it was somewhat weak, and could never have strengthened beyond category 1. Three conditions made it happen (i) there was an unusually strong vortex over Cuba which kick-started the rotation, (ii) the high level atmospheric shear, which usually kills off fledgling hurricanes, just didn't appear on this occasion, (iii) the storm initially followed the Gulf Stream Drift, and hence gained energy from higher than usual sea surface temperatures - though still a lot cooler than normal. After the hurricane reached central north Atlantic it veered off to the northwest, where it met much colder sea-surface temperatures. It was this cold water that killed it off.
Alex was a weird development, being only the second ever recorded in any month to form north of 30°N and east of 30°W, the only other being Hurricane Vince in 2005. It developed over waters cooler than normally required for tropical development at 20-22°C, however upper level air was remarkably cold for the latitude above where Alex developed, giving a strong temperature gradient which allowed convection to consolidate and form a warm core tropical hurricane. For references see Bob Henson at Wunderground Astounding Alex..., Alex Becomes... Temperatures in the upper atmosphere in that area were extremely cold, down to around minus -60 degrees Celsius, or minus -76 degrees Fahrenheit. Andrew Freeman, Mashable Specifically the upper troposphere (6 miles up).Liam Dutton
Tropopause temperatures were unusually cold. This allowed a steep enough lapse rate despite cool sea surface temperatures of 68 degrees F, and was one of the factors that helped Alex to form despite such cool sea surface temperatures. Hurricanes are heat engines with the hot source at the ocean surface and the cold sink at the tropopause. Thus an unusually cold tropopause can compensate to some extent for sea surface temperatures lower than the usual minimum value of 78 degrees F required for hurricane formation.