With an average of nine named storms during the 90-day period, the Atlantic region of the tropics (the large area of tropical motion where tropical development is a possibility) has had five hurricanes, four of which have been classified as major hurricanes. Such events were uncommon in the US during 2016, when nine hurricanes with sustained winds of 111mph or more occurred in the Atlantic.
Shallow water and warmer waters made the 2017 season unusually mild compared with previous years, providing low atmospheric pressure over the warm waters near the equator, the ingredients for tropical development. This year was exceptionally stable, so that a storm that was named Katia three weeks before hitting Puerto Rico would be inactive when approaching the Leeward Islands. To the relief of meteorologists, this hurricane season has started well, with at least 10 tropical systems in the Atlantic, including four hurricanes.
A weak monsoon may have weakened the tropical cyclone that hit Puerto Rico, which was left heavily flooded and without power due to 3ft (90cm) of rain. If it has a negative impact on tropical storm formation this year, it would have been due to La Niña conditions.
The most active period in the 20th century was 1967-2001, with 22 named storms. In the 1997-2000 and 2005-2010 seasons, no hurricanes formed at all.