The graphic you see above paints a nice, neat picture of a normal hurricane season. Things start out slow and progressively get more active in August and September. By late October the season is quieting down. Notably, in an average season, there’s never more than one tropical storm out there in the Atlantic at any given time. That’s certainly not how this season went.
The National Oceanic and Atmospheric Administration’s definition of hurricane season spans half the year, from June 1 through Nov. 30. Those dates were chosen to encompass the vast majority of storms and activity, but it doesn’t always catch everything. There are sometimes preseason storms, such as this year’s Arlene, which formed in April, and there are postseason storms — such as everything after Tropical Storm Delta in 2005.
November, while still “in season,” is typically rather quiet. This year is already among the most active on record, but what does that tell us about the remainder of the season?
From Arlene through Philippe, the 2017 season has had 16 named storms, including 10 hurricanes and six major hurricanes. All of those stats are above average. Another way to quantify the season is accumulated cyclone energy, or ACE, which is at about 229 percent of average through the end of October. This puts the season well into the “hyperactive” category.
Before 2017, the top 10 highest-ACE seasons were 1933, 2005, 1893, 1926, 1995, 2004, 1950, 1961, 1998 and 1887. Six of those 10 years were before reliable weather satellite coverage and are almost certainly underestimates of activity. This year is already the seventh most-active year, nudging 1887 out of the list.
The map below shows tracks of tropical storms and hurricanes that formed during November and December just during those top-10 years. There are fifteen storms on this list, for an average of 1.5 per year. The average number of November and December storms during the remainder of the years is about 0.4 per year. So during these hyperactive seasons, we are nearly four times more likely to get a named storm after Halloween.
While that map actually looks pretty benign — the storms didn’t get very strong and they generally didn’t impact land — the post-October time frame is capable of producing some very potent hurricanes. Since 1851, there have been eight known major hurricanes during November, and none during December. But these eight did generally impact land. What’s peculiar is that there is no overlap between the two maps; the late-season major hurricanes did not occur during the top 10 hyperactive seasons.
So we do not know for certain if this hurricane season will produce any more storms. But the odds favor at least one more, and the next name on the list is Rina.