09 September 2021

When is the peak of hurricane season?

When is the peak of hurricane season?  It turns out there is no single or correct answer.  It depends on what metric you prefer and which time period you use.  But no matter how you slice it, the peak of the Atlantic hurricane season occurs during the second week of September.  For all of the plots and data presented here, I use a 7-day centered average, because there is quite a bit of noise when strictly using daily values. Let's break down what all of those curves are in the chart above.

One common metric would be the daily average of named storm (NS) activity.  Using the new 1991-2020 "climate normal", that peaks on September 12th, with a secondary peak on August 31st.  But the daily average of major hurricanes peaks on September 3rd, and one might argue that those are much less prone to being over/under counted and are definitely more impactful when close to land.

Using the past fifty years, 1971-2020 (which is still entirely during the satellite era), that chart smooths out a bit and the peak is a tie between September 11th and 12th. There's a secondary peak on August 31st-September 1st.

But zooming out and using the full 1851-2020 period of record (with all the nuances and disclaimers about data from the pre-satellite era), it looks like this: a definitive peak on September 11th for named storms, and a September 5th-7th peak for major hurricanes.

So for named storms, one could conclude that the peak falls on September 11-12, but the September 1-14 period encompasses the various peaks that arise depending on the choice of time period.

Another common metric is the Accumulated Cyclone Energy (ACE).  That is not dependent on the number of storms, but rather is an integrated measure of the intensity and duration of all storms.  Weak, short-lived storms barely make a dent in it, while long-track intense hurricanes make large contributions.

Again beginning with the 1991-2020 climate normal, that metric peaks on September 14th. Also notice the much more pronounced and dramatic rise and fall surrounding the peak of the season compared to the named storm counts. This speaks to the point that the real "meat" of hurricane season typically falls between mid-August and mid-October -- those two months account for 75% of the total ACE of the six-month-long season! 

Using the longer 1971-2020 period, the data expectedly smooth out a bit, and there are three dates that are essentially tied: September 14th, September 13th, and September 1st, though September 14th is technically the highest by a hair.

And the full 1851-2020 period of record results in a much smoother time series, with a broad peak centered on September 10-11.  An exaggerated feature of this period is the abrupt increase of activity during early August, then a much more relaxed decline of activity into November.

If we think of the peak of the season as when 50% of the total ACE has occurred, the peak is September 12th for all three of the time periods considered here (1991-2020, 1971-2020, and 1851-2020).

In summary, rather than assigning a specific date to it, we can conclude that the peak of the Atlantic hurricane season is the second week of September...  accounting for different metrics, different averaging periods, evolving observing technology, and the relatively short period of record.  But if you feel the need to assign a specific date to the peak, it would logically be September 12th.

Keep in mind that this is all based on historic storms and the best record we have of their existence and intensity.  Individual seasons will rarely follow the "average", and very significant hurricanes can and have occurred outside of the peak of the season.  Even the six-month hurricane season doesn't always encompass all of the activity, nor was it designed to.

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