Tropical Atlantic Update

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.

Larry weakens, Mindy forms during peak of season

Since my previous post on Monday, Hurricane Larry has weakened only slightly but is still a large and powerful hurricane located just east of Bermuda. Then, the disturbance I first wrote about on September 1st down near Colombia (later tagged Invest 91L) was upgraded to Tropical Storm Mindy just before landfall on the Florida panhandle on Wednesday afternoon.

Once Larry clears Bermuda today, it's not done with land impacts. A hurricane watch is in effect for eastern Newfoundland, and Larry will reach the island on Friday night... very likely as a hurricane. It's reminiscent of Igor in 2010, and that storm's encounter with Newfoundland earned Igor's retirement as a name.

From the track map at the top of the post, we can see the 10-day journey that Mindy took from the coast of Colombia to the Florida panhandle... it's been a long time coming but it finally got organized on Wednesday afternoon and quickly wrapped up to become a tropical storm. This radar loop (latest available at http://bmcnoldy.rsmas.miami.edu/tropics/radar/) shows the quick development and landfall.

Mindy has since weakened to a tropical depression and will re-emerge over the Atlantic later this morning. Due to increasing vertical wind shear, it is not likely to regain tropical storm status, but could be marginal immediately upon exit, over the Gulf Stream. It's moving quickly so significant rainfall isn't expected, just some heavy rain over eastern Georgia and South Carolina as it passes through.

In terms of named storm count, Mindy is the season's 13th named storm. The average formation date of the 13th named storm is October 25th, so this is really exceptional. Only four other years reached this mark by September 8th since 1851: 2020, 2012, 2011, and 2005. Keep in mind that it's only September 9th and we have already used 13 of the 21 names in the regular list! The table below shows the 1991-2020 average formation dates.

In terms of ACE (Accumulated Cyclone Energy), the season is at about 155% of average for the date, using the past fifty years as the baseline. This season has already accrued more ACE than some entire seasons (2015, 2014, 2013, 2009, 2002, 1997, etc), and Larry is largely to thank for that boost, as that one storm has contributed roughly 40% of the total!

Looking farther east, there's a strong easterly wave just about to exit the African coast, and model guidance favors it for development by the end of the weekend. Once over the ocean, it should track toward the northwest, no threat to the Caribbean or areas west. When the time comes, the next name on the list is Nicholas.

06 September 2021

Larry still a major hurricane, also watching Gulf coast

The easterly wave that would become Larry left the African coast on August 30th. It quickly became the season's 12th named storm on September 1st, then the season's 5th hurricane on September 2nd, and the season's 3rd major hurricane on September 4th. All of those dates are 1-2 months ahead of where an average season would be, keeping the 2021 season activity well ahead of average too.

Larry has been a large Category 3 hurricane since Saturday morning, and it's expected to remain that way for another 3 days or so. It's also expected to remain far from land, with the exception of a brush with Bermuda on Thursday and Newfoundland on Saturday. These are the hurricanes we all can enjoy: huge beautiful storms far from creating any havoc on land.

However, even these distant long-lived powerful hurricanes do have a noticeable impact on the U.S. east coast. This comes in the form of large swells and wave energy. These elevated water levels will affect the U.S. east coast from Wednesday through the weekend:

Moving on from Larry, we've been keeping an eye on a disturbance for the past week, since it was just off the coast of Colombia. It has since passed over Nicaragua, Honduras, Belize, and Mexico's Yucatan peninsula and is now centered in the Bay of Campeche... tagged as Invest 91L.

Although environmental conditions aren't too favorable for its development, there is a chance of it becoming the next tropical cyclone later in the week as it approaches the northeast Gulf coast or after crossing the Florida peninsula. If it happens to reach tropical storm status, the next name on the list is Mindy. The average date of the 13th named storm formation is October 25th.

Overall, the 2021 hurricane season is at about 148% of the average ACE (Accumulated Cyclone Energy) for the date, and recent seasons that were more active by now include 2017, 2012, 2010, 2008, 2005, etc. It's well ahead of the 2020 season.