We’re expecting another quiet week in store for the Atlantic, which begs the question: when does the hurricane season typically pick up the pace?
It’s been since July 3 that the Atlantic’s observed a named storm, and all of us would welcome another month (or two or three) without one, but history tells us not to underestimate the tropics, especially this time of year.
There’s an unwritten rule among hurricane forecasters that the active part of the hurricane season commences in late August.
Dr. Bill Gray, one of the forefathers of tropical meteorology and the architect of seasonal hurricane forecasting, would ring a bell every Aug. 20 around his research labs at Colorado State University to signal the beginning of the active part of the season.
Here's a video I took of Dr. Gray ringing the bell on this day in 2014 at the CIRA Tropical Discussion https://t.co/H1yFxMVYIL pic.twitter.com/DypQAqISEt— Dan Lindsey (@DanLindsey77) August 20, 2021
Although there’s no official designation for late August on the hurricane season calendar, we can back up the observations of Gray and others by analyzing the historical records.
About 90 percent of hurricanes form after Aug. 1 and about 75 percent of hurricanes form after Aug. 20, but neither helps to explain the significance of late August. For that we want to examine when tropical activity ramps up.
To gauge overall tropical activity, we measure both the strength and duration of tropical storms and hurricanes. This hurricane season yardstick is called Accumulated Cyclone Energy (or ACE), where strong, long-track hurricanes score highest.
The ACE gives us the overall speed of the hurricane season. In the plot below you can see the speed usually ramps up in August.
To dig a little deeper, we can calculate exactly how quickly the speed of the hurricane season typically accelerates or decelerates through something I call ACE acceleration.
It resembles a seismograph or a hospital EKG strip. The ACE acceleration gets us to the answer we’re seeking for when hurricane season activity picks up. On average, the big ramp up to the most active part of the season happens the last week of August, centered around Aug. 28.
This is an average and seasons can ramp up sooner or later than this, with some seasons having multiple peaks, but it does help us assign a date to the traditional start of the most active part of the hurricane season.
Bill Gray’s bell is a good reminder that hurricane season is backloaded. There’s often a lot of tropical activity compressed in a few short weeks between the end of August and middle October.
We’ll continue to stay vigilant but for now enjoy the pause before Mother Nature picks up the pace.