Eyewall Replacement Cycle

Footage of rough surf courtesy of an easterly fetch produced by high pressure and Hurricane Erin at Manasquan, New Jersey on August 19, 2025.

Over the course of Hurricane Erin’s trek through the Atlantic during the middle of August 2025, the storm underwent a process called an eyewall replacement cycle.

The eyewall replacement cycle (ERC) process occurs in major hurricanes, or tropical cyclones that achieve a level of intensity that is Category Three or higher on the Saffir-Simpson Scale.  An ERC happens when a hurricane develops two concentric eyewalls, and the outer eyewall that develops overtakes the inner eyewall.

What is an eyewall?  An eyewall is the ring of thunderstorms in the hurricane’s core that contains the most powerful winds before you enter the eye of the storm.  During an eyewall replacement cycle, an outer ring of storms emerges within the hurricane’s CDO, and that eventually becomes the primary eyewall of the system going forward.

Eyewall Replacement Cycle in Hurricane Maria back in 2017.
Figure 1: Explanation of an Eyewall Replacement Cycle courtesy of the National Weather Service Office in Little Rock, Arkansas.

When major hurricanes go through an ERC phase, they weaken somewhat in terms of intensity but grow in size.  As the eyewall replacement cycle takes place, the storm undergoes a form of reorganization that causes its pressure to rise and its sustained winds and gusts to decrease.  In order to conserve angular momentum, hurricanes increase in diameter.

Such was the case when Erin grew to a Category Five Hurricane on August 16, 2025.  The storm rapidly intensified to have maximum sustained winds of 160 miles per hour and a minimum central pressure of 915 millibars, or 27.02 inches of Hg (Mercury).  Within 24 to 36 hours, Erin underwent an ERC, and it weakened.

Hurricane Erin at Category Five strength - August 16, 2025
Figure 2: Satellite image of Hurricane Erin at Category Five strength on August 16, 2025.

As a result of weakening from its ERC, Erin’s winds decreased to Category Three strength at 125 miles per hour.  At the same time, Erin’s size began to increase.  Conditions were also still favorable for re-strengthening, and the storm was able to regain Category Four intensity with 140-mile-per-hour winds.  

Erin also went on to grow into one of the largest tropical cyclones on record in the Atlantic with a diameter of over 800 miles across.  The combined size and power of Hurricane Erin played a significant role in the very dangerous surf and rip currents along the Eastern Seaboard of the United States during the period of August 18-22, 2025.

Footage of the rough surf from Hurricane Erin at Sandy Hook, New Jersey on August 21, 2025.