Hurricane Matthew is poised to affect parts of Florida that have never experienced a landfalling Category 4 or 5 storm. Disney World is closing on Friday and many Waffle Houses in Florida (both unusual). According to BBC 264 lives have been lost in Haiti, and this number is likely to rise. I am watching Matthew approach the East Coast of Florida tonight. The storm is well within radar range and something happened Thursday evening that many tropical meteorologists are familiar with. Hurricane Matthew completed an Eyewall Replacement Cycle (ERC). What exactly does that mean and what are the implications for storm strength?
Hurricane Matthew approaching Florida coast on Thursday evening at 7:22 pm EDT. Source: NWS Miami
According to the National Oceanic and Atmospheric Administration (NOAA) GOES-R training online textbook,
Tropical cyclone (TC) eyewalls contract as they strengthen to the intense TC threshold. After the existing eyewall has contracted to its minimum size for that threshold intensity, the TC enters a weakening phase. All other factors being equal, the TC weakens when an outer eyewall forms; some of the moisture and momentum is taken from the existing eyewall, which dissipates. The outer eyewall contracts gradually and the TC regains its original strength or becomes stronger.
On October 6th (Thursday evening) Hurricane Matthew went through this process. In the radar image above, the smaller eyewall near the center of the storm was eroding while the outer eyewall was becoming the dominant feature. Such concentric or dual eyewall structures can temporarily reduce the strength of the storm. At the time of writing Hurricane Matthew was still at Category 4 but its winds at 8 pm EDT were reported at 130 mph (down from previous updates during the afternoon), which still represents a life-threatening storm.
Hurricane Matthew 8 pm EDT update on October 6th. Source: NOAA NHC
Forecasters at the National Hurricane Center recognized in the 5 pm EDT forecast discussion that this process was likely
A NOAA Hurricane Hunter plane just reached Matthew and measured 121 kt at the 700-mb level and a minimum pressure of of 936 mb. Until the plane finishes sampling the circulation, the initial intensity is kept at 120 kt. There some indications that an outer eyewall is trying to form, and perhaps an eyewall cycle will occur. If so, some weakening could occur, but there could also be fluctuations in intensity while the hurricane moves toward the east coast of Florida….
The presence of concentric eyewalls is a very interesting “Jekyll and Hyde” transition process in TCs. With Matthew it could end up meaning slightly lower (but still Category 4 winds) but over a broader area (which means parts of Florida may experience hurricane force winds sooner). The Eyewall replacement cycle for Matthew is happening over a time span of 6 to 12 hours, but they can take up 2 to 3 days. Some storms can undergo several cycles over a life span.
Double eyewall structures are more common in West Pacific storms because they general encounter more favorable conditions for intensification (less land, warm water, and less wind shear). Typhoon Sudal (below) is a good example of a typical Pacific Eyewall replacement cycle.
Typhoon Sudal Eyewall replacement cycle in the Western Pacific. Source: NOAA and Naval Research Laboratory (NRL).
One of the producers of The Weather Channel’s Sunday talk show Weather Geeks, which I host, happens to be one of the world’s foremost experts on Eyewall replacement cycles. In 2011 Dr. Matthew Sitkowski, Senior Coordinating Producer at The Weather Channel, published a key paper examining the process. His analysis with colleagues examined aircraft data for 79 Atlantic basin hurricanes over the period 1977 to 2007. It provides one of the most robust climatologies of the dynamics and structure of Eyewall replacement cycles.
I should have emailed him for a quote but I know that he is busy covering Matthew. This hurricane has already made history and may continue to do so. For now I remain concerned about the safety of people in Florida, Georgia, and South Carolina going forward. I had to write something to calm my nerves.
Dr. Marshall Shepherd, Dir., Atmospheric Sciences Program/GA Athletic Assoc. Distinguished Professor (Univ of Georgia), Host, Weather Channel’s Sunday Talk Show, Weather (Wx) Geeks, 2013 AMS President