With or Without a Landfall, Category 4 Matthew Will Pose an Extreme Threat

Hurricane Hunters found on Wednesday afternoon that Hurricane Matthew was hanging on to its formidable strength, despite a less-than-textbook structure on satellite. Matthew’s central pressure was as low as 936 millibars late Wednesday afternoon. Top sustained winds dropped to 130 mph in the 8 pm EDT advisory from the National Hurricane Center. Dry air flowing into the storm from the northwest took a clear bite out of Matthew’s core of showers and thunderstorms (convection) on Wednesday afternoon, leaving the convection mostly concentrated on Matthew’s south side. However, after about 5:00 pm EDT, the convection rapidly filled in on Matthew’s north side, making for a much more symmetric hurricane.


Figure 1. A radar image from WU’s Storm app at 6:20 pm EDT Thursday showed Matthew’s center just southwest of Freeport, The Bahamas. Matthew’s small eye and eyewall were surrounded by a broader eyewall whose west side was located about halfway between Freeport and the Florida coast.


Figure 2. Visible satellite image from 5:45 pm EDT Thursday, October 6, 2016. Image credit: NOAA/NESDIS.

Complicating things: a possible eyewall replacement cycle
Matthew has been an extremely difficult storm to forecast, and was exhibiting extremely complicated and rapid changes in its structure Wednesday afternoon. In the early afternoon, Matthew began forming concentric eyewalls--typically the prelude to an eyewall replacement cycle (ERC), where the inner eyewall collapses and a new larger-diameter outer eyewall takes over. At 3 pm EDT, the Hurricane Hunters found two concentric closed eyewalls--an inner eyewall 12 miles in diameter, and an outer eyewall 75 miles in diameter. Just three hours later, the inner eyewall had shrunk to 8 miles in diameter, and the outer one to 57 miles in diameter. At this time, a portion of the inner eyewall passed over Settlement Point on Grand Bahama Island, bringing sustained winds of 75 mph, gusting to 86 mph (and as high as 105 mph at 6:11 pm EDT). Around 7 pm EDT, Miami radar indicated that the inner eyewall had reconnected to the outer eyewall via a spiral band, while the outer eyewall continued to contract. It appeared some dry air may have gotten wrapped into the circulation at this time, which would potentially cause weakening. However, at 7 pm the core of the hurricane was crossing the axis of the very warm Gulf Stream Current, which may give it a boost of energy.


Figure 2. Sea surface temperatures are exceptionally warm, in the upper 80s Fahrenheit, just east of the Florida peninsula, as shown in this map derived from data collected on October 5, 2016, by the European MetOp-B satellite. Image credit: Rutgers University Coastal Ocean Observation Lab.

If Matthew retains both eyewalls, it has the potential to strengthen before making landfall. It’s also quite possible that Matthew’s inner eyewall will collapse and the outer one will become predominant at some point before Matthew gets very near the Florida coast. If so, that would be good news and bad news. The good side: Matthew’s peak winds would likely decrease, perhaps by 10 to 20 mph. On the other hand, hurricane-force winds would expand to cover a much larger area, encompassing much of the outer eyewall. Furthermore, the large western side of the outer eyewall would give a multi-hour battering to buildings along the coast as the hurricane moved northward, increasing the chances of building failure. It’s not a sure thing that the inner eyewall will collapse before landfall; eyewall replacement cycles sometimes take several days to complete.

The bottom line: Matthew is more likely to weaken than to strengthen because of the potential ERC that is underway. If the inner eyewall completely collapses by Friday morning, landfall as a Category 3 storm instead of a Category 4 storm will be likely.


Figure 4. Official NHC forecast for Matthew as of 5:00 pm EDT Thursday, October 6, 2016.

Matthew’s track
NHC’s Hurricane Warning as of 8 pm extended from Boca Raton, FL, to South Santee River, SC, including Lake Okeechobee and Orlando. As of 8 pm EDT, Matthew was moving toward the northwest, but slightly more northward than westward, at about 13 mph. Given that Matthew was then located about 75 miles east of Palm Beach, we can be confident that Matthew will not make landfall south of Palm Beach. (A reminder: NHC defines landfall as the center of a tropical cyclone reaching the coast. A hurricane’s eyewall can move along the coast without it being considered landfall of the storm itself.)

Matthew’s path should undergo a very gradual rightward arc as it approaches the angled Florida coast, which makes it very difficult to say exactly where landfall might occur. The more important question may be where Matthew’s strongest winds come ashore. Especially if Matthew undergoes an ERC, parts of its eyewall could affect the entire coastline from around Palm Beach north to the Georgia coast for extended periods, although these winds may only be around Category 1 strength. A broader field of sustained winds above tropical-storm strength (39 mph) can be expected well inland, including Lake Okeechobee and the Orlando area. As we discussed in our our post on Wednesday morning, the most likely point for landfall--if landfall occurs--is Cape Canaveral, which juts about 10-15 miles into the Atlantic.

Extreme storm surge possible in northern Florida, Georgia, and South Carolina
Even a broader, weaker Matthew would retain its ability to produce severe storm surge north of its path, especially along Florida’s First Coast (including St. Augustine and Jacksonville) and the coast of Georgia. As of late Thursday afternoon, the entire coast from near Boca Raton, FL, to Cat Island, SC, was under a storm surge warning under a prototype NHC system expected to become operational next year. As Matthew churns northward, the northeast winds ahead of it will pile water against the coastline, leading to what could be record or near-record storm surges in some areas.


Figure 5. Potential maximum inundation (amount of water above ground) that could result from Hurricane Matthew through 5 pm EDT Sunday, October 9, 2016. Displayed flooding values indicate the water height that has about a 1-in-10 (10%) chance of being exceeded. This prototype graphic was issued by NHC at 11 am EDT Thursday, October 6. Please consult local advisories for official information about the expected storm-surge threat in particular areas. Image credit: NOAA/NHC


At 7 pm EDT Thursday, persistent onshore winds associated with Matthew’s circulation were already pushing a storm surge of 2.7 feet to the Florida/Georgia border at Fernandina Beach, 1.9 feet to Jacksonville, Florida at Mayport Bar, 1.8 feet to Savannah, Georgia near Fort Pulaski and 1.8 feet to Cape Canaveral, Florida, as seen on our wundermap with the “Storm Surge” layer turned on, or the NOAA Tides and Currents storm page for Matthew, or storm surge expert Dr. Hal Needham’s U-SURGE page for Matthew.

We’ll be back with our next update late Thursday night.

Jeff Masters and Bob Henson