The structure of Hurricane Matthew changed on Thursday as it strengthened on its way toward Florida.  A second larger outer eyewall formed around the small tight inner eye and eyewall.   At 5:00 p.m. EDT on Thursday the center of Hurricane Matthew was located at latitude 26.2°N and longitude 78.6°W which put it about 25 miles (40 km) south-southeast of Freeport, Bahamas and about 100 miles (160 km) east-southeast of West Palm Beach, Florida.  Matthew was moving toward the northwest at 13 m.p.h. (20 km/h).  The maximum sustained wind speed was 140 m.p.h. (220 km/h) and there were wind gusts to 165 m.p.h. (270 km/h).  The minimum surface pressure was 938 mb.  Hurricane Matthew was a Category 4 hurricane on the Saffir-Simpson Scale.

A Hurricane Warning is in effect for the portion of the coast from Golden Beach, Florida to South Santee River, South Carolina including Lake Okeechobee.  A Tropical Storm Warning is in effect for the portion of the coast South Santee River, South Carolina to Surf City, North Carolina.  A Tropical Storm Warning is in effect for the portion of the coast from Anclote River, Florida to Suwanee River.  A Tropical Storm Warning is in effect for the portion of the coast from Chokoloskee to Golden Beach, Florida and for the Florida Keys from Seven Mile Bridge eastward including Florida Bay.  A Tropical Storm Watch has been issued for the portion of the coast from Chokoloskee, Florida to Anclote River.  A Hurricane Warning is in effect for the northwestern Bahamas including the Abacos, Andros Island, Berry Islands, Bimini, Eleuthera, Grand Bahama Island and New Providence.

Hurricane Matthew began to strengthen early on Thursday morning.  The minimum surface pressure decreased from 961 mb at 11:00 p.m. EDT on Wednesday night to 940 mb at 8:00 a.m. EDT on Thursday morning.  A decrease of 17 mb in nine hours is a very rapid pressure fall, but the maximum wind speed only increased from 115 m.p.h. ((185 km/h) to 125 m.p.h. (205 mk/h) during that time period.  The rapid decrease in pressure and the increase in the pressure gradient force around the center of Hurricane Matthew contributed to the reformation of a small circular eye at the center of the hurricane.  Once the new eyewall was established, the wind speed increased to 140 m.p.h. (225 km/h) within three hours.

The structure of the circulation of Hurricane Matthew continued to improve during the day on Thursday as the hurricane moved over the very warm water around the Bahamas.  A circular inner eye with a diameter of 18 miles (29 km) was surrounded by a ring of strong thunderstorms.  A rainband wrapped all the way around the original eye and eyewall and a second outer eyewall formed.  The outer eyewall has a diameter of 70 miles (105 km).  The strongest winds in Hurricane Matthew are occurring in the inner eyewall.  Hurricane Matthew has maintained a double eyewall structure in recent hours.

Matthew is moving through an environment that is favorable for hurricanes.  The Sea Surface Temperature is near 30°C.  There upper level winds are weak and there is not much vertical wind shear.  Hurricane Matthew is likely to maintain its current intensity as long as the inner eyewall remains intact.  If the inner eye starts to dissipate, then the strongest winds would be found in the outer eyewall, but those winds are not as strong as the winds in the inner eyewall.  So the intensity of Hurricane Matthew would decrease if that happened.  However, the wind field of a hurricane typically gets larger if the outer eyewall becomes the strongest part of the circulation.

Hurricane Matthew is moving around the western end of a subtropical high pressure system which is steering it toward the northwest and that general motion is expected to continue until the hurricane gets close to the coast of Florida.  When Hurricane Matthew gets close to the coast, it will be at the western end of the high and the hurricane will turn north.  It is still unclear if the turn will occur just before Matthew reaches the coast or whether it will occur after the center of Matthew moves on shore.  Eventually, when Hurricane Matthew moves farther north, an upper level trough approaching form the west will steer it toward the northeast, but the timing of that turn is still uncertain too.

The future track of Hurricane Matthew will be a critical factor in determining the damage it causes.  If Matthew turns north before it reaches the east coast of Florida, the the strongest winds would stay over the Atlantic Ocean.  The same holds true for the turn toward the northeast.  If the turn occurs before Hurricane Matthew reaches Georgia and South Carolina, then the strongest winds would stay offshore.  A much more destructive scenario would unfold if Hurricane Matthew reaches the coast of Florida and then turns north.  If Matthew grinds its way north along the coast, the strongest winds and highest storm surge would occur.  Matthew could move over cities like Daytona Beach and Jacksonville.  If the turn to the northeast is delayed, then Matthew would also move near Savannah, Georgia and Charleston, South Carolina.

The Hurricane Intensity Index (HII) for Hurricane Matthew is 28.2.  The Hurricane Size Index (HSI) is 18.4 and the Hurricane Wind Intensity Size Index (HWISI) is 46.6  The indices indicate that Hurricane Matthew is stronger than Hurricanes Frances and Jeanne were when they hit southeast coast of Florida in 2004.  Matthew is smaller than Frances and Jeanne were in 2004.  The HWISI for Matthew is similar to the ones for Frances and Jeanne, which would suggest that Matthew is capable of causing similar damage.  Hurricane Frances cause 11 billion dollars of damage in 2004.  Hurricane Jeanne caused about 8 billion dollars of damage.  So, Hurricane Matthew has the potential to be a very destructive hurricane, but the damage it does will be determined by its ultimate path.  Matthew will do much less damage if the core of the hurricane stays offshore.

Hurricane Matthew will bring gusty winds and heavy rain to the southeast coast of Florida on Thursday night.  It will do the same to the northeast coast of Florida on Friday.  Strong easterly winds on the northern side of the center of circulation will drive water toward the shore and create dangerous storm surges as Hurricane Matthew moves along the coast.  The heavy rain is likely to cause fresh water flooding in some locations.  Hurricane Matthew could have the greats impact on the coasts of Georgia and South Carolina on Friday night and Saturday.