That day, Dennis weakened slightly before making a final landfall on Santa Rosa Island, Florida as a Category 3 hurricane.

In the Caribbean, the outer bands of Hurricane Dennis brought tropical storm-force winds and heavy rain to parts of Haiti, resulting in floods and landslides.

The resultant floods severely affected Jamaica's agriculture industry, killing nearly 160,000 livestock and leaving around 100,000 customers without electricity.

Additionally, Dennis produced a 6 to 9 ft (2 to 3 m) storm surge, which caused significant flooding along Apalachee Bay, well east of the landfall.

In neighboring Alabama, Dennis brought hurricane-force winds, a 3–6.5 ft (1–2 m) storm surge, and heavy rainfall peaking at 12.80 in (325 mm) in Camden.

The depression soon crossed the island country of Grenada[1] before entering the Caribbean, where increasingly favorable environmental factors, such as low wind shear and high sea surface temperatures, fueled intensification.

[1] The formation of a well-defined eye and central dense overcast signaled Dennis's intensification into a Category 3 hurricane on July 7, as it traversed the Jamaica Channel.

[1] Interaction with the mountains of Cuba caused significant weakening;[10] once Dennis emerged over the Gulf of Mexico on July 9, it quickly reorganized in favorable conditions.

The hurricane reached Category 4 strength for the third time on July 10 as it approached Florida, attaining its lowest barometric pressure of 930 mbar (hPa; 27.46 inHg).

[11][12] Weakening ensued as the hurricane approached the Florida Panhandle, the storm ultimately making landfall over Santa Rosa Island on July 10 as a Category 3.

Dennis' remnant circulation remained, traversing the river valleys of the Mississippi and Ohio before finally dissipating over Ontario on July 18.

The nation was still recovering from the deadly Hurricane Jeanne in September 2004 when Dennis arrived, with about 550,000 people receiving assistance from the World Food Programme.

[13][14] The Haitian National Red Cross Society mobilized 300 personnel and identified ten potential shelters in the threatened region.

[15] The Pan American Disaster Response Unit branch of the Red Cross was equipped with supplies for 10,000 people and prepared to deploy.

A tropical storm warning extended east of the area to Anclote Key, and westward to Grand Isle, Louisiana, including New Orleans.

[34] At Cape Canaveral, NASA made preparations to potentially move Space Shuttle Discovery from the launch pad, but ultimately let the vehicle ride out the storm.

[38][39] Alabama Governor Bob Riley ordered traffic on Interstate 65 south of Montgomery to be northbound only on July 9, a process known as contraflow lane reversal, to provide additional capacity for evacuations.

[71] Dennis also disrupted the nation's agriculture industry, ruining about 360,000 tons of citrus fruit, destroying 11,000 hectares (27,000 acres) of bananas, and killing about 73,000 livestock birds.

[71][77] Throughout the southeastern United States, Hurricane Dennis resulted in 17 fatalities – 14 in Florida, 1 in Mississippi, 1 in Alabama, and 1 in Georgia – as well as about US$2.5 billion in damage.

[83][84] The most significant coastal flooding occurred along Apalachee Bay, with a storm tide of 8.11 ft (2.47 m) recorded in Apalachicola, about 175 mi (282 km) east of Dennis' landfall.

The water levels in the region were about 3.5 ft (1.1 m) higher than anticipated by the Sea, Lake, and Overland Surge from Hurricanes computer model.

The NHC determined that the high tides were "likely triggered by an oceanic trapped shelf wave that propagated northward along the Florida west coast.

[1][90] Across southern Florida, gusty winds and rainfall knocked down trees and power lines, with some minor coastal flooding.

[91] One beneficial effect of Hurricane Dennis was that it flipped over the former USS Spiegel Grove, which was deliberately sunk in 2002 in the Florida Keys National Marine Sanctuary in an attempt to create an artificial reef; for three years the navy ship had been upside-down.

[98] The rains caused numerous rivers to overflow, resulting in flash flooding that entered homes and covered roads up to 5 ft (1.5 m) deep.

[1] Offshore in the Gulf of Mexico, the Thunder Horse oil platform tilted to a 30º list after the vessel was evacuated and the structure's ballast tanks were flooded.

[105][106][107] The hurricane's passage also caused an algae bloom in the eastern Gulf of Mexico, although conditions returned to normal within ten days of the storm.

[114][115] In Woodstock in Cherokee County, the Noonday Creek swelled to a record crest of 16.3 ft (5.0 m), surpassing the peak set during Hurricane Ivan.

[53][45] Médecins Sans Frontières, the United Nations Stabilization Mission in Haiti, the World Food Programme, and other international agencies mobilized for relief efforts on this day.

[154] Voluntary agencies such as AmeriCorps, the Christian Contractors Association and the United Way provided assistance to residents who had temporary roofing and repair needs.