The Category 3 storm has wind speeds of 215 kph and has enough power to dump one metre of rain in parts of North Carolina. Storm surges could also reach three feet above land, according to Athena Masson, a meteorologist and hurricane specialist currently based in Canada.
So what is a hurricane made up of and how did Florence get so strong?
Hurricanes are giant storms that use warm, moist air as fuel, which is why they only form over waters near the equator, according to NASA.
WATCH: Raleigh, N.C., prepares for possible devastation from Hurricane Florence
There are several stages to a hurricane.
1. Starts off as a tropical wave
Hurricanes usually start off as a “tropical wave,” a low-pressure area that moves through moisture-rich areas, possibly causing rain and thunderstorm activity, according to the National Ocean Service.
Hurricanes can only begin in tropical regions where the ocean is at least 26 C because they require warm, moist air as fuel to form.
As this weather system moves, the warm ocean air rises into the storm, creating an area of low pressure underneath. This causes more air to rush in.
The air then cools and condenses, forming clouds and thunderstorms. Intense low-pressure air then suck in air, causing very strong winds.
2. Hurricane starts forming
When winds reach 120 kph, the storm is classified as a hurricane. The whole system then spins and grows, fed by the ocean’s heat and water evaporating from its surface.
WATCH: Hurricane Florence: South Carolina governor warns of days of rainfall
As the storm rotates faster, an eye forms in the centre that is very calm and clear. The eye is calm because the strong winds that converge towards the centre never reach it. The Coriolis force deflects the wind slightly away from the centre, just like how toilet water swirls away from the centre when flushed.
3. Storm hits land
Hurricanes typically weaken when they hit land, because they are no longer “fed” by warm ocean waters. However, they can inflict massive damage with wind and rain as they weaken. Hurricanes can downgrade to a tropical storm once winds drop below 120 kph.
Key ingredients for a hurricane
Masson said a storm of this magnitude needs two key ingredients.
The first is the ocean temperature. Masson said the sea surface needs to be 26 C or higher. Below this temperature, hurricanes will not form or will weaken rapidly. The waters surrounding Florence are around 30 C, Masson said.
“It’s like a warm bath that is fueling the storm,” she said.
WATCH: Hurricane Florence: preparations underway in Myrtle Beach as storm nears
The second ingredient is the low wind shear.
Strong upper-level winds destroy the structure of the storm by displacing the warm temperatures above the eye. Hurricanes will not form when the upper-level winds are too strong.
Difference between categories
A category 1 hurricane has winds that vary between 119 kph and 152 kph. They often cause minor damage to properties, such as to the roof, and can cause short-term power outages.
A Category 2 hurricane has winds from 154 kph to 177 kph. They can cause significant property damage, flooding, multi-day power outages and threats to people due to falling debris.
A Category 3 hurricane has winds from 178 kph to 209 kph. They can destroy mobile and frame homes and cause extensive flooding. Often, evacuations are called for Category 3 hurricanes, and electricity and water can be unavailable for up to several weeks. Hurricane Florence is currently a Category 3 hurricane as it nears the U.S. east coast.
A Category 4 hurricane has winds from 210 kph to 249 kph. They can cause irreparable damage to houses and shopping centres, pose a serious risk of death in certain areas and cause long-term power outages and water shortages. Hurricane Harvey that hit Texas in 2017 was a Category 4 hurricane — the second costliest hurricane to hit the U.S.
A Category 5 hurricane has winds faster than 249 kph. They cause complete destruction of homes, uproot trees, major flooding and can put power and water out for months. Hurricane Katrina that hit New Orleans in 2005 was a Category 5 hurricane.
— With files from Eric Stober