With maximum winds of 298km/h, Irma is tied for the second strongest storm ever observed in the Atlantic.
The hurricane is expected to remain at least a Category 4 for the next few days with minor fluctuations in intensity. It could even become slightly stronger, but has neared the theoretical limit for how strong it can get given ocean temperatures.
It cannot be overstated that Hurricane Irma is extremely dangerous and will produce the full gamut of hurricane hazards across the Caribbean and potentially in South Florida, including a devastating storm surge, destructive winds and dangerous flash flooding.
All of Florida - especially South Florida and the Keys - should be preparing for a major hurricane landfall on Sunday. Tropical-storm-force winds are expected to arrive as soon as Friday.
Computer models are in strong agreement that by Saturday, Irma will be approaching the Florida Keys - where dangerous storm conditions are likely. Then, they show a sharp northward turn. The precise timing and location of the turn has huge implications for Florida.
It is impossible to say with certainty whether Irma will track up along the eastern side of the Florida Peninsula, the western side, or straight up the peninsula. Models have generally shifted westward with the storm's forecast track, which means interests along the coast of the Gulf of Mexico should also closely monitor this storm.
For a major hurricane, the exact track of the storm's eyewall - the zone surrounding its calm centre - is critical as it will determine where the most severe effects tend to concentrate. The most violent winds coincide with the eyewall, and the biggest storm surge occurs just to its right (or north).
But as Irma is such a large and powerful hurricane, very dangerous weather will also occur up to 320km away from the eyewall - including coastal surge, flooding rains and potentially damaging winds.
"The hurricane force winds in Irma are wider than Florida," tweeted Bryan Norcross, hurricane specialist at the Weather Channel. "You won't need a direct hit to get Wilma-type winds & storm surge on both coasts."
Beyond the weekend, the scenarios really depend on which side of Florida it tracks. But for now, it's safe to say that the southeast United States, including the Florida panhandle, Georgia and the Carolinas, should also brace for potential impacts, such as flash flooding, storm surge and strong winds.
The powerhouse storm was positioned 210km east of the island of Antigua in the northern Leewards, where it is forecast to make a direct impact early Wednesday. The storm was moving westward at 24km/h and the Hurricane Centre warned weather conditions would soon deteriorate.
Destructive winds as well as heavy rain that can produce flash flooding and mudslides are possible in the warning areas. Along the coast, the storm surge height - or rise in water above normally dry air - could reach 3.3m - especially just north of the storm centre.
Irma is likely to become the strongest hurricane on record to hit the Leeward Islands, even more intense than David, which raked across the central Leeward Islands in 1979.
"David was a horrible hurricane for Leeward Islands: 56 fatalities in Dominica," tweeted Phil Klotzbach, hurricane expert at Colorado State University.
Antigua, Barbuda, Saint Kitts and Anguilla - in particular - are right in the path of the storm.
Areas affected by the core winds near the storm's eye face devastating wind destruction.
The US Virgin Islands and Puerto Rico may remain south of the storm's centre, so less prone to Irma's most hostile conditions. But even so, damaging winds and torrential rains are likely along with a significant storm surge at the coast.
