By John Hammond
BBC broadcast meteorologist
Hurricane Dean is not yet as severe as Hurricane Katrina, which devastated Louisiana and Mississippi in 2005.
But it is up there with the most damaging storms, such as Hurricane Gilbert in 1988 - the last hurricane to directly hit Jamaica.
Katrina was a Category Five storm - winds higher than 155mph - (though downgraded to a Category Three when it hit New Orleans) whereas this one is at the top end of Category Four, with winds higher than 145mph.
That is already enough to cause severe damage, and as Dean moves west, encountering warmer waters, it could build further and reach Category Five status.
During the rest of Sunday, Dean won't intensify much, so over Jamaica it will be a Category Four storm.
We will see copious amounts of rain, up to 20in (50cm) - this really puts the UK floods in shade.
After Jamaica, the storm will encounter the Cayman Islands in the early hours of Monday morning.
While Jamaica will get a direct hit, the Caymans may well get just a glancing blow before Dean moves on to Mexico's Yucatan peninsula on Tuesday.
It will then move back into the open waters of the Gulf of Mexico, where the water temperature is the highest it will encounter, giving it scope to intensify into a Category Five storm.
Then - and this is two to three days off, making it much harder to forecast - Hurricane Dean is expected to hit other coastal areas of Mexico, then Texas.