Tropical storm Gustav began pounding Jamaica Thursday, prompting evacuations and relief efforts as the strengthening storm continued a collision course with the Gulf of Mexico, where officials in Mississippi and Louisiana were already on high alert..

After barreling through Haiti and the Dominican Republic with hurricane-force winds and rain earlier in the week, Gustav was somewhere shy of 80 miles off the coast of east Jamaica on Thursday and gaining steam. Heavy rains that signaled its arrival led Jamaican officials to open shelters and warn residents to seek cover.

Forecasters said the storm had winds of roughly 70 miles per hour, but would accelerate toward Category 3 speeds of 111 mph as it continued west toward the Gulf. Hurricanes are rated on a scale of 1 to 5, with Category 1 storms reaching winds of 74 mph and Category 5 storms exceeding speeds of 155 mph. Forecasters said Gustav would grow to a hurricane by the end of the day.

Emergency preparations for the storm along the Gulf Coast come almost three years to the day that Hurricane Katrina slammed New Orleans, destroying levees, flooding 80 percent of the city, and killing nearly 1,500 people. If Gustav were to strike the region, it would be the first major hurricane to threaten the Gulf since 2005.