Tropical Storm Melissa is expected to rapidly intensify over the weekend into a Category 4 hurricane, according to the National Hurricane Center. The slow-moving storm is forecast to bring "life-threatening and catastrophic flash flooding and landslides to southern Hispaniola and Jamaica through the weekend.

At 11 a.m. Saturday, the National Hurricane Center issued an advisory stating that Tropical Storm Melissa is in the Caribbean Sea, 155 miles southeast of Kingston Jamaica and 235 miles southwest of Port Au Prince Haiti. The system, with maximum sustained winds of 70 mph, is moving west-northwest at 1 mph.

Melissa is currently the only active tropical system in the Atlantic basin. As of Friday night, the storm remains nearly stationary, drifting north at just 2 mph. Maximum sustained winds are around 65 mph—just below the 75 mph threshold needed to reach Category 1 hurricane status.

At 5 a.m. Saturday, the National Hurricane Center issued an advisory stating that Tropical Storm Melissa is in the Caribbean Sea, 180 miles southeast of Kingston Jamaica and 245 miles southwest of Port Au Prince Haiti. The system is moving northwest at 3 mph, with maximum sustained winds of 65 mph.

Dozens of people were already in shelters in the Dominican Republic, and schools, businesses and government agencies were closed.

The two categories combined make up about 17 percent of all hurricanes in recorded history. Those reaching Category 5 — like Melissa — historically make up more like 4 percent of hurricanes. And fewer than half of the strongest two categories of hurricanes go on to hit land at such intensity.