Authorities posted hurricane warnings for Puerto Rico and the Dominican Republic on Sunday as forecasters predicted Tropical Storm Irene would strengthen as it neared the Caribbean islands.

Irene was nearing the U.S. Virgin Islands with top winds of 50 mph Sunday, the National Hurricane Center in Miami reported. At 5 p.m. ET, the storm was centered about 25 miles east of St. Croix and about 120 miles east-southeast from Puerto Rico, moving west-northwest at 17 mph, forecasters said.

Tropical storm warnings covered the U.S. and British Virgin Islands, the northern coast of the Dominican Republic and Haiti.

The storm was expected to pass south of Puerto Rico overnight, strike the Dominican Republic on Monday and could reach the southeastern United States by Thursday, forecasters projected.

“Some strengthening is forecast during the next day or so, and Irene could become a hurricane on Monday,” the hurricane center reported.

The hurricane center warned that Irene could dump heavy rain on the Dominican Republic and neighboring Haiti, where previous storms have caused deadly mudslides down deforested hillsides. The NHC said Haiti and the Dominican Republic could see up to 20 inches of rain in isolated areas, with six to 10 inches possible across the rest of Hispaniola.

Meanwhile, the storm known as Harvey weakened to a tropical depression, but continued unleashing heavy rainfall over parts of Guatemala and eastern Mexico on Sunday.

At 2 p.m. ET, Harvey was about 125 miles east-southeast of the Mexican coastal city of Coatzacoalcos, packing top winds of about 30 mph. The storm could dump another two to four inches of rain over southeastern Mexico, with some areas seeing as much as 10 inches, forecasters said.

“These rains could produce life-threatening flash floods and mudslides, especially in areas of higher terrain,” the Hurricane Center said.

Harvey struck land Sunday on the coast of Belize as a tropical storm, with top winds near 60 mph. It is expected to dissipate over southern Mexico by early Monday.