(CNN) -- Authorities posted tropical storm warnings for parts of the Bahamas and hurricane warnings for Puerto Rico and the Dominican Republic on Sunday, as Tropical Storm Irene picked up steam near the Caribbean islands.
The center of Irene was moving past St. Croix, with maximum sustained winds of about 60 mph (95 kph), the National Hurricane Center in Miami reported. At 8 p.m. ET, the most recent update, the storm was centered about 10 miles (15 kilometers) northwest of St. Croix and about 90 miles (145 kilometers) east-southeast from Puerto Rico, moving west-northwest at 15 mph (24 kph), 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 near or over Puerto Rico overnight and be near hurricane strength Monday, forecasters said. It's expected to move near or over the Dominican Republic on Monday, and forecasters expect it will weaken as it does.
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.
The Christian aid group World Vision is prepositioning supplies and mobilizing its staff to fan out across camps in Haiti Monday, to help residents prepare for Irene.
"We are concerned that vulnerable families still living in temporary camps won't treat this storm as a serious threat, after Tropical Storm Emily didn't hit Port-au-Prince as predicted," warned World Vision's Meg Sattler in Port-au-Prince. "If flooding hits, people living in unsafe housing will be the worst affected. Heavy rain has the potential to worsen sanitation conditions in camps, which, with cholera still so prevalent in Haiti, is a major concern."
Meanwhile, the storm known as Harvey strengthened slightly after weakening to a tropical depression. The system continued unleashing heavy rainfall over parts of Guatemala and eastern Mexico on Sunday.
At 8 p.m. ET, Harvey was about 60 miles (95 kilometers) north-northeast of the Mexican coastal city of Coatzacoalcos, packing top winds of about 35 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.
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.