MEXICO CITY (AP) — Tropical Storm Julia is gaining strength heading westward in the southern Caribbean, and authorities are preparing for a possible hurricane on Colombian islands and in Nicaragua.

Julia's maximum sustained winds had increased to 45 mph (75 kmh) late Friday, the U.S. National Hurricane Center said. The storm was centered about 360 miles (575 kilometers) east of Providencia Island and moving west at 17 mph (28 kph).

Julia was forecast to pass near or over Colombia’s San Andres and Providencia islands Saturday night on its way to landfall in Nicaragua on Sunday morning.

Colombian President Gustavo Petro said via Twitter that the government was preparing shelters on the islands. Officials on San Andres announced a curfew for residents beginning at 6 a.m. Saturday to limit people in the streets.

Yolanda González, director of Colombia's Hydrology, Meteorology and Environmental Studies Institute, said Julia could be a Category 1 hurricane when it passes the islands.

A greater threat than Julia’s winds were rains of 5 to 10 inches (13 to 25 centimeters) — up to 15 inches (38 centimeters) in isolated areas — that the storm was expected to dump across Central America.

“This rainfall may cause life-threatening flash floods and mudslides through this weekend," the U.S. National Hurricane Center said.

The storm's remnants were forecast to sweep across Nicaragua, Honduras, El Salvador, Guatemala and southern Mexico, a region already saturated by weeks of heavy rains.