Tropical Storm Laura, which killed at least 13 people when it struck Haiti and the Dominican Republic, was set to track just south of Cuba on Monday.
“Gradual strengthening is expected, and Laura is forecast to become a hurricane by early Tuesday,” the US National Hurricane Center said, predicting it would make landfall in the US state of Louisiana later in the week.
Storm Laura brought heavy rain over the weekend, killing nine in Haiti.
The storm also killed four in the Dominican Republic’s capital Santo Domingo, said the country’s Center of Emergency Operations.
A woman and a child died at home, while a young man died when a tree fell on his home.
The storm flooded houses, cut off remote villages and left more than one million Dominicans in the dark.
In neighboring Haiti, a 10-year-old girl was among the nine dead as homes were flooded and evacuations were underway.
Energy companies suspended some oil and natural gas production in the Gulf as the weather deteriorated.
Storm Marco was downgraded from a hurricane overnight and was forecast to approach the coast of Louisiana later Monday, weakening to a tropical depression on Tuesday.