Tropical Storm Katia strengthened rapidly on Wednesday, approaching hurricane status, but it was too soon to tell whether it would threaten land.

Katia formed off the Cape Verde islands near the northwest coast of Africa early Tuesday, and the National Hurricane Center reported that the storm had sustained winds of 65 miles per hour as of 5 a.m. on Wednesday and was moving west-northwest at 21 miles per hour. It was expected to become a hurricane, with winds of at least 74 miles per hour, later on Wednesday.

Forecasters said Katia would probably reach the vicinity of Puerto Rico on Sunday, Sept. 4. Current projections show it passing well north of the Caribbean islands without threatening land, but projections are not always reliable when a hurricane is still so far out in the Atlantic Ocean, and there was no way to tell yet whether Katia would threaten the United States.

The National Hurricane Center predicted that Katia could become a major Category 3 hurricane, with winds of 110 miles per hour or higher, on Sept. 4.

The name Katia replaced Katrina in the seven-year rotation of tropical storm and hurricane names after Katrina was retired in response to the devastating 2005 storm.