Erin was about 280 miles (451 kilometers) west-northwest of the Cabo Verde Islands, with maximum sustained winds of 45 mph (72 kph), the NHC said. It was moving west at about 20 mph (32 kph).
Gradual strengthening was forecast over the next several days.
Hurricane Henriette was a Category 1 storm on Monday. Henriette had maximum sustained winds of 85 mph (137 kph), the NHC said, and was about 525 miles (845 kilometers) northwest of Honolulu.
Additional strengthening was possible Monday, with weakening over the next few days.