Vince turned into a hurricane on Sunday in an unusual location near the mid-Atlantic Madeira islands, about 2,000 km (1,200 miles) southwest of Portugal. Hurricanes normally form over much warmer waters well to the south. Vince was downgraded to a tropical storm on Monday by the US National Hurricane Center in Miami.
"This storm continues to lose force, reaching national territory tomorrow (Tuesday), as it dissolves," Portugal's Meteorological Institute said. "Even so, the forecast is of worsening meteorological conditions, with rain and strong winds."
The US hurricane center said that by 5 p.m. EDT (2100 GMT), Vince's maximum sustained winds had dipped to 40 mph (65 kph) as it raced east-northeastward at 25 mph (41 kph). The cyclone was about 170 miles (275 km) west-southwest of the southwesternmost tip of Portugal.
"Vince is expected to become a tropical depression shortly and degenerate into a remnant low later tonight," the hurricane center said.
This Atlantic hurricane season has been uncommonly severe, with hurricanes Katrina and Rita wreaking havoc on the US Gulf coast.
(Additional reporting by Michael Christie in Miami)