Matthew, a violent Category 4 storm packing 145 mile-per-hour (230 kph) winds, is due to pound the western tip of Haiti in the early hours of Tuesday, the U.S. National Hurricane Center said.
Already, late on Monday, the outer bands of the cyclone reached the area, flooding dozens of houses in the town of Les Anglais when the ocean rose, the local mayor said. In the town of Les Cayes on the southern coast, the wind bent trees and the power went out.
The storm is forecast to spread hurricane force winds and up to three feet of rain across denuded hills prone to flash floods and mudslides, threatening villages as well as shanty towns in the capital Port-au-Prince.
The cyclone comes at a bad time for Haiti, where tens of thousands of people still live in tents after a 2010 earthquake that killed upwards of 200,000 people. Cholera introduced by U.N. peacekeepers is expected to rise in the October rainy season, and the country was due to hold a long-delayed presidential election in five days time.
Matthew is forecast to sweep over Cuba to the Bahamas on Tuesday and possibly reach Florida by Thursday as a major hurricane, though weaker than at present. Governor Rick Scott declared a state of emergency for Florida on Monday, designating resources for evacuations and shelters and putting the National Guard on standby.
One of the most powerful Atlantic hurricanes in recent history, it was about 100 miles (155 km) south of westernmost Haiti at 11 p.m. EDT (0300 GMT) on Monday, the U.S. hurricane center said.
Crawling north at about 7 miles-per-hour (11 kph), the storm threatens to linger long enough for its winds and rain to cause great damage.