MIAMI, Sept. 18 (UPI) -- Tropical Storm Rita gained strength as it moved closer to the Bahamas, with top sustained winds of 50 mph about 295 miles southeast of Nassau.

The National Hurricane Center said it could become a Category 1 hurricane by late Monday.

Forecasters said Rita continued to gain strength as it moved west-northwest at near 10 mph Sunday night, with tropical storm force winds extending outward up to 70 mph. The storm was expected to move over the southeast and central Bahamas Sunday night and Monday.

A hurricane watch for the Florida Keys was upgraded to a hurricane warning late Sunday. A hurricane warning is now in effect for all of the Florida Keys and Florida Bay from Ocean Reef to Dry Tortugas.

At 11 p.m. EDT, the center of Tropical Storm Rita was located about 295 miles southeast of Nassau and less than 500 miles from Ocean Reef on Key Largo.

Visitors and non-residents have been ordered to evacuate the lower Keys.

The Monroe County Emergency Management Center will be fully activated Monday morning.

Weather conditions in the Florida Keys are expected to deteriorate slowly Monday, with tropical storm force winds possible by evening and hurricane force winds of 74 mph or greater as early as Tuesday morning.