Tropical Storm Isaac has strengthened slightly as it churns through the Caribbean toward the southern coast of Haiti.
A Friday night update from the U.S. National Hurricane Center
placed the storm 160 kilometers southeast of Port Au Prince with maximum sustained winds of 100 kilometers an hour.
Isaac is expected to pass over Haiti on the western half of Hispaniola Friday night, before moving to the northwest over Cuba on Saturday and Sunday.
Long lines were seen outside supermarkets Friday afternoon in Port Au Prince, a locale where 400,000 people have lived in tent camps since an earthquake devastated the capital city nearly three years ago.
President Michel Martelly urged Haitians to heed civil defense advice, as the impoverished country braced for the storm.
Computer models show Isaac approaching the southern tip of Florida by Monday, making landfall somewhere between the Florida Panhandle, in the western part of that state, and New Orleans, further west on the Gulf of Mexico.
New Orleans and other parts of the Gulf were devastated by Hurricane Katrina in 2005.
Organizers of next week's Republican National Convention in Tampa, Florida, are keeping a watchful eye on Isaac. Forecasters say it is not yet clear what, if any, impact the storm will have on the four-day event on Florida's Gulf Coast.