Disaster prevention officials issued an "orange" alert for Friday and five following days based on a weather system now forming near Cuba.
The alert is their second-highest warning. At-risk areas include Port-au-Prince, Jacmel, Leogane and others ravaged by the Jan. 12 earthquake that displaced an estimated 1.5 million people.
No evacuation plans exist for the hundreds of thousands still living in makeshift camps prone to floods, torrents of mud and potential spread of disease.
Authorities say residents should avoid flooded rivers and be prepared to move if necessary.