Tropical Storm Isaac is expected to dump up to 12 inches of rain on Hispaniola, the island shared by Haiti and the Dominican Republic.
A forecaster with the U.S. National Hurricane Center says "that kind of rain is going to cause some life-threatening flash floods and mudslides."
Forecasters predict the storm will blow ashore in flood-prone Haiti Friday night. The country's prime minister is urging people to avoid crossing rivers, to tape their windows, and to stay calm. He says "panic creates more problems."
The government has set aside about $50,000 in emergency funds and has buses and 32 boats on standby for evacuations.
A woman who lives in a tent camp in Port-au-Prince says many Haitians don't have homes that will stand up to a hurricane. About 400,000 people still live in the camps following Haiti's devastating 2010 earthquake.