updated 10/28/2007 2:11:09 PM ET 2007-10-28T18:11:09

Slow-moving rains that have drenched the Caribbean strengthened into a tropical storm on Sunday and forecasts said it could send flash floods gushing down denuded hills in Haiti.

Tropical Storm Noel, which formed in the Caribbean early Sunday afternoon, poses a serious threat to an impoverished nation that is still recovering from floods earlier this month that killed at least 37 and sent more than 4,000 people to shelters.

The storm had sustained winds of about 50 mph.

Flood concerns on Saturday forced three U.S. senators to cut short a trip to survey damage caused by earlier storms.

"It was just raining like mad," Sen. Tom Harkin of Iowa told The Associated Press before flying out of Port-au-Prince Saturday evening. Senators Jeff Bingaman of New Mexico and Tennessee's Bob Corker were also on the fact-finding trip.

Widespread deforestation and poor drainage mean even moderate rains can cause devastation in Haiti, where thousands build ramshackle homes in flood plains.

Flash flood warnings also were posted for Puerto Rico, while the Dominican Republic's national meteorological office warned residents to expect nearly 8 inches of rain and told ships to remain in port.

The governments of Cuba and Jamaica have issued storm watches, meaning tropical storm conditions are possible, generally within 36 hours.

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