Mr Preval said he would hold emergency talks with donor countries to appeal for aid.
Two other major storms - Hurricane Ike and Tropical Storm Josephine - are following in Hanna's path.
'Ripping up trees'
Hurricane shelters were being readied across the Bahamas archipelago as residents in the northern town of Freeport stocked up on emergency supplies in anticipation of Hanna's arrival, reports said.
At 2100 GMT on Wednesday, Hanna was about 60 miles (95km) south-east of Great Turk island and moving northwards, the US National Hurricane Center said.
It forecast Hanna would head over the Bahamas and towards the south-east coast of the US in the next few days.
In Haiti, the northern city of Gonaives bore the brunt of Hanna's maximum sustained winds of 100km/h (65mph) on Tuesday, with people on roof-tops screaming for help as floods reached depths of 2m (6.5ft).
"There are a lot of people who have been on top of the roofs of their homes over 24 hours now. They have no water, no food and we can't even help them," Haitian Interior Minister Paul Antoine Bien-Aime told Reuters.
UN peacekeepers and aid workers have been trying to reach stranded survivors.
"The situation is as bad as it can be," the UN's Vadre Louis told the Associated Press news agency.
"The wind is ripping up trees. Houses are flooded with water. Cars can't drive on the street. You can't rescue anyone, wherever they may be."
Storms have killed more than 100 people in Haiti in the last three weeks.
The impoverished Caribbean island was first drenched by Tropical Storm Fay, before Hurricane Gustav wreaked havoc last week, with torrential rainfall over heavily deforested and hilly terrain causing floods and mudslides.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.