Hurricane Wilma weakened slightly overnight to a Category 4 storm, but forecasters warned Thursday morning that it was still "extremely dangerous" and likely to regain strength before it nears Mexico's Yucatan Peninsula.

• INDEPTH: Tropical storms and hurricanes

An Oct. 19 satellite image of Wilma. (Courtesy NOAA)

As of 4 a.m. ET, the hurricane had maximum sustained winds of 240 km/h, down from more than 280 km/h a day earlier. Its eye was located about 315 kilometres southeast of the Mexican resort areas of Cozumel and Cancun.

"If the centre of Wilma makes landfall on the Yucatan Peninsula, coastal storm surge flooding of seven to 10 feet [two to three metres] above normal tide levels, along with large and dangerous battering waves, can be expected near and to the north of the centre," the U.S. National Hurricane Center warned on its website.

A day earlier, the same website said Wilma was the most intense hurricane ever recorded in the Atlantic basin, with minimum central pressure estimated at 882 millibars. The lower the pressure at the eye of a hurricane, the more powerful its winds will be.

• FROM OCT. 19, 2005: Wilma grows to most intense Atlantic storm ever

An Air Force reconnaissance plane measured the pressure at 900 millibars Thursday morning, the hurricane centre said.

The storm's heavy rains have already been blamed for the deaths of at least 13 people in Haiti and Jamaica, due to flooding and landslides.

Wilma's path is characterized by "wobbles," according to the U.S. National Hurricane Center update, but its eye looks likely to arrive on Mexico's eastern coast by early afternoon Friday.

After that, it will head into the Gulf of Mexico and could pick up strength again before hitting southwestern Florida Saturday or Sunday.

High risk of big storm surge

Forecasters warned Wednesday that a "tremendous" storm surge as high as 7.5 metres could swamp the southwest coast of Florida after the eye passes over the state. They also say the storm could go on to hit New England.

"What could happen here is eventually this gets out of the water and turns northwards toward the Maritime provinces, in which case it would not be a very merry time up there," AccuWeather forecaster Joe Bastardi told CBC News Thursday morning.

Before that happens, Bastardi said, he expects Wilma "to beat the Yucatan Peninsula like a rented mule over the next couple of days."

Tourists have been leaving Mexico, Cuba, Belize and Florida in droves over the past 24 hours, respecting the potential power of the hurricane.

Canadian tourists being flown out

Some Canadian-based travel companies have been sending special flights to those regions to pick up vacationers.

Air Transat spokesman Michel Lemay said his airline sent five planes to Cuba Wednesday to pick up 900 travelers from Canada in advance of the storm. All should be back in Canada by noon Thursday.

"Safety comes first, so yesterday we decided that the best thing to do was to evacuate our passengers," Lemay told CBC News on Thursday morning.

Residents of the Florida Keys have also started getting ready to evacuate, fearing their low-lying islands will be swamped.

Store windows are being boarded up and sheets of plywood are flying off the shelves in local hardware stores.