Wilma, the record-tying 21st tropical storm of the season, formed in the Caribbean on Monday, and forecasters warned it could become a powerful hurricane (search) and hit somewhere along the U.S. Gulf Coast as early as the weekend.

"I think the message is that the season is certainly not over. People in the Gulf Coast are going to have to watch Wilma," said National Hurricane Center (search) Director Max Mayfield.

The Atlantic hurricane season runs from June 1 to Nov. 30.

Wilma reached tropical-storm strength before dawn, tying the 21-storm record set in 1933. By Tuesday, it could become the season's 12th hurricane, which would match another record: There were 12 hurricanes in 1969, the highest number since Atlantic record-keeping started in 1851.

At 5 p.m. EDT, Wilma had sustained winds of about 50 mph, up 10 mph from earlier in the day. It was centered about 255 miles southeast of Grand Cayman.

Click here to track Tropical Storm Wilma.

A hurricane watch was issued for the Cayman Islands (search), meaning hurricane conditions could be felt there within 36 hours. The storm was expected to bring 2 to 6 inches of rain in the Caymans, Cuba, Haiti, Honduras and Jamaica, with as much as 12 inches possible in some areas.

"We're on alert but we're not panicking," said Tootie Eldemire, owner of the Eldemire Guest House on Grand Cayman.

Some computer models had Wilma heading west to Mexico's Yucatan Peninsula by Friday, while others showed it curving north into the Gulf of Mexico and northeastward toward Florida over the weekend. However, such models typically have large errors this far in advance.

"There's no scenario now that takes it toward Louisiana or Mississippi, but that could change," Mayfield said. The Gulf Coast was already battered this year by Hurricanes Katrina (search), Rita (search) and Dennis, and Emily hit Mexico.

The hurricane center said Wilma could strengthen into a major hurricane, with winds over 110 mph.

Mayfield noted that there have been 10 late-season hurricanes of Category 3 or higher since 1995.

Wilma is the last on the list of storm names for 2005; there are 21 names on the yearly list because the letters Q, U, X, Y and Z are skipped. If any other storms form this season, the Greek alphabet will be used, starting with Alpha.

That has never happened in the roughly 60 years that forecasters have been regularly naming Atlantic storms.