MIAMI — A record-breaking 22nd named tropical storm formed in the Caribbean yesterday and could bring life-threatening floods and mudslides to Haiti and the Dominican Republic, the U.S. National Hurricane Center said.

The storm was called Tropical Storm Alpha, the first time the hurricane center has resorted to using the Greek alphabet since it began naming tropical cyclones in 1953.

The 2005 hurricane season has had so many storms that all the storm names preassigned for this year were used up with Hurricane Wilma, which pounded the Mexican resort of Cancún yesterday and was expected to head to Florida.

Alpha made 2005 the most active hurricane season since records began 150 years ago, and the 2005 season still has five weeks. The 1933 season had 21 named storms.

The World Meteorological Organization, the U.N. agency that names storms, said it had run out of letters from the Roman alphabet. Names beginning with Q, U, X, Y and Z are never used.

Alpha was not expected to be a threat to the United States or U.S. oil and gas facilities in the Gulf of Mexico. Nor was it expected to become a hurricane, with sustained winds in excess of 74 mph, the hurricane center said.

But the system, which was situated 210 miles west-southwest of San Juan, Puerto Rico, at 5 p.m. EDT , was expected to cross the island of Hispaniola, shared by the Dominican Republic and Haiti, and bring rainfall of up to 12 inches to the two countries.

Haiti, which is largely deforested, is particularly vulnerable to floods and mudslides. Last year, Hurricane Jeanne killed up to 3,000 people in the Haitian port city of Gonaives while it was still a tropical storm.

A tropical-storm warning was in place for Haiti and parts of the Dominican Republic, and a tropical-storm watch was in effect for the Turks and Caicos islands and the southeastern Bahamas.

Since 1995, the Atlantic has been in a period of higher hurricane activity, a cycle expected to last at least another 10 years.

Scientists say the cause of the increase is a rise in ocean temperatures and a decrease in the amount of disruptive vertical wind shear that rips hurricanes apart.

The busy seasons are part of a natural cycle that can last for at least 20 years, and sometimes 40 to 50, forecasters at the hurricane center say. The current conditions, they say, are similar to those in the 1950s and '60s.

The U.S. Gulf Coast has been battered this year by Hurricanes Katrina, Rita and Dennis — and Wilma will be next. It had sustained winds of about 100 mph as it moved over the Yucatán Peninsula yesterday and was expected to turn northeast, pushed by a strong wind current, and approach southern Florida tomorrow. A hurricane watch was in effect for the state's entire southern peninsula.