Tropical storm expected to form off U.S. East Coast

MIAMI (Reuters) - Hurricane Bertha defied cool Atlantic waters to cling to hurricane strength on Saturday while a tropical depression just off the U.S. East Coast was set to become the 2008 hurricane season's third tropical storm.

By 11 a.m. EDT, Hurricane Bertha, an unusually resilient Atlantic storm for so early in the six-month hurricane season, was located about 450 miles east-southeast of Cape Race, Newfoundland, the U.S. National Hurricane Center said.

The season's first hurricane -- well on its way to being one of the longest-lived Atlantic storms on record -- was speeding toward the northeast at 25 miles per hour (41 km per hour) in the general direction of Iceland. But it was forecast to lose its tropical nature long before reaching there.

Bertha's top sustained winds clung tenaciously to just over hurricane strength at 75 mph (120 kph) although sea surface temperatures underneath it were already chilly, the hurricane center said. Tropical storms need waters above 79 degrees Fahrenheit (26 Celsius) if they are to sustain themselves.

Bertha formed near Africa on July 3, signaling an early start to the Atlantic hurricane season, which rarely gets into gear before August. Hurricane forecasters have predicted an average or above average season this year.

The tropical Atlantic on Saturday looked more like what it ought to be in September than in July, with a depression about to become the season's third tropical storm off the coast of South Carolina and another weather system south of Jamaica expected to make its way to the Gulf of Mexico.

The depression was located 90 miles east of Charleston, South Carolina, and a tropical storm warning was in effect from South Santee River in that state to the North Carolina/Virginia border, the Miami-based hurricane center said.

The system was moving to the northeast on a track that would likely see it graze the coast as a tropical storm. Storm-force winds are unlikely to pose much of a threat to the Carolinas.

"The depression could become a tropical storm later today, with continued strengthening possible on Sunday," the hurricane center said. It would be called Cristobal when it becomes a storm with top winds of at least 39 mph (63 kph).

An area of thunderstorms in the Caribbean Sea, meanwhile, had not managed to get better organized by Saturday morning but still had the potential to develop as it moved westward toward Mexico's Yucatan Peninsula and the oil and gas rigs of the Gulf of Mexico beyond, the center said earlier.

Oil markets watch Atlantic storms very carefully because of their potential to affect oil and gas production in the Gulf, where the United States produces a third of its crude.

A series of devastating hurricanes in 2004 and 2005, including Katrina, the hurricane that swamped New Orleans, pushed oil prices to then record highs after toppling oil rigs and severing undersea pipelines.

(Reporting by Michael Christie, Editing by Eric Beech)