Storm Cristobal forms off U.S., Bertha weakens

MIAMI (Reuters) - The third tropical storm of the 2008 Atlantic hurricane season, Cristobal, formed off the U.S. East Coast on Saturday, with gale-force winds and heavy rains expected to lash the Carolinas as the storm grazes the shoreline on a northeasterly path.

Hurricane Bertha, meanwhile, finally weakened over cool Atlantic waters but a strong tropical wave south of Jamaica was expected to develop into a depression -- the precursor to a storm -- as it headed toward Mexico's Yucatan Peninsula and the oil rigs of the Gulf of Mexico beyond.

Cristobal strengthened after forming but then held steady and was not seen becoming a hurricane, which requires winds of at least 74 mph, the U.S. National Hurricane Center said.

Cristobal was about 45 miles southeast of Cape Fear, North Carolina, by 11 p.m. EDT (0300 GMT) and moving to the northeast parallel to the coastline at 6 mph with 45 mph winds, the Miami-based hurricane center said.

A tropical storm warning was in effect from north of Little River Inlet to the North Carolina/Virginia border and up to 5 inches of rain could be expected in coastal areas, the center said.

Bertha, far to the east in the open Atlantic, had displayed an ominous resilience as it raced over chilly waters in the direction of distant Iceland. By late evening, the hurricane finally weakened back into a tropical storm.

Now on its way to becoming one of the longest-lived Atlantic storms on record, Bertha formed on July 3 near the Cape Verde islands off the coast of Africa, signaling an early start to what might be an active six-month hurricane season.

GETTING INTO GEAR

The season begins on June 1 but rarely gets into gear before August.

By 11 p.m., Tropical Storm Bertha was about 545 miles (875 km) east of Cape Race, Newfoundland, the hurricane center said. It was speeding toward the northeast at 24 mph with winds of 70 mph.

The tropical Atlantic on Saturday looked more like what it ought to be in September than in July.

"The level of tropical activity this past week has been quite remarkable," Jeff Masters, co-founder of meteorological Web site The Weather Underground, wrote in a blog.

"It's a very good thing that sea surface temperatures are more the 1 degree Celsius (1.8 degrees Fahrenheit) cooler than during the record-breaking hurricane season of 2005."

Tropical storms and hurricanes need water temperatures of at least 79 Fahrenheit (26 Celsius) to sustain themselves.

Particularly warm waters in 2005 allowed 28 storms to form, including Katrina, the hurricane that swamped New Orleans and killed 1,500 people on the U.S. Gulf Coast.

In addition to Bertha and Cristobal, the hurricane center was watching an area of thunderstorms in the Caribbean that appeared likely to become a tropical depression as it moved westward toward the Yucatan Peninsula and the Gulf of Mexico.

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 pushed oil prices to then record highs after toppling oil rigs and severing undersea pipelines.

(Editing by John O'Callaghan)