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

Hurricane Bertha, meanwhile, defied cool Atlantic waters and remained a hurricane while 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 despite being over only marginally warm waters but was not seen becoming a hurricane, which requires winds of at least 74 miles per hour (119 km per hour), the U.S. National Hurricane Center said.

It was about 125 miles east of Charleston, South Carolina, by 5 p.m. EDT and moving to the northeast parallel to the coastline at 7 mph with 45 mph winds, the Miami-based hurricane center said.

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

Bertha, far to the east in the open Atlantic, continued to display an ominous resilience as it raced over chilly waters in the direction of distant Iceland.

The first hurricane of the season -- 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 turn out to be an active six-month hurricane season.

GETTING INTO GEAR

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

By 5 p.m., Hurricane Bertha was located about 490 miles east-southeast of Cape Race, Newfoundland, the hurricane center said.

It was speeding toward the northeast at 25 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 than 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 degrees Fahrenheit (26 Celsius) to sustain themselves.

Particularly warm waters in 2005 allowed 28 storms to form during the season, 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.