By Zachary Fagenson

MIAMI (Reuters) - A combination of cooler seas and a quiet West African monsoon season made for a less active Atlantic hurricane season, giving the South and East Coasts of the United States one of their lengthiest reprieves in history from a major hurricane, forecasters said on Monday.

"This is the longest without a major hurricane hitting the U.S. since the Civil War era," said Jeff Masters, chief meteorologist for Weather Underground.

The Atlantic Basin, which includes the Caribbean and the Gulf of Mexico, saw only eight named storms, including six hurricanes, two of which reached major Category 3 status, during the season that began June 1 and closes Nov. 30, according to an end of season summary by the National Oceanic and Atmospheric Administration.

The two major storms - considered to be Category 3 or above with winds hitting at least 111 mph (178 kph) - included Gonzalo, which pummeled Bermuda. The other Category 3, Edouard, never threatened land but was the first major storm to form in the Atlantic since superstorm Sandy in 2012.

Wilma in 2005 was the last major hurricane to make U.S. landfall. Sandy was not a hurricane by the time it reached land.

Only one hurricane made U.S. landfall this year, Arthur, which grazed the Outer Banks of North Carolina with 100 mph (160 kph) winds, disrupting the July 4 holiday weekend.

“There’s been a whole sequence of conditions that suppress these storms,” said Gerry Bell, NOAA’s lead seasonal hurricane forecaster.

Among factors that tamped down storm formation were below-average temperatures in the tropical Atlantic and an active Pacific storm season that saw more than 20 named storms in its most active season since 1992.

“It’s a seesaw effect; often when the Atlantic is more active the Pacific will be suppressed,” Bell said.

At the start of the season forecasters had predicted up to 13 tropical storms with winds topping 39 mph (63 kph), and at least one major storm.

This year forecasters rolled out two new tools, a storm surge map showing potential flood risk and a longer term five-day color-coded tropical weather outlook map to give earlier warning of possible storm formation.

(Editing by David Adams and James Dalgleish)