HACKENSACK, N.J. — Federal weather forecasters said Thursday that conditions are right for an “active or extremely active” hurricane season in the Atlantic this summer, with as many as 20 named storms and up to six major hurricanes.

The six-month hurricane season begins June 1, and the National Oceanic and Atmospheric Administration estimates a 70 percent chance of 13 to 20 named storms, with up to 11 possible hurricanes, including as many as six with winds of 111 mph or higher.

“That’s a pretty bold forecast,” said David Robinson, the state climatologist. “The odds are greater, then, that something could impact the Atlantic coast.”

NOAA experts say three climate factors that strongly control hurricane activity are expected to come together to produce an active hurricane season this summer — a strong West African monsoon, warmer than average water temperatures in the tropical Atlantic Ocean and Caribbean Sea, and the lack of an El Nino. El Nino is an unusually warm flow of water in the Pacific Ocean toward the western coast of South America It generates strong winds out of the southwest that can suppress the formation of Atlantic hurricanes.

Robinson said a fourth factor is also in place to help hurricane building — lower than average atmospheric pressure over the Atlantic Basin.

These four elements work together to create hurricanes: a strong west African monsoon provides energy and helps create spin up into the atmosphere; warm ocean water provides fuel; and low pressure helps to lift the energy to join the spin in the atmosphere, Robinson said. And with no El Nino, there’s less tendency for winds to develop that can shear hurricanes and break them apart before they have a chance to intensify.

Last spring, NOAA had predicted a “near-normal” Atlantic hurricane season for 2012, with nine to 15 named storms and one to three major hurricanes. In fact, there were 19 named storms and two major hurricanes.

An active hurricane season this year could mean big damage from a tropical storm for the third year in a row, following Superstorm Sandy in 2012 and Tropical Storm Irene in 2011.

“As we saw firsthand with Sandy, it’s important to remember that tropical storm and hurricane impacts are not limited to the coastline,” said Kathryn Sullivan, NOAA’s acting administrator. “Strong winds, torrential rain, flooding and tornadoes often threaten inland areas.”

The NOAA forecast predicts only the likely number of storms – not their path. Over the next several weeks, forecasters should get a better sense of whether any storms that do develop would be more likely to head up the Atlantic coast or west into the Caribbean and Gulf of Mexico, Robinson said.

If a ridge of high pressure forms from Bermuda to Texas, that tends to block hurricane movement up the coast and instead forces storms west, into the Gulf. But if the “Atlantic alley” is clear, the storms can move north. “The opportunity will probably be there multiple times this summer for a hurricane to head north, but if there happens to be no storm right at those moments, we will have dodged a bullet,” Robinson said.

New Jersey is situated in such a way on the coast that hurricanes rarely make landfall — they tend to get pushed east out to sea as they head north. The last tropical storm to make landfall in New Jersey while still a hurricane was in 1903.

In August 2011, weather experts initially thought Irene had made landfall as a hurricane, but later they decided it wasn’t strong enough. Superstorm Sandy had hurricane strength when it made landfall last October, but by then it had already merged with another powerful cold-air system coming from the west, so it was no longer officially considered a tropical storm or hurricane.