NEW YORK — A slower-than-usual hurricane season is expected this year because of an expected El Nino, federal forecasters said Thursday. But they warned that it takes only one storm to wreak havoc and urged Americans to be prepared.

The El Nino, which warms part of the Pacific every few years and changes rain and temperature patterns around the world, will likely reduce the number and intensity of tropical storms and hurricanes, the National Oceanic and Atmospheric Administration said.

Cooler temperatures on the surface of the Atlantic Ocean compared with recent years will also lower the probability of hurricane formation.

“El Nino helps to reduce the ability of storm systems coming off Africa to strengthen into tropical storms and hurricanes,” said Dr. Gerry Bell, NOAA’s top hurricane season forecaster.

But Bell cautioned that El Nino has not yet developed and officials have not yet issued any forecasts for it.

Officials expect about eight to 13 named tropical storms and three to six hurricanes. One or two major hurricanes with winds over 110 mph are forecast.