Washington ? The ice and snow storms that staggered parts of the East last week may portend more wintry conditions to come, federal weather forecasters said Thursday.

A moderate-strength El Niño is under way in the Pacific Ocean. The condition can set the stage for cold and wet weather in the eastern states, according to the National Oceanic and Atmospheric Administration’s winter outlook.

“El Niño is one of the driving forces behind these kinds of winter storm systems, which develop in the South and head east,” said NOAA Administrator Conrad C. Lautenbacher.

El Niño occurs when sea surface temperatures in the equatorial Pacific Ocean remain above average for more than several months. This can affect wind patterns high in the atmosphere resulting in changed weather in many parts of the world.

NOAA forecasters said the nation could expect warmer-than-normal temperatures across the northern half of the country, wetter and stormier-than-normal weather across the south from California through the Carolinas, and drier-than-normal conditions in the northern Rockies and Midwest.

“Over time, these precipitation patterns can reduce lingering drought conditions that have plagued the Southeast and Southwest in recent years,” said Jack Kelly, director of the National Weather Service, an arm of NOAA.

However, he added, “Drought may intensify in the northern Rockies and parts of the Midwest.”

About 9 percent of the area east of the Mississippi River remains in drought, while 53 percent of the region west of the river is in drought.

As recently as September, about 55 percent of each region was affected by drought, Kelly said.

“El Niño most strongly impacts U.S. weather patterns during the winter by shifting the jet stream and storm track toward the southern tier of the country,” said Jim Laver, director of NOAA’s Climate Prediction Center. That causes the increased storminess in the South.