TRAVERSE CITY, Mich. — Water levels in the Great Lakes should remain mostly above average over the next six months as a powerful El Nino gives the region a break after two bitterly cold winters, but it’s unclear whether there will be longer-term effects, federal scientists said Thursday.

El Nino, the warming of surface waters in the Pacific Ocean that can significantly influence weather in North America, is expected to bring milder temperatures and less snow across the lakes that hold nearly one-fifth of the world’s surface fresh water, National Weather Service hydrologist Jim Noel said.

Those conditions are unlikely to alter the seasonal pattern of Great Lakes levels, which tend to rise in spring, peak in summer and decline in fall, said Keith Kompoltowicz, hydrology chief with the U.S. Army Corps of Engineers’ district office in Detroit. But the fluctuations may be less pronounced than usual, partly because of El Nino’s effects on evaporation and precipitation.

“We don’t see any extreme events in terms of lake levels on the horizon,” Kompoltowicz said as scientists released an updated forecast for the Great Lakes, which have rebounded sharply after a prolonged slump that lasted from 1998 through 2012.

Lakes Huron and Michigan have enjoyed the most dramatic improvement, rising nearly three feet — their biggest two-year jump on record. It’s been a relief for cargo shippers, marina owners and property owners along shorelines where retreating water had given way to muck and weeds.

Mild air temperatures this fall may lower evaporation rates, resulting in less of a drop-off in water levels than usual, Kompoltowicz said. But if snowfall declines this winter as expected, the spring rise in water levels might also be reduced. Spring runoff of the snowpack is a key factor in determining lake levels.

Lakes Superior, Huron, Michigan and Erie should remain at above-average levels for the next six months, he said. Lake Ontario, which is controlled by a hydroelectric dam on the St. Lawrence River, is expected to be near or slightly below its long-term average.

The scientists were reluctant to extend their prediction beyond six months, although Kompoltowicz said a return to colder temperatures in fall 2016 could bring greater-than-usual evaporation.