Long-term weather forecasters say it is now unlikely that a strong El Niño will develop this fall, dimming hopes in California for heavy rains that might bring relief from a severe drought.

In its latest monthly forecast, the federal Climate Prediction Center in College Park, Md., said that while there was still about a two in three chance that El Niño would develop, perhaps in the next two months, it would most likely be weak.

This year, some scientists had said rising water temperatures and changing wind conditions in the Pacific suggested that a major El Niño event, perhaps as strong as the one of 1997-98, was possible. That winter was one of California’s wettest on record, with widespread flooding and frequent mudslides.

“Certainly expectations have declined from what folks were saying earlier in the year,” said Mike Halpert, acting director of the center, which is part of the National Oceanic and Atmospheric Administration.