LOS ANGELES — A soaking El Niño weather system is in the forecast, promising to pummel California with torrents of rain by the end of the year. That would seem like Champagne-popping news as this state suffers through its worst drought in a millennium.

But in the latest sign of the meteorological, behavioral and political challenges that California leaders face in managing this crisis, the prospect of months of heavy rain has become a cause for concern as well as celebration — a cloud on the horizon that could bring this state more than just moisture.

It is forcing state emergency officials to turn from trying to manage water rationing and forest fires to preparing for floods, mudslides and debris flows that follow extended dry periods, made worse by the fact that forests have been denuded by drought-fueled blazes. And after months of success at persuading Californians to conserve water — the most recent figures show that residents pared back their water use 31 percent in July, compared with July 2013 — the prospect of El Niño-as-savior threatens to lull Californians back to long showers and daily dousings of lawns.

And it is emerging, officials said, as a decided disincentive to the crisis-inspired attempts by state regulators to try to change entrenched rules on how water is distributed to farms and communities.