The possible return of La Niña threatens to give the drought-ravaged U.S. West another winter without much rain or snow.

The U.S. Climate Prediction Center issued a watch for La Niña on Thursday, saying there’s a 66% chance the phenomenon will return for a second straight year some time in the November-January period. La Niña occurs when the equatorial Pacific Ocean cools, triggering an atmospheric chain reaction that can cause droughts across the western U.S. and roil weather systems globally.