El Niño refers to the warming of ocean surface temperatures in the equatorial Pacific, which impacts global weather patterns.
El Niño winters tend to bring warmer and drier conditions across the southern United States.
The warm air masses move northward, meaning less snow for areas like the Ohio and Tennessee Valleys.
Places like the Pacific Northwest into the Northern Plains still tend to see cold air and snow during El Niño.
So for states like Oregon, Montana and the Dakotas, El Niño may not decrease the odds of a white Christmas.
But from the Midwest into the Northeast, El Niño tilts the odds toward a brown Christmas without much snow on the ground.
Historical data shows that during some past El Niño years, less than 20% of the contiguous U.S. had snow cover by Christmas.
No two El Niños are alike, so seasonal snowfall forecasts can still vary widely, but the overall pattern favors reduced early-winter snow with El Niño.