Most people think avalanche danger comes from fresh snow, but avalanches need a weak layer in the snowpack plus a trigger.

When rain falls on snow, or temperatures spike, meltwater loosens the bond between snow layers. That can release wet slab avalanches, which can break wide with little warning and run into terrain that normally feels safe.

(MORE: Widespread snow blankets resorts from California to Montana)

Rain triggers a wet slab avalanche

A wet slab avalanche happens when water works its way down through the snowpack and weakens a layer buried below the surface. The snow above that layer, the slab, can then release all at once.

Rain makes this more likely in two ways:

• First, it adds weight to the slab.
• Second, the meltwater it creates can loosen the bonds that hold snow layers together.

The result is a heavy slab sitting on a weaker layer, with nothing holding it in place.

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Wet slabs are harder to predict than many other avalanche types. They can break wide with little warning, and because the snow is heavy and wet, they can be destructive.

El Niño could shape the setup this winter

NOAA's Climate Prediction Center says chances "remain high for a historic event," with an 83% chance for October-December.

In winter, the subtropical jet stream is usually turbocharged in a stronger El Niño. For the West Coast, that typically means a wetter winter in California. It could also bring more mountain snow if the air is cold enough, and rain instead of snow if it isn't.

Farther north, the Northwest typically trends milder and drier. That could mean less snow overall, and when storms do arrive, milder air makes it easier for the rain-snow line to climb.

(MORE: Our complete coverage of El Niño)

The rain-snow line is key

The rain-snow line is the elevation where rain changes to snow. Colder air sits north of it and warmer air sits to the south.

Warmer air pushes that line higher up the mountain. That means rain can fall on slopes that were getting snow just days earlier.

This is the setup for a wet slab. Rain soaks into a snowpack that has been building for weeks, adding weight and loosening the layers underneath.

It can also be easy to miss. The slopes may look the same as they did yesterday, and there's no fresh snow to warn you. The weak layer is already buried.

Skiers, snowboarders, hikers and mountain pass drivers should know that rain doesn't mean you're in the clear. Rain on snow can raise the avalanche danger.

Faiza Mohammed is a meteorologist intern at weather.com. She studied geography and worked with the Weather Dawgs team at the University of Georgia. She also received her master's in Earth and Atmospheric Sciences from the Georgia Institute of Technology.