Rain at 10,000 feet. In the Bridger Range outside Bozeman, in the Sawtooths above Ketchum, across the high basins of central Idaho where winter snowpack is supposed to be dry and cold and light underfoot. The rain falls, soaks the surface, and stops. Temperature drops overnight. The wet layer locks into a smooth, hard crust. Then new snow buries it.

What sits above the crust doesn't know the crust is there. Neither does the skier crossing the slope. Not until the whole slab releases on that glassy surface and moves downhill with a speed and mass that continental snowpacks weren't supposed to produce.

A study published this winter in ARC Geophysical Research by University of Washington researchers Clinton Alden, Benjamin Sullender, John Stimberis, and Jessica Lundquist mapped this transformation. Using data from 53 snow monitoring sites and a high-resolution hydrologic model, the team simulated what happens to snowpack structure under 2°C and 4°C of warming. The finding splits along a geological fault line: the Cascade Range.

West of the crest, warming makes snow wetter. At Snoqualmie Pass, where co-author Stimberis supervises avalanche forecasting for Washington State DOT, rain-on-snow is already routine. The surface saturates, goes granular, softens into slush that a boot sinks through. It doesn't refreeze hard because ambient temperatures stay too warm. Avalanche risk from ice crusts actually decreases. Forecasters there have spent careers reading this kind of snowpack.

East of the crest, the same warming produces the opposite. Temperatures climb just enough to turn snowfall into rain at elevation, but the continental cold snaps still come. The rain refreezes. The crust forms. And everything that accumulates on top of it sits on glass.

This winter, the projections read like field notes. Oregon's statewide snow water equivalent hit a record low of 2.9 inches in early February. Washington's snowpack sat at the 5th percentile of historical conditions. October through January was the warmest start to a water year on record for the state.

But the snow that did fall inland told a different story than simple absence. In the Gallatin National Forest surrounding Bozeman, avalanche center observers documented a "thick and robust ice crust layer" in the Bridger Range. Stability tests fractured repeatedly on old snow crystals trapped beneath crusts buried 16 inches deep. At Bridger Bowl, a ski cut triggered a slide that ran on a melt-freeze crust, entraining more mass than the forecaster expected. The center's season summary referred to "the nasty persistent weak layers of this winter." Nasty carries weight from people whose professional vocabulary runs toward understatement.

In central Idaho, Scott Savage, director of the Sawtooth Avalanche Center, described conditions after December storms dumped more than a foot of snow onto an already compromised snowpack:

Beneath the new snow sat weak layers from rain crusts and facets. Over the past decade, Idaho has become the state with the second-most avalanche fatalities in the country, behind only Colorado.

Stimberis works the maritime side, where the wet snowpack is familiar, and fields calls from colleagues in the continental ranges encountering rain-on-snow for the first time.

That second question is harder to answer with ice crusts. Crusts trap water inside the snowpack. When that water moves, it can trigger large avalanches that travel faster and farther than expected. The instability can persist for weeks.

An Idaho water official described this season as the first time the state experienced a "wet snow drought." Precipitation totals near normal, but so much of it falling as rain that the snowpack barely registered. Plenty of water, all of it wrong.

The forecasters in the inland ranges are learning to read a snowpack that no longer behaves the way their training taught them it would. Their hands probe snowpits and find layers that shouldn't be there, smooth and hard and wrong. The snow has become something else entirely, in mountains where the people who read it for a living are still learning its language.