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.
The UW Climate Impacts Group projects the Pacific Northwest will warm 2°C to 5°C by 2050 compared to pre-2000 temperatures. Even at the low end, the study shows a sharp divergence: fewer ice crust days at maritime sites, more across the inland ranges. At 4°C, the inland increase intensifies considerably.
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:
"Stacking textbooks on a layer of potato chips."
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.
"They tell me they're getting rain at 10,000 feet, which they've never seen before. They want to know when to expect the onset of avalanches and when to expect the return to stability."
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.
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Record snow drought downstream: Oregon's snowpack crisis extends well beyond avalanche risk, with statewide snow water equivalent nearly 30% below the previous record low set in 2015, threatening water supply, hydropower, agriculture, and an earlier, longer wildfire season across the Northwest.
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Early heat, unadapted bodies: An unprecedented March heat wave broke dozens of temperature records across the western U.S., and doctors warn that bodies take roughly two weeks to acclimatize, making early-season heat disproportionately lethal compared to late-summer events.
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Warming has measurably accelerated: A Potsdam Institute analysis stripped El Niño, volcanic, and solar noise from temperature records and found that global warming's underlying trend has picked up speed since 2015, potentially breaching 1.5°C before 2030.
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Hidden wildfire carbon underground: Researchers found that smoldering peat fires in boreal forests can release enormous stores of ancient carbon largely invisible to satellites, with models underestimating emissions by up to a factor of 14 in peat-rich areas.