On February 17, 2026, nine backcountry skiers died in an avalanche near Lake Tahoe's Castle Peak. The same storm that killed them dropped over five feet of snow on Mammoth Mountain in three days, forced a 60-mile closure of Interstate 80, and shut down every major ski resort in the Sierra Nevada. It took four days to recover all the bodies. Another storm rolled in before searchers could finish.

This wasn't a freak accident in a low-snow year. It happened during one of the most intense atmospheric river events the Sierra has seen in years, part of a pattern that's rewriting the rules of avalanche forecasting.

The Paradox Nobody Expected

Climate change is doing something strange to alpine snowpack. The overall picture shows declining snow depth worldwide, particularly at lower elevations where the snowline keeps creeping upward. But at the highest elevations—above 9,000 feet in the Sierra Nevada, above 3,000 meters in the Swiss Alps—something different is happening.

"We do expect that in the highest elevations in the Sierra, for example, there to actually be more snowfall," says Ned Bair, a researcher at UC Santa Barbara and former research chairman of the American Avalanche Association. The atmosphere holds more moisture as it warms, and when storms do hit, they hit harder.

This creates a split-screen future: less snow where people live and play at moderate elevations, but potentially more dangerous conditions at the peaks where backcountry enthusiasts are increasingly venturing.

When Atmospheric Rivers Intensify

The key factor isn't just temperature or total precipitation. It's how the snow arrives.

Atmospheric rivers—high-altitude currents of moisture flowing from tropical ocean regions—have always shaped Sierra weather. But their intensity is changing. Instead of steady snowfall building stable layers, these supercharged storms dump enormous amounts of precipitation in short windows. Mammoth received 27 to 39 inches during the February 2026 event, with another 20 to 26 inches forecast immediately after.

"What really matters with the avalanches is the intensity of the atmospheric rivers," Bair explains. Rapid loading creates textbook avalanche conditions: heavy new snow piling onto older layers before the snowpack can adjust. The Sierra Avalanche Center's warning on February 17 was blunt: "Travel in, near, or below avalanche terrain is not recommended today. A widespread natural avalanche cycle is expected over the next 24 hours."

They weren't wrong. Beyond the Castle Peak tragedy, two ski patrollers were injured at Mammoth Mountain on December 26, 2025, during avalanche mitigation work after another massive storm cycle dropped five feet of snow in three days.

Mapping a Moving Target

Swiss researchers published the first comprehensive quantification of climate change's impact on avalanche distribution in April 2024. Using snow cover simulations spanning 1997 to 2100, they created spatial risk maps showing where avalanche danger will increase and where it will fade.

The findings confirm the paradox. At lower altitudes, avalanche risk should decrease as snow becomes less reliable. Some side valleys that currently require avalanche protection infrastructure may eventually need none at all. But at high elevations, some climate models suggest increased snow accumulation leading to larger avalanches with greater destructive force.

The research used sophisticated tools—the RAMMS avalanche model combined with the CLIMADA risk assessment platform—to produce maps that account for changing snow patterns, temperature shifts, and terrain features. But models only work if the underlying climate scenarios prove accurate, and atmospheric rivers are notoriously difficult to predict decades in advance.

The Speed of Safety

Ski resorts have always dealt with avalanche risk, but the operational challenges are intensifying. When Palisades Tahoe, Mammoth, and other Sierra resorts shut down on February 17, it wasn't just about guest safety. The mountains themselves became too dangerous for the professionals who manage them.

"Teams work at the speed of safety," Palisades Tahoe explained, describing the process of digging out lifts, performing avalanche mitigation, assessing terrain, and preparing slopes after major storms. That speed has slowed. More extreme storms mean longer closures, more explosive charges needed for mitigation, and more uncertainty about when terrain is truly safe.

Mt. Shasta Ski Park offers a glimpse of the other side of this shift. The resort had been closed for lack of snow—then suddenly reopened on February 17 when the atmospheric river finally reached them. Feast or famine, with less middle ground.

Backcountry's Dangerous Allure

The Castle Peak avalanche killed experienced backcountry skiers, not beginners wandering into terrain beyond their skill level. As resorts close more frequently during the most intense storms, and as lower-elevation snow becomes less reliable, more skiers are pushing into higher, steeper terrain during the narrow windows when conditions align.

The math is grim. Avalanche warnings now regularly cover hundreds of square miles—the February 17 warning stretched from Yuba Pass to Ebbetts Pass, encompassing the entire Lake Tahoe basin. The danger level was HIGH, meaning natural avalanches likely and human-triggered avalanches very likely. Yet backcountry use continues to grow.

The Sierra Avalanche Center and similar organizations worldwide face an impossible task: predicting snow behavior in a climate that's rewriting the patterns they've studied for decades. Old assumptions about storm cycles, snowpack structure, and seasonal timing no longer hold. The historical record that avalanche forecasters rely on is becoming less relevant with each passing year.

The nine skiers who died near Castle Peak were recovered by February 21, after the storm finally cleared enough for helicopters to reach them. Another atmospheric river is already forming over the Pacific.