Extreme heat and snow drought preconditioned the August 2026 Nepal rock–ice avalanche flood

High-mountain floods can originate from slope failure rather than intense rainfall, yet the environmental states that precede such failures remain poorly resolved. On 26 August 2026, a large rock–ice avalanche from Langtang Lirung generated a destructive flood along Nepal’s Bhote Koshi–Trishuli corridor. Here we show that the failure followed an exceptional, short-lived combination of atmospheric heat and depleted seasonal snow. Precipitation was negligible around the reported failure time despite normal-to-wet antecedent moisture conditions. By contrast, July–August above-freezing thermal accumulation reached approximately the 98th percentile of the 1979–2025 record, while recent snowfall and August snowpack storage were strongly depleted. Regional snowmelt was not exceptionally high, indicating a snow-limited rather than melt-dominated state. The concurrence of exceptional heat and acute snow drought provides evidence of climatic preconditioning of the slope, without identifying the immediate trigger of failure. This antecedent state reveals a pathway to cascading mountain hazards that rainfall metrics or geological analysis alone may not capture, underscoring the need to integrate short-timescale heat and snow diagnostics with direct monitoring of unstable slopes.