Outburst floods from glacial lakes have predominantly occurred during ablation seasons, with few documented cases for frozen lakes during winter. The rarity of winter failures has led to the perception that glacial lakes with frozen surfaces and limited meltwater are generally regarded as safe. Here we report a winter-outburst flood from a frozen proglacial lake in the Central Himalaya on 16 December 2024. Two lateral rockfalls with a total volume of ~4.29 Mm<sup>3</sup> broken ~0.35 m thick lake ice below and triggered an “ice tsunami” at the lake terminus, unleashing a partial drainage of the ice-covered lake and triggering a flash flood that travelled roughly 140 km downstream. Although the lake-ice cover had blunted catastrophic overtopping by damping impact energy and wave amplitude, this winter outburst case suggests that the occurrence window for future GLOFs will temporally extend in high mountain areas as paraglacial slope failures increase and lake ice diminishes during winter. This single event therefore reveals a systematic blind spot in current GLOF risk assessments: frozen lakes are not inherently safe. We therefore urge a call to heighten public awareness of this emerging dangerous and future GLOF risk assessments/early-warning systems should incorporate lake-ice condition as one of monitoring parameters under relevant safety protocols.