• Flash droughts are an emerging type of drought that has newly emerged as climate change intensifies hydrologic extremes. Within days to weeks, they can rapidly deplete water resources due to a combination of precipitation deficits and high temperatures that increase evapotranspiration.

• These droughts are already occurring in South Korea, with a marked rise in heatwave-induced flash droughts during summer. Their impact goes beyond agriculture, affecting municipal water supplies as well.

• However, the conventional drought definition emphasizes a slow-onset, long-duration phenomenon. This leaves flash droughts largely invisible within existing early warning, monitoring, and response systems. As a result, South Korea's current monthly drought statistics and warning systems often fail to detect them, even when real-world damage is occurring during officially ‘drought-free’ periods.

• Due to their ability to rapidly exhaust water resources, flash droughts pose a major threat to sustainable water management. Addressing them requires a new operational definition. We also need high-temperature-based indices and integration of weekly forecasts into drought monitoring, statistics, and response frameworks.