Flash floods are sudden, powerful surges of water that can rapidly move through gorges, valleys and other low, narrow areas. The article cites recent severe losses and more than 800 missing people in Nepal, while noting that such floods have also been reported in Bangladesh and elsewhere. Unlike slower river flooding after prolonged rain, flash floods can turn dry riverbeds into overflowing channels after only three to four hours of rainfall.

Heavy rain linked to thunderstorms is a main trigger, while rapid snow and ice melt, dam failures and landslides can also cause flash floods. Steep terrain and impermeable surfaces such as rock, roads and roofs accelerate runoff toward lower areas. The report says warmer air can hold more moisture, potentially increasing heavy rainfall, though scientists say it is still too early to determine climate change's role in Nepal's event.

Experts say flash floods can be forecast, but warning time is limited and data are scarce in remote mountain basins. Radar, satellites and automatic sensors can provide real-time information on lake levels and dam conditions. Nepalese specialists called for stronger international data systems, satellite monitoring in uninhabited risk zones and avoiding settlement in vulnerable areas.