A flash flood can transform a river within minutes. This is what communities near the Nepal-Tibet border witnessed on Wednesday.A massive flash flood ripped through villages along the Bhote Koshi–Trishuli river system carrying mud, debris and fast-flowing water. It has claimed the lives of at least 469 people and left hundreds more missing, including 39 Australians. Preliminary investigations suggest the flood was caused by a major collapse of ice and rock high in the Himalayas.During such disasters, every extra minute is valuable time to reach higher ground. So how can we ensure communities are prepared in the face of such rapidly developing hazards?Our research suggests early warning systems may help.How do early warning systems work?Early warning systems use various tools, data and communication channels to detect natural disasters before they strike.These systems do much more than send a text message after a flood or fire has started. They are designed to identify a potential hazard early, track it as it develops, and forecast where it may go. The most effective early warning systems communicate this information quickly, enabling the public and emergency responders to act.These systems have saved lives. In the Swiss village of Blatten, monitoring enabled authorities to evacuate residents before a massive rock-ice avalanche swept through the region in 2025. In Iceland, monitoring and established emergency procedures allowed more than 800 people to escape sudden glacial floods during the 2010 Eyjafjallajökull eruption.Tracking where a disaster travels and how it develops is crucial. In the case of floods, river-monitoring stations at strategic intervals can measure water level and discharge. Cameras and other sensors can provide information about flow velocity and debris. Combined with real-time flood modelling, these observations can indicate when a flood may reach downstream communities, how severe it could be, and which areas may be inundated.This information also helps emergency responders. If authorities know one settlement may have 20 minutes before a flood arrives while another has 50 minutes, they can prioritise evacuations, position rescue teams, and identify roads and bridges that may soon become inaccessible. Read more: Nepal’s flash floods show the devastating power of ‘cascading hazards’ When warnings aren’t enoughIn Nepal, emergency warnings were sent out but were not enough to save local villages.The cascading disaster was triggered by the collapse of a massive glacier in Nepal’s Langtang national park at about 8:40am. Nepal’s Flood Forecasting Division soon received information that a major flood had entered the Bhote Koshi river system from Tibet.Within minutes, more than 600,000 mobile warning messages were sent to the public. By then, the Syabrubesi monitoring station in northern Nepal had already stopped transmitting. As the flood moved downstream, authorities continued to track it and issue warnings.Nepalese authorities have known about the risk of such a disaster occurring for years. A 2020 assessment by the International Centre for Integrated Mountain Development and the United Nations Development Programme identified 47 potentially dangerous glacial lakes across the region’s main river basins. This was based on data from satellite images and other remote-sensing technologies used to monitor glacier activity. High-risk areas can then be monitored using cameras, lake-level gauges, weather stations, and sensors measuring ground or ice movement.The Nepali government has rolled out similar technology at Imja Lake, a fast-growing glacial lake in the Himalayas. Authorities installed sensors and automated sirens in 2016, but these have since fallen into disrepair. Read more: Tsunami of dirt: what caused Nepal’s deadly glacial flood? Not foolproofThe catastrophic Nepal flood reveals the limitations of early warning systems.For communities close to the source, flood warnings simply came too late. This is why warning systems should be automated wherever possible. When sensors detect dangerous conditions, information should immediately reach forecasting and emergency authorities. Location-based mobile alerts must then be sent to people in threatened areas.But mobile networks and electricity can fail, so phones cannot be the only solution. Sirens, radio and trained local task forces provide essential backup and ensure people know where to go and which evacuation routes to follow.The monitoring network itself must be resilient. In Nepal, several weather stations stopped transmitting or were swept away during the recent floods. Monitoring infrastructure must be designed to keep functioning even if some instruments fail. This may involve establishing more strategically spaced weather stations, communication pathways, and backup power systems. Read more: No power, no phone, no radio: why comms dropped out during the central Victorian fires More work neededEarly warning systems cannot prevent natural disasters or fully protect vulnerable communities. They must be used alongside hazard mapping, which identifies areas that are more vulnerable to a particular disaster. Resilient infrastructure and established evacuation routes are also vital to prepare for natural disasters.Research must focus on what interventions are most effective and in what regions. In the wake of the recent Nepal floods, several important questions remain, including how closely river-monitoring stations should be spaced, and how much warning time different communities need to evacuate safely.Communities may have only minutes before a disaster strikes. An effective early warning system is about making every one of those minutes count.Prashidha Khatiwada does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.