Natural Resources Wales (NRW) said it was concerned by long-range forecasts indicating an increased likelihood of wetter, windier weather than normal over the coming months. The warning, reported by BBC Wales, urges communities to brace for conditions that could cause significant flooding.
Residents living in areas previously hit by flooding told BBC Wales they were worried and had already started preparing.
NRW said one in seven properties in Wales is classed as being at risk of flooding, yet only 27% of those eligible to receive flood warnings to their phone or email are currently signed up for the service. The agency is encouraging more households to register so they can be alerted quickly if rivers rise or coastal conditions become dangerous.
The call to "prepare for the unthinkable" reflects the heightened risk expected this autumn, with the agency pointing to seasonal forecasts as the basis for its concern. Communities can often face danger from both rivers and the sea, and the coming months are expected to test flood response preparations across Wales.