When disseminating ensemble flood forecasts, what key piece of information do they provide that a single deterministic forecast cannot, regarding future flood extent?

A single deterministic forecast provides one specific prediction of the future flood extent, representing what is considered the single most likely outcome. It presents a solitary boundary for the anticipated flood area without indicating the confidence in this prediction or the likelihood of alternative scenarios. In contrast, an ensemble flood forecast comprises multiple individual forecasts, known as 'members,' each generated from slightly varied initial conditions, model parameters, or different model physics within the forecasting system. These variations reflect the inherent uncertainties in weather observations, hydrological processes, and model limitations. The key piece of information ensemble flood forecasts provide that a single deterministic forecast cannot is the *quantification of uncertaintyand, consequently, the *probability of various possible future flood extents*. Instead of a single flood boundary, the ensemble allows forecasters to assess the range of potential flood extents and assign probabilities to these different outcomes. For example, it can indicate that there is a 70% chance the flood will reach a certain elevation or geographic point, a 50% chance it will reach a further point, and a 10% chance it will reach an even more extreme extent. This probabilistic information empowers decision-makers to understand not just one potential future, but a spectrum of possibilities, along with their likelihoods, enabling more robust risk assessment and preparedness strategies.