Seasonal outlooks for coastal high-tide flooding

Chronic “high-tide” flooding is a growing concern for coastal communities in Australia and globally as sea levels rise. Early warnings of such events can help coastal managers prepare for their impacts. Building on previous work, we construct predictions of daily coastal sea level and threshold exceedance probability that provide information on high-tide flooding risks over the coming season/s. Our trial forecast system combines tide predictions and relative sea level rise projections with statistical representations of storm surges and ensemble monthly-mean sea level anomaly predictions from a dynamical seasonal forecast model. We assess the system’s performance at tide gauges around the Australian coastline and in the western Pacific islands. The system out-performs tide-only and climatological reference forecasts for daily maximum sea level at all locations, and beats a best constant reference forecast for 99th percentile threshold exceedance probability at most locations. Including seasonal sea level anomaly predictions from the dynamical model improves performance by up to $$\sim \,20$$ ∼ 20 % in north-west Australia and $$\sim \,40$$ ∼ 40 % in the Pacific islands, decreasing with lead time. Introducing a reliability calibration significantly improves threshold exceedance forecast skill at locations with complex tidal dynamics where harmonic tide predictions struggle to capture the highest high tides. We show that forecast skill is sensitive to the choice of impact-based (e.g., minor) or likelihood-based (e.g., 99th percentile) flood threshold, highlighting the importance of defining locally-relevant thresholds for application of the warning system. The development of real-time forecast guidance products to suit a range of potential stakeholders, from emergency services and meteorological agencies to coastal and ecosystem managers and port authorities, is ongoing.