Storm surge

Storm surge is an abnormal rise in water level caused by a storm, measured above the predicted astronomical tide. Storm tide is the observed water level resulting from the combination of storm surge and the astronomical tide. These terms describe water levels, not the additional effects of waves breaking over the water surface. In forecasts and public communication, definitions and reference levels matter: a stated water height may be measured relative to normally dry ground, a tidal datum, or another baseline.

Strong winds blowing onshore push water toward the coast. In deep water, storm-driven circulation can move water vertically; near the coast, the seabed limits that downward motion and water can pile up and move inland. Storm-related air pressure and other processes can also affect sea level, but wind is the primary driver of surge. The resulting flooding is shaped by the storm's interaction with local ocean and land geography.

Surge depends on several interacting factors, including storm intensity, size and forward speed, its angle of approach, the shape of the coastline, and the width and slope of the continental shelf. A broad, gently sloping shelf can favor higher surge than a steep shelf. Bays, inlets, barrier islands, rivers and other local features alter how water enters and moves through a coast. Tides also matter: when surge coincides with a high astronomical tide, the resulting storm tide is higher. Waves and wave-driven setup can raise water levels and intensify impacts beyond the surge alone.

Coastal inundation can flood homes and infrastructure, damage ports and boats, erode beaches and undermine roads or building foundations. Water can move through waterways and reach locations well away from the immediate shoreline. Saltwater flooding can affect estuaries, wetlands, freshwater supplies and soils. Storm surge is a major hazard during tropical cyclones, but storm-driven coastal water rise can occur with other kinds of storms as well.

Forecasts combine storm predictions with models of tides and coastal flooding. Because a storm's track, intensity, size and speed are uncertain, forecast maps can represent a range of possible inundation rather than a single guaranteed waterline. Instruments deployed along the coast, tide gauges, high-water marks and post-storm surveys help document what occurred. Forecast products and warnings should be interpreted using their stated baselines and probability information; local emergency management instructions take precedence for protective actions.