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What to expect from Alfred's dangerous storm tide

Source Weatherzone Thu 06 Mar 2025

Coastal areas in southeast Queensland and northeast NSW are already being hammered by huge waves and abnormally high tides, but Tropical Cyclone Alfred’s peak storm tide is still on the way. While wind and rain get a lot of attention when tropical cyclones are about to make landfall, the cyclone’s storm tide and storm surge are often less understood. However, storm tides and surges can be some of the most dangerous impacts from a landfalling tropical cyclone, particularly for low-lying areas near the coast. What is a storm surge and storm tide? Tides go up and down every day due to gravitational forces exerted on the earth by the moon and the sun, with coastal areas experiencing two high tides and two low tides on most days throughout the year. However, weather systems can also influence the tides by causing water levels to get higher or lower than the expected astronomical tide. Tropical cyclones are known to have a strong influence on tides because of their powerful winds and low atmospheric pressure: The ferocious winds associated with a landfalling tropical cyclone forces water to pile up along the shoreline as the cyclone approaches and crosses the coast. The decrease in atmospheric pressure near the centre of the tropical cyclone allows the water level to rise because there is less air pushing down on the water’s surface. As a rule of thumb, every 10 hPa drop in air pressure corresponds to a 10 cm rise in water level. Storm surge refers to the additional rise in sea level above the expected astronomical tide caused by the presence of a storm or tropical cyclone. The storm surge is the difference between the expected tide level and the observed tide level. Storm tide refers to the height of the water when you combine the astronomical tide and the storm surge. For example, an expected astronomical tide of 4 metres with a storm surge of 3 metres will result in a storm tide of 7 metres. Storm surges usually peak as a tropical cyclone makes landfall due to the abrupt spike in wind speed and drop in atmospheric pressure. However, the magnitude of the resulting storm tide will depend on the timing of landfall in relation to the astronomical tide. A storm surge has its most severe impact when it occurs around the time of the astronomical high tide. Some tide gauges along the coast of southeast Qld are already registering more than half a metre difference between expected and observed tide levels, largely due to the wind and waves being pushed towards eastern Australia by Alfred. Image: Tide predictions and observations for Maroochydore, showing the observed tide running about 40 to 60 cm above the expected tide in the last couple of days. Source: Queensland Government. Tropical Cyclone Alfred is currently expected to make landfall at some point on Friday or Saturday. The highest astronomical tides in the next few days will occur in the early morning hours of each day (overnight), with a secondary smaller high tide each afternoon. This means an overnight landfall on Friday night would cause a higher storm tide than landfall during mid-to-late morning Friday or Saturday. Wave heights have also started to increase substantially as Alfred edges towards the coast. Maximum wave heights in excess of 15 metres were recorded near North Stradbroke Island and Tweed Heads on Wednesday and Thursday. A maximum wave height of 16.52 metres was reported off the Gold Coast at 9am on Thursday. However, the Queensland Government has since removed this data from their website, stating: "Unfortunately the Gold Coast wave buoy has sustained some damage during this weather event. Data is unreliable and therefore has been removed." Even small changes in the timing and location of Tropical Cyclone Alfred’s landfall will have a big influence on which areas see the biggest storm surge, storm tide and waves. Be sure to stay up to date with the latest tropical cyclone advisories and severe weather and flood warnings in the next few days. - Weatherzone © Weatherzone 2025