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.
https://www.weatherzone.com.au/news/what-to-expect-from-alfreds-dangerous-storm-tide/1890430

We are a long way from the Cyclone Alfred, but our son and family live in Kalangur, QLD (northern part of Brisbane) and he has said they have prepared for the worst.

Enjoy,
Paul

https://www.rnz.co.nz/news/world/544126/live-ex-tropical-cyclone-alfred-downgraded-to-tropical-low
Ex-tropical cyclone Alfred has been downgraded to a 'tropical low', according to Australia's Bureau of Meteorology.

Violent winds have toppled power lines as Alfred inches towards Australia's eastern coast, swelling rivers, sparking evacuation orders and leaving nearly a quarter of a million of homes without electricity.

It crossed the islands off the coast of Queensland overnight, crawling towards the densely populated coastline at "walking speed", government forecasts said.

No deaths have been reported, but the impacts are already being felt across southeast Queensland and northern New South Wales.

Ex-cyclone Alfred slowly crossing mainland; Flooding fears as huge rainfall expected; More than 300,000 homes without power.

Utility companies said 295,000 properties in southeast Queensland and another 42,600 in New South Wales were without power, warning that floods could hamper repairs.

"That's the largest ever loss of power from a natural disaster in Queensland's history," said the state's premier, David Crisafulli, estimating that about 750,000 people had been impacted since the blackouts began.

The potential dangers facing electricity crews working to restore power has been illustrated by this steel roof sheeting that landed on a powerline overnight.




'the emergency was not over'

Although the weather system "stalled and began weakening", the bureau of meteorology warned that intense rain and damaging wind gusts were a risk throughout the weekend.

"Rivers are already starting to respond to the heavy rainfall, with many Minor to Major Flood Warnings current," the bureau said in a statement.

Evacuation orders had been issued for thousands of people in New South Wales, where 30 flood rescues have been carried out over the past 24 hours, emergency services said.

New South Wales Premier Chris Minns told a news conference the emergency was not over.