Here's how the massive storm smashing Sydney formed and why it's so dangerous

Large swell and strong winds smash Sydney several times a year. Photo: Getty Images

The brutal storm lashing Sydney’s coast is creating havoc for thousands of commuters, bringing down trees, causing severe property damage – and sadly, taking lives.

A severe weather warning remains in place for Sydney, with another storm cell off Newcastle bringing continued heavy rain. More flash flooding is predicted around the city’s CBD.

Conditions are anticipated to ease later today. But how did it form?

The Bureau of Meteorology’s (BoM) manager of NSW Weather Services Andrew Treloar told Business Insider it’s an intense low pressure system, often referred to as an East Coast Low (ECL).

“They are quite frequent visitors to the NSW coast — on average several times a year -– however some years can be quieter and others busier than average,” Treloar says.

Treloar said the last significant system to hit Sydney was in October 2014 when there were wind gusts of around 160km/h recorded to the south of Sydney.

“Luckily, the damage from that event wasn’t as widespread,” he said.

“They [ECLs] often develop in the autumn/winter seasons, generally within an existing low pressure trough.

“They are very ‘dynamically’ driven and will have a significant upper-air disturbance to their west over inland NSW. The warm water of the east Australian current often provides ‘fuel’ for their development.”

Warnings for this event began with a tweet from the BoM last Friday about deteriorating conditions ahead for the NSW coast.

“Formal warnings began on Sunday and have been updated throughout the event. Our first flood watch went out Monday morning,” Treloar said.

Why are these types of low pressure system so dangerous?

The BoM says ECLs can cause:

  • Gale or storm force winds along the coast and adjacent waters.
  • Heavy widespread rainfall leading to flash and/or major river flooding.
  • Very rough seas and prolonged heavy swells over coastal and ocean waters which can cause damage to the coastline.
  • Falling trees and flash flooding, leading to fatalities on the land. Many small craft have been lost off the coast and larger vessels have run aground during these events.

Here’s what the ECL looks like as it tracks south across Sydney: