Sydney lashed by torrential rains and flash floods

Context

Australia to declare east coast floods a national emergency.

About

About Australia floods 2022:

Sydney, Australia's most populous city, bracing for more rain after being drenched by heavy downpours over the past week.
A wild weather system that dumped more than a year's worth of rainfall over a week in southern Queensland and northern New South Wales (NSW).
It has brought widespread destruction, leaving thousands of people in the states displaced and sweeping away property, livestock and roads.
Thousands of residents in Sydney’s south-west were ordered to evacuate overnight as heavy rain swelled rivers.
Australia's east coast summer has been dominated by the La Nina weather phenomenon, typically associated with greater rainfall, with most rivers at near capacity even before the latest drenching.
Sydney has received nearly 900 mm (35 inches) of rain so far in 2022, or about 80% of its annual rainfall, official data showed, the wettest start to a year since records began.

Rain:

Rain refers to water droplets forced to fall down to earth by gravitational push.
The droplets form when atmospheric water vapour condenses to water.
Rain is essential for human, plant, and animal life.
For instance, rainwater provides water for plant irrigation and hydroelectric power.

Torrential rain:

Torrential rain refers to the heavy downpour of rain.
There is no definite definition of it other than the definition provided by the National Weather Service (NWS).
The NWS defines torrential rain as rain that accumulates at a rate of three tenths of an inch or more per hour.
Causes of Torrential Rain:

Flood:

An overflow of water onto normally dry land.
The inundation of a normally dry area caused by rising water in an existing waterway, such as a river, stream, or drainage ditch.
Ponding of water at or near the point where the rain fell.
Flooding is a longer term event than flash flooding: it may last days or weeks.

Flash flood:

A flood caused by heavy or excessive rainfall in a short period of time, generally less than 6 hours.
Flash floods are usually characterized by raging torrents after heavy rains that rip through river beds, urban streets, or mountain canyons sweeping everything before them.
They can occur within minutes or a few hours of excessive rainfall.
They can also occur even if no rain has fallen, for instance after a levee or dam has failed, or after a sudden release of water by a debris or ice jam.