When this moisture is carried south by humid north-easterly winds and cooled by the colder air in the upper-level low-pressure system above,it condenses and falls as rain.
The phenomenon was seen repeatedly throughout Sydney’s wettest year on record in 2022,which brought widespread flooding across the state.
How much rain will fall in Sydney?
On Thursday,Weatherzone meteorologist Yoska Hernandez said the phenomenon would bring very dark skies and anywhere from 100 to 300 millimetres of rain through to Saturday afternoon.
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“The amount of moisture in the atmosphere is going to be very elevated,so the clouds are going to look black because of the amount of moisture available,” she said.
“Most of the rain is going to fall on Friday and into Saturday morning.”
Those humid northeasterly winds would bring gusts up to 80km/h along the NSW coast from late afternoon on Friday to Saturday morning,Hernandez said.
The “Black Nor’easter” is a different system to the one that dumped a month’s worth of rain on Victoria in less than 24 hours earlier in the week,Hernandez said,but would bring more rain and wild weather to the state on Saturday and Sunday.
What emergency warnings have been issued?
On Thursday afternoon,the weather bureau issued a flood warning for more than 30 rivers,creeks and lakes across the state which are expected to experience minor to major flooding in the coming days.
The Hawkesbury and Lower Nepean rivers were expected to experience minor to major flooding from Friday,while in Sydney’s south the Georges and Woronora rivers are expected to experience minor to moderate flooding. Moderate to major flooding was forecast along the Colo River in the Blue Mountains.
The NSW State Emergency Service (SES) issued 34 warnings on Thursday for locations across NSW,mainly for possible flooding. It is advising people around Sydney,Parramatta,Gosford,Wollongong,Nowra,Bowral,Batemans Bay,Katoomba and Goulburn to “prepare now” for damaging winds and heavy to locally intense rainfall.
NSW emergency services had rescued seven people and received more than 550 calls by Friday morning.
Warragamba Dam,which supplies much of Sydney’s drinking water,is expected to reach capacity over the next few days. It is currently 96 per cent full.
“The upper end of the forecast would result in the dam experiencing a more significant spill,” Water NSW said on Thursday.
A 71-year-old grandfather died when his ute was washed away in floodwaters in Brisbane’s south on Thursday.
Nine News identified the man as local Peter Wells,whose car was found submerged in waters at Greenbank,near Logan southwest of Brisbane,where more than 100 millimetres of rain fell in an hour on Wednesday night.
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