Eastern Australia has experienced an active thunderstorm pattern in recent days. Amid the broader storm activity, slow-moving thunderstorms affected the Sydney Metropolitan, Illawarra, Central Tablelands and Hunter regions on Friday, prompting heavy rain warnings. This story looks at the nature of these storms, focusing on why they became slow-moving and how the atmospheric setup supported heavy rainfall. 

Thunderstorms on Friday produced very intense short-duration rainfall, particularly across parts of the Central Coast, Hunter and Sydney regions. 

Image. Satellite, lightning and radar imagery over central eastern NSW on Friday morning. Source: Weatherzone. 

Some of the more notable short-period rainfall observations on Friday included: 

Martinsville 81.0 mm in the 3 hours to 4:04 p and 57.5 mm in the 60 minutes to 2:45 pm; 

Lake Macquarie 76.0 mm in the 2 hours to 2:58 pm; 

Norah Head 44.2 mm in 30 minutes to 7:58 am; 

Linden 61.0 mm in the 2 hours to 7:30 am; 

Kangy Angy 74.5 mm in the 2 hours to 6:17 am; 

Lake Macquarie 70.0 mm in the 3 hours to 3:30 am. 

Across NSW, some of the higher observed rainfall totals during the 24 hours to Saturday 9am included: 

Martinsville 98.5 mm; 

Bulahdelah 63.5 mm; 

Port Kembla 61.5 mm; 

Broken Back (Cedar Creek) 52.4 mm; 

Lostock 42.0 mm; 

Holsworthy (Defence) 40.0 mm; 

Sydney Olympic Park 38.8 mm; 

Badgerys Creek 28.8 mm; 

Camden 28.7 mm; 

Bankstown 27.6 mm; 

Kiama 25.2 mm; 

Maitland Airport 24.0 mm; 

Newcastle Nobbys 21.4 mm; 

In southeast Queensland, thunderstorms also delivered areas of heavy rain, with standout totals including: 

Coolangatta 71.0 mm; 

Inner Beacon 41.0 mm; 

Brisbane Airport 26.4 mm; 

Gold Coast Seaway 24.4 mm; 

Brisbane 22.2 mm; 

Cape Moreton 22.2 mm. 

 Why rainfall rates were so intense in some places? 

To better understand why these storms behaved the way they did, we’ll take a closer look at a skew-T diagram from Friday. Meteorologists use skew-T diagrams to analyse the vertical structure of the atmosphere. The diagram is called a skew-T because temperature lines are drawn at an angle, or “skewed”, rather than vertically, which makes it easier to interpret temperature changes with height. Skew-T diagrams help assess thunderstorm potential, storm intensity and the types of storms that may produce hazards such as heavy rain, large hail or damaging winds. 

As shown in the modelled skew-T diagram below for northern parts of the Illawarra, the atmosphere on Friday was set up in a way that strongly favoured heavy rainfall. Steering winds, which are the winds that control how fast storms move, were weak through the lower and middle levels of the atmosphere, generally less than about 10-15 knots, causing storms to drift slowly rather than move away quickly. The air was also very moist, with precipitable water values close to 30-35 mm, meaning there was a large amount of moisture available to be converted into rainfall. Clouds also formed at very low levels (roughly 300-400 metres above sea level). This meant rain had only a short distance to fall through drier air, reducing evaporation and allowing rainfall to reach the ground more efficiently.  

CAPE, or Convective Available Potential Energy, is a measure of how much energy is available for air parcels to rise once storms develop. In this case, moderate instability was present, with CAPE values around 400-500 J/kg. While not extreme, this was sufficient to support thunderstorm development and maintain sustained updrafts when combined with deep moisture through the atmosphere. This setup allowed storms to produce very intense rainfall rates, and because the storms moved slowly, rainfall accumulated rapidly, leading to flash flooding, particularly across urban areas and along the escarpment and ranges.

Image. Modelled skew-T diagram for Friday 12th December around midday for a location in northern parts of the Illawarra, where thunderstorms with heavy rainfall developed. 

What’s to come for today, Saturday? 

A trough supported by an upper-level disturbance is triggering scattered severe thunderstorms across parts of inland and eastern NSW, including Sydney, with warnings for damaging winds, large hail and heavy rainfall in place as of 2pm EDT. This system will continue to move east through the remainder of today, bringing severe thunderstorms into southern Qld. Storm activity is likely to persist into Sunday, potentially impacting Brisbane as well as broader areas of northern and eastern Qld.