Dangerous supercell storms to bring wild weather and potential flooding to eastern parts of Australia

The severe weather system has already brought storms and heavy rain to Adelaide. (Supplied: Katrina Jamieson)

A major storm system developing over south-east Australia has the potential to generate not only widespread flooding but also significant damage from violent, rotating supercell thunderstorms that could even spawn tornadoes.

The system already dumped a month’s worth of rain in an hour on parts of Adelaide on Tuesday morning, and is now heading towards the eastern states with rain and storm activity increasing rapidly on Tuesday afternoon from northern Victoria to central Queensland.

The greatest risk of severe weather through the remainder of Tuesday is across the eastern inland but will spread further east on Wednesday to the eastern seaboard including Brisbane, Sydney and Canberra.

The storms are forming just one component of a significant low-pressure system which will bring heavy rain and flooding this week from the northern tropics down to coastal Victoria.

Tornado risk from explosive thunderstorms

By Tuesday afternoon the most intense thunderstorms were moving across the border to western NSW and north-west Victoria, triggering warnings for flash flooding and damaging gusts.

The storms should intensity further through Tuesday afternoon under a jet stream, a band of very strong winds about 10 kilometres above the ground, and have the potential to reach supercell status from roughly Mildura to Bourke.

“Large changes in wind speed and direction in the low to middle levels of the atmosphere can cause the updrafts to rotate and produce violent winds, large hail, commonly known as supercell thunderstorms,” Milton Speer, a meteorologist based at the University of Technology Sydney, said.

Supercells are the least common but most dangerous type of thunderstorm — they are long lasting cells capable of generating hail greater than 5 centimetres in diameter and destructive wind gusts in excess of 125 kilometres per hour.

Occasionally the rotation in the cloud stretches to the ground to form a tornado, and while Australia only see around 1 per cent the number of tornadoes seen each year in the USA, we average more than 10 per year.

The system is close to the perfect storm for Australia, with warm humid air from the Coral Sea undercutting the powerful cold winds aloft, creating an environment favourable for one or two twisters during the next 48 hours.

The most intense storms will continue moving east on Wednesday as the low-pressure system drifts from the inland towards the NSW coast.

Wednesday’s potential supercell zone will stretch from roughly Sydney to Brisbane with a peak threat over northern NSW directly under the powerful jet stream.

However, even non-supercell thunderstorms have the potential to bring hail, damaging wind gust and flash flooding.

“The strong winds associated with jet streaks can reach the earth’s surface in severe storms even without rotation of updrafts in the form of straight line downbursts,” Dr Speer said.

Flood risk for NSW and Victoria

The second component of the low-pressure system is widespread rain and possible flooding.

While storms bring localised flash flooding, the system will also deliver hours of heavy soaking rain to central and southern NSW along with eastern Victoria, prompting a flood watch for more than 40 rivers.

The NSW South Coast is likely to receive the highest rain totals on Wednesday — potentially up to 150 millimetres in 24 hours.

The heaviest rain on Thursday may shift to Gippsland if the low drifts slightly south, although significant falls are still possible on the NSW South Coast.

The low is likely to only move slowly away from the coast from Friday to Sunday, however relatively widespread showers, and even a few storms, are likely to persist along the coast and ranges of eastern Victoria and NSW through the weekend.

In the meantime, thunderstorms will continue to fire up each day over much of Queensland , completing an exceptional wet week for eastern Australia.