Spring tracking will be the wettest on record for South East Australia as forecast for a wet weekend

Spring 2022 is on track to be the wettest on record for South East Australia as another round of heavy rain and flooding arrives this weekend.

In a repeat of the pattern seen several times since August, a band of thunderstorms will first develop across central and southern Australia on Saturday, then expand to the eastern states through Sunday and Monday, bringing heavy rain from north-west Queensland to Tasmania.

Heavy thunderstorms on Thursday and Friday

Before the main event arrives, a storm is already bringing severe weather to the southern states.

On Thursday, violent thunderstorms dropped torrential rain on towns in western Victoria and south-west NSW, including 28mm in Yanac by 6pm.

Thunderstorms on Friday are likely across much of Victoria and NSW, including Canberra and Melbourne.

A weekend of wild thunderstorms and torrential rain

Just as quickly as Friday night’s first band of storms is dissipating, another region of thunder and lightning is set to flare up across central Australia, but this time development will be aided by a low-pressure system in the bay, a replica of the previous flood event in late October.

Saturday’s focus is on South Australia and the Northern Territory, where potentially dangerous storms could form, particularly in the region from Adelaide to Alice Springs.

The storms will spread to western Victoria and far western NSW later on Saturday before engulfing the eastern states on Sunday and early Monday. Heavy rain, gusty winds and hail are possible across a wide band from around Mount Isa to Melbourne.

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Major floods for dozens of rivers

Long before the flood from the previous rain event has receded, renewed flood peaks are likely to form after the weekend flood.

An average gradient of 30 to 70 mm should fall along the slopes and ridges of the Great Divide, the source of downstream flood peaks.

In the higher areas around the Alps, totals could exceed 100mm, although the array is helping to create storm clusters that could cause isolated falls to 100mm anywhere but the east coast this weekend.

Of prime importance is the central and southern Murray-Darling Basin where major flooding is possible along numerous rivers including the Gwydir, Castlereagh, Namoi, Macquarie, Lachlan, Murrumbidgee, Ovens, King and Goulburn rivers

This next round of flooding will also send additional water into the Murray River.

The current flow rate along the Murray in South Australia is already exceeding 90 gigalitres per day and is expected to reach 135 gigalitres in December, the highest flow rate since 1975.

Why rainbands form so regularly

The upcoming soak follows the same familiar pattern as previous events this spring:

• Tropical moisture flows south through Australia due to a sustained La Niña and negative dipole in the Indian Ocean
• The moisture will interact with a low pressure system and front to create thick clouds and thunderstorms
• The resulting rain will fall on already wet catchments, leading to significant runoff into already swollen rivers and through full dams
• Rivers will burst their banks and cause significant flooding

The rain will ease by Tuesday, but there are already signs that the pattern will repeat next weekend.