The moisture and instability that has been avoiding SA in recent weeks, if not the past month or two, is set to change from this week, with a pattern flip sending the moisture over NSW and QLD into the state from a northeast flow. Another port of moisture is expected to pass through from the north via the trough over the NT. All of this converging into our trough positioned through western and central SA bringing a large scale storm event with widespread to follow into early next week.

Forecast confidence remains low to moderate at this time, but there is better agreement this morning on the above mechanics coming to pass, it will just come down to the placement of the troughs and the speed of which it moves eastwards, and whether we see a low pressure system form on the trough next week which will increase rainfall for the state.

But this is a heads up for a dramatic shift in weather conditions for the state.


Severe Weather Watch

Not much change from yesterday with a significant storm outbreak developing from Thursday and lasting through to early next week. The heaviest impacts are expected to be over the Eyre Peninsula through to the Northwest Pastoral District but it is likely to spread through to the remainder of the Agricultural Areas during the weekend and into next week with heavy rainfall and damaging winds the main concerns at this time. Riverine flooding and ponding over the outback is likely as well. The weather will clear east by Australia Day at this time.

Flood Watch - Friday through Tuesday

Significant thunderstorm activity for a number of days over the same areas could lead to flooding for Outback areas of SA and extending into the Eyre Peninsula and possibly the remainder of the eastern districts, depending on the trough evolution and low pressure development. At this time the higher risk is for areas over the EP and points north.

Rainfall Next 7 Days

Rainfall is expected to be heavy and extensive throughout the state but more likely to be heaviest over the Eyre Peninsula and Northwest Pastoral Districts and perhaps the West Coast as well, but this may change. The rainfall will extend further east, and impact all Agricultural Areas, with the heaviness of that rainfall possibly leading to some areas of flooding as well. More clarity will come to hand as we get closer to Thursday and Friday as we see how the trough is behaving in real time.


18Z GFS - Surface Pressure Pattern and Rainfall Distribution - Next 7 Days

The persistent upper trough and deep moisture leads to major storm activity over the same region for a number of days in a row. This will lead to flood risks. The trough will gradually move eastwards over the weekend taking the thunderstorm risk to eastern and southeast areas. Another wave develops on the trough during early next week which could lead to a low pressure system forming and leading to heavy rainfall and thunderstorms redeveloping for much of the state before contracting east around Australia Day. Significant rainfall, some areas could see all their Summer rainfall in a sitting, possibly more.

18Z GFS - Precipitable Water Values - Next 7 Days

This paints the picture of the moisture engulfing the nation and how the ridge has kept most areas drier. But once we lose the ridge, it opens the door for that moisture to surge south with the upper level winds tending northeast to northerly. As an upper level disturbance comes into play, you can see that moisture goes nowhere, becomes stationary for a number of days which will lead to extensive rainfall and thunderstorm activity. This will result in some areas recording near record humidity values but also possibly some record rainfall.

18Z GFS - Rainfall Accumulation - Next 7 Days

You can see the spread of rainfall is slow to move east, so gradually the thundery rain and heavy falls will lift out of the Eyre Peninsula and West Coast and impact the remainder of southern SA during Sunday at this time. Then a more potent area of low pressure will pick up the moisture and the residual trough and a low will form, that is where you see the more widespread heavy rainfall develop for the state with some areas likely to see 1-3 months worth of rainfall with this one system. As advertised, this is was and how the Summer rainfall was going to be unfolding under the current climate guide.

More details to come on this - but remain very aware of the forecasts. The weather will evolve rapidly in coming days and the forecasts will be playing catch up.

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