The pattern is turning dynamic in the lead into Christmas over northern Australia with troughs over much of the interior as well, that in direct response to the stifling heatwave conditions over the outback - which is not a shocker for this part of the world, but the heat levels are extreme, which means the low pressure that develops will be broad in nature.

The moisture that is building over northern Australia will likely be drawn into these troughs creating showers and thunderstorms most afternoons through this period, the coverage and spread dependent upon the location of the troughs in relation to moisture, but the overall signal on that front is unchanged.

Tropical waves over the Arafura/Timor Seas may develop further as we edge closer to New Years Eve which may enhance rainfall over parts of the north. If a tropical low/cyclone forms, this will increase rainfall and storm activity south and east of this feature across the nation.

Finally, the southern parts of the nation will remain dry at this stage, but that said, there is a risk of a few showers and thunderstorms through the period, but the further you are inland of the coast, the more likely you are to feel the humid weather and see showers and thunderstorms.

Lets take a look


Tropical Depression Risk - December 23-30 2021

The tropics are expected to become more active during this period, with a few tropical waves embedded in the easterly flow up north to whip up the rainfall and thunderstorm activity. A Rossby wave is expected to move over the Arafura and Timor Sea through the end of the year and this may support the development of a tropical low offshore the NT. If this develops this may help to drag in a northwest/monsoonal flow over northern Australia. The MJO is currently placed over the Western Pacific and is likely to move on by to the east during this period with the tail of that monsoonal flow briefly coming onshore the NT but more likely Cape York.

Moisture Watch Next Week - December 23-30 2021

No change in the overall guidance from yesterday with moisture expected seep further south and east during this time, and will be connected to the tropical activity over northern Australia. Where that low/tropical wave forms, the moisture may spread south and east of this feature as it works through the nation. Otherwise, the easterly winds over the eastern states will propel moisture back and seed a trough over the inland areas producing showers and thunderstorms. There will be another trough over in the west of the nation that will also be fed by modest moisture from northern Australia. The far south and southwest will be dry and settled for most of this time, however I am watching the impacts of a deep trough moving through VIC, NSW and QLD towards the end of the year with the chance of a more widespread rainfall event emerging.

Rainfall Next Week - December 23-30 2021

Rainfall is expected to become more widespread as the period drags on, especially from Christmas onwards, with troughs deepening and slowly meandering around the inland in response to the high heat of this week into early next week. Moisture will be deeper over the northern and eastern areas of the nation to begin with leading to more rainfall over these areas but as we approach the end of the year, the rainfall may move over southeast areas of the nation with moderate to heavy falls possible. At this stage, the southern areas of SA and much of the SWLD of WA and up along the west coast will remain dry at this time.

Rainfall Anomalies - December 16-30 2021

The rainfall anomalies are expected to lean above average over the northern and eastern parts of the nation, but more likely to see above average rainfall through pockets of QLD and northern NT. The shower and thunderstorm coverage will also cause central areas of the nation extending into NSW and western QLD to be above average through this period, but the overall trend is not as high as what we saw back in November. The drier bias persists for the south and southwest of the nation with ridging nearby.

Temperature Anomalies Next Week - December 23-30 2021

The temperatures still look to moderate through this period after this vicious high heat over much of the interior. The increase in cloud cover and rainfall will assist in this. The weather is expected to turn cooler over NSW and QLD through the ACT into eastern VIC with onshore winds, while the southern portions of SA and through WA may be above average.

DATA - This is looking primarily at the period December 23-31 2021

18Z GFS Surface Pressure Pattern and Rainfall Distribution - Next 16 Days

More details in the video.

18z GFS Precipitable Water Anomalies - Next 16 Days

Note the deepening moisture over the northern tropics at levels not seen so far this season which indicates a very wet phase may be around the corner for the north and east.

18z GFS Rainfall Accumulation - Next 16 days

This is still the driest out of all the data sets today but it is starting to pick up the monsoonal weather north of Australia.

12z CMC Rainfall Accumulation - Next 15 days

More details in the video - but very good agreement with the Euro

12z Euro Rainfall Accumulation - Next 15 days

More details in the video - but this is in good agreement with CMC for the medium term.