December Outlook 2024

December outlook 2024 SA

*Forecast prepared between December 3rd-8th 2024

Review:

G’Day everyone, I hope you have all been keeping well. I apologise for the lack of a November mid-month update and a later December outlook, but things have become beyond busy of late around the traps. I trust you have also been flat out finishing the harvest and dodging the rain where possible.

November as a whole finally saw a change in the dry pattern that has ravaged SA and VIC for much of this year. This at least did pan out as expected with the mid to late November and early December period the main talking point for this uptick in moisture, driven by tropical infeeds with humid air resulting in showers and thunderstorms with areas of rain as well. Many areas received average to well above average rainfall across the Pastoral areas and right down into very dry agricultural areas including the Riverland and Upper North – two regions that were in severe drought. Let's get straight to the charts below to see how the rainfall fell and how it stacked up to the official November outlook.

Figure 1: November 2024 rainfall deciles – source BoM http://www.bom.gov.au/climate/

Figure 2: November 2024 rainfall percentages – source BoM http://www.bom.gov.au/climate/

Figure 3: November 2024 rainfall anomalies – source BoM http://www.bom.gov.au/climate/

Now the main thing that stands out from the above charts is the strong sweeping band of very much above average rainfall from the NT border areas of the NW Pastoral down to the northern Riverland. Much of this area can be attributed to a single rain event that was very slow moving over 2 to almost 3 days with sustained and focused convergence lifting up very moist tropical air. Some occasional thunderstorms were embedded in this band which traversed these regions on the 23rd and 24th. Renmark, which has been incredibly dry this year received a very welcome 53.8mm. Yunta further north up the Barrier received 77.6mm across a 4-day block with numerous totals in the 50-100mm range across the Southern NE Pastoral towards the backside of the Flinders and into the Upper North. Whilst this is a mere drop in the ocean to what is needed to erase years of deficits in these regions, the immediate benefits will certainly be felt with renewed growth of grass, vegetation and boosting any dry dams and water storages. Furthermore, cooling the upper portion of the soil profile to reduce the potential of extreme heat locally and improving evapotranspiration for convective development on days of instability – it all helps on various scales.

Here is the November outlook for reference: “My overall November outlook is for average rainfall, tending to above average with northern and especially NW extent. Towards the SE Districts there is an increased risk of below average rainfall. Most of the above average chances will be attributed to thunderstorms over mostly the second half of the month. Locally above average totals are possible over agricultural areas, chiefly over the Eyre Peninsula and Upper North.”

Generally, for the most part, this panned out quite well with the regions and trends of wetter areas to drier areas, especially with the wetter Pastorals and Upper North with drier in the SE. The main misses were the drier areas more over the southern Lower North to Northern Adelaide Plains. There certainly has been a very noticeable trend of showers and thunderstorms weakening over Central areas in the last 3-4 weeks, dying on approach and reforming to the east with subsidence present over the gulf areas. I have noticed this in the past, especially in the 2002-2003 period but these things do come and go in waves so the wheel will turn.

Meteorological Discussion:

The current situation is on a waning phase after a good burst of moisture through the mainland from late November to early December. We now enter a lull where our state will lose focus for rain events with the shift heavily focused into WA as tropical air pours into there which will shift the main heat oven east across the central interior of the country and this includes the Far North of SA. This period leading up to about Christmas give or take a few days should allow most of you that are able to, to finish the harvest with minimal disruptions apart from perhaps 2-3 hot to very hot days but these will be short-lived.

For now, until about the 14th-15th period, we should see an extended period of settled weather as a slow-moving ridge of high pressure settles into the Bight blocking moisture infeeds and fronts at the same time. This high finally moves east around the 14th which will allow heat to advect south across most district raising temperatures into the high 30’s to low 40’s. A cold front will then move eastwards quickly clearing the heat away and perhaps generating a band of weakening mid-level showers and isolated thunderstorms (more likely in the Far West).

So really not a great deal to discuss despite a later outlook this month but the indications are for the MJO to shift east towards mid to late December and into January which will allow moisture to also shift eastwards through more Central Australia and then QLD. However, given the relatively mobile pattern below our country with the westerly belt, I won’t be expecting significant long term blocking patterns until at least early to mid-January, but I will review that in time.

Given the late outlook, no official December rain outlook but I would have expected a wetter first half trending drier middle and possibly moisture to increase late. It looks to me like the northern half of the Pastorals will see above average rainfall and potentially the far west coast however most agricultural areas are likely to be in the average to below average category.

Climate Drivers:

The Pacific has continued to hold in the cool neutral area, close to La Nina but not officially getting there as we can see via Figure 4 – the cooling is general across the Central to Eastern Pacific, but the degree is not to the thresholds required for La Nina. Nevertheless, trade winds continue to be average to slightly above at times despite the strong MJO pulse that went through the area in early November. With the approach of the MJO again, these trades have strengthened and this will at least keep the cooling going across the Pacific. Slowly but surely we will see things lift back to a more textbook neutral. I should also note the Tasman Sea is extremely warm which will favour a wet NSW/QLD coastal area this summer and also into NE/E VIC. When we see periods of Positive SAM, this will likely result in humid air entering SA from the E/NE. I do expect more humid episodes like we had late November/early Dec through Jan and Feb.

Now the Indian has continued to remain in a negative IOD state and is one week off officially being declared though in the past, 7 weeks has been enough for one to be declared but now it seems to be 8 weeks. The warming has continued to increase off the NW shelf in the last 3 weeks, peaking about a week ago before starting to cool thanks to cloud associated with the approaching MJO and monsoon. The westerly burst from this will continue to cool the area so getting the final week of anomalies below –0.4C may be a stretch, however to me this has been a negative IOD period, albeit a very late one. The major rain increase through late November can be attributed to this however given it has been so late, the traditional big rains over agricultural areas we normally see from a negative IOD have been skewed further inland with more tropical influence and less colder air interactions from the westerly belt.

Figure 5: Current SST anomalies – source Tropical Tidbits https://www.tropicaltidbits.com/analysis/ocean/cdas-sflux_ssta_global_1.png

Turning to the MJO and currently it is hovering just off the NW shelf of Australia causing a strong monsoon flow into Indonesia with significant heavy monsoonal rain. The bulk of this flow however whilst increasing general rain areas across the top end of Australia has largely stopped short of getting right into Australia with a broad convergent area of NE to E trades still established. The MJO will remain slow moving over the coming weeks but slowly weaken as it inches eastwards in the coming 2 weeks through phases 4-6. Heavy rain is still expected across more of the northern coastal extremities and adjacent inland of WA/NT/QLD with a strong surge of moisture likely into WA where there are slight indications of a Tropical Cyclone developing from this burst. The WA heat trough is very active right now and will draw a lot of moisture into it. Together with the convergence between the E trades and NW to W monsoonal flow over Indonesia and the very warm SST’s off WA. we should see an environment ripe for Tropical Cyclone development should cyclogenesis occur or at least some tropical lows.

The SAM had a period in late November where it was positive, and this allowed some good moisture infeed. Lately however, we have seen things trend neutral and now negative which is causing some wild fluctuations of weather. But the main take home from this is it will completely halt moisture coming into our state for at least the coming 2 weeks. Fast moving weak systems will fly by the coastline but inland, heat will build. That is going to be enhanced thanks to the moisture piling into WA, shifting the main oven further east, especially if a Tropical Cyclone/low moves down over that area – traditionally, the heat goes east of that. Whilst it will be tempered as mentioned above re the wetter soils, it still will be hot enough to send temperatures inland into the mid to high 40’s and the low to mid 40’s further south as per the section above. The SAM is likely to stay neutral ish to negative for the remainder of December, especially once the MJO shifts east of the mainland.

Districts:

Now normally I would do some districts as usual but due to the shorter period to the mid-month and the lack of any major change till the 15th, I will keep that for the next update. The main take home is we will slowly heat up with the 14th-16th period roughly being the peak of it before a cold front moves through. However further north, the interior is likely to remain well above 40C.

Final Notes

• Wet Season across Northern Australia off to a good start and likely to be above average rainfall wise by the end
• Significant inland rain for WA with current MJO pass but Negative SAM will limit rainfall through SA
• Negative IOD one week from being official but for mine is already here
• Very warm SST’s around the mainland will support humid air advecting inland through summer – expect more humid setups with afternoon/evening thunderstorms from Mid-Jan and through Feb.
• 2025 outlook is for wetter than 2024 (won’t be hard) but there should be a broad shift back to average rainfall at the very least. Better trends will present by early new year.

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Happy Farming and Cheers from The Weatherman

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