May Outlook 2024 SA

May Outlook 2024 SA

*Forecast prepared between May 2nd to 6th 2024

Review:

G’Day everyone, definitely time for an important update that I am sure many of you have been waiting for given the state of the autumn season to date. As usual there is good and bad news but will go into more detail in the discussion and drivers section.

April did not finish off well, and was very dry with high pressure dominating. There were some onshore showers about chiefly exposed coasts and a lucky 14mm or so near Cowell from a training mid-level cell ahead of an upper low but all in all, meagre totals less than 1-4mm for most districts which was fairly close to expectations from the last update.

WA has been very dry as well however just recently, hints of a change in patterns has impacted the SW portion of the state with a tight upper low bringing showers and thunderstorms on the 30th and 1st of May followed by a sliding cold front with heavier showers and thunderstorms chiefly south of Perth. A lot of this has only meekly penetrated inland to the main wheatbelt area although some patchy handy falls to the NE did occur with 25mm at Morawa. There is however a very long way to go for WA to get remotely close to where they should be given the significant deficits over the last 6-8months.

Now quickly back to SA and a look at April via the charts:

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

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

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

As we can see, it is not a pretty picture overall with heavy subsidence for the entire month resulting in below average to well below average rainfall over 90% of the state. The only area that had some rainfall near average was the southern NE Pastoral region near the border as a result of some thunderstorm activity. When adding on April to the severe deficits we saw in February and March the overall picture is very grim as seen in Figure 4 below:

Figure 4: February – April rainfall deciles showing severe rainfall deficiencies right over agricultural areas – source BoM http://www.bom.gov.au/climate/

The clear trend is very dry right over the agricultural areas although the NE corner has been above average thanks to the El Nino breakdown rains, they simply just could not penetrate south with powerful ridging and subsidence which is still occurring. There is not a lot more than needs to be said as the results are self-explanatory with the consistent subsidence across much of mainland Australia right through autumn to date so far. I will explain further down about the why’s and what to expect going forward as there will be consequences from this persistent high pressure.

Meteorological Discussion:

A relentless dry spell continues over much of the nation with the exception of the eastern seaboard where the dominant high pressure has allowed a constant easterly flow to be lifted into the Great Divide, and give showers with occasional upper troughs turbocharging the moisture and helping provide heavy falls.

We are completely blocked in a strong meridional pattern with this log jam extending into the Pacific. The result is a strong long wave trough parked around the New Zealand longitude with another strong longwave trough parked half way across the Southern Indian Ocean. Typically, there is movement in these, even after previous long considered blocks of 3-4 weeks but not this one, it has doubled down in the final week of April, and this continues to this date with a behemoth 1043hPa high anchored well south of the mainland. Figure 5 below shows the pressure anomalies for March 2024 (April report is not ready yet however it is essentially the same but even more pronounced).

Figure 5: March 2024 pressure anomalies @500hPa showing high pressure sitting below the mainland – source NOAA https://www.ncei.noaa.gov/access/monitoring/monthly-report/global/202403#precip

Note the chart above is for the mid-level at 500hPa however the surface chart will be quite similar. It is a good level to analyse the longwave trough and ridge and we can clearly see here this pattern I just talked about. This block has still not moved. I have been aiming for a Mid-May uptick when that

whole pattern begins to start moving east however with the power of the subsidence occurring, that is going to be a slow struggle with multiple attempts made before we see the rain here in SA and indeed into SW/W VIC. WA has seen some small off shoots from this longwave trough with rainfall beginning, which is great but I will not call it a seasonal break yet for them, let alone anywhere further east.

In short, WA will see that longwave trough and frontal influence before SA and VIC does and that looks to me to be appearing around the 10th-13th window before those fronts push east but diving SE and weakening as they do so. That is the main timeframe for the start of this block to break down but as mentioned before it will take a number of attempts before we see that knock out the high pressure over SA and general mainland Australia.

Once we break this block down, it should allow moisture that is building up over the Indian and parts of Indonesia to flow S then SE ahead of and into some of these cold fronts with the peak of this signal present throughout June. From earlier in the year, I expected this to come more in May but unfortunately that is more June now – not ideal when running on a solid deficit.

Now as for current proceedings we have a very large and slow-moving high sitting well south of the mainland which is maintaining this block and keeping us in a SE-NE flow for the most part with fine and mild to warm days and cool to occasionally cold nights. Even some moderate downslope gully winds have been present on the west side of the ranges due to dominance of the easterly flow -these sorts of winds very rarely continue into May.

Figure 6: Satpic for 060524 with overlay schematic – source Weatherzone

I thought I’d just go back and add this one in quickly to help explain how the air is moving at this current point in time. In short, the big H inside the circle over the mainland represents the dominant high pressure and sinking/downwelling air. The red arrow pointing NW is where strong SE winds are blowing out of this high under the bottom side of Indonesia causing largely cloud free skies with dry air from mainland Australia. The two black boxes with L inside represent the main up motion in our region of the globe and it is clear there is still quite a lot of it explaining that we have a turbocharged Hadley circulation with air rising to our north and sinking to our south however the up motion is splitting into two areas with that gap between the two an area to watch, especially if the trade winds continue to blow. The thunderstorms in the western most box are still a good source of tropical moisture feeding into the Central and Southern Indian and this area as explained further down in climate section is above average in SST for this time of the year. The main as can be seen is at this point in time all that good moisture is simply being directed around, down and under mainland Australia however that area is shifting east slowly – but just weakening as it does so and hence my expectation of a frustrating, slow and patchy break to the season. Without this powerful and persistent high pressure, a lot of that moisture would be coming down closer to our NW by now, it is not as if moisture is lacking in the tropics, it is infact fairly strong in both the eastern and western Indian Ocean – indicative of the widespread above average ocean temperatures. In any case, we need to keep an eye on the daily satellite, if the thunderstorms currently west of Sumatra (in the IOD Box) weaken and shift west and allow the clear area to extend in from the SE then this will assist in an early Positive IOD. However, if these persist, then that Positive will have a hard time forming properly but all of the IOD activity is explained better in the Climate Drivers section.

Weatherwise synoptically, we currently have a weak upper low spinning over Southern WA generating some showers and isolated thunderstorms though falls are spasmodic and patchy due to the nature of the system. This upper low will weaken and broaden out as it extends eastwards in the coming days. Meanwhile as touched on further up, a very strong high-pressure system which was at one stage 1043hPa, is still anchored well south of the mainland keeping us largely mild to warm and dry. A weak surface trough will form within this high around the 9th in response to the cooling uppers from the broad upper low and although these features will remain weak, they will still combine to generate showers and isolated thunderstorms, tending to areas of rain at times across the NE Pastoral District, chiefly the NE/E portions and into far NW and W NSW. The back edge of where all this comes down is a little hard to be sure on but I would suspect a bulk of it will be over the border leaving a mix of isolated to scattered showers and patchy light rain with minimal totals for the very far eastern parts of the Riverland and northern Mallee through the 10th before the system clears eastwards into NSW where far more significant rainfall will fall, increasing with eastern extent generally speaking. I should note that very heavy rainfall, with flooding is likely for coastal regions south of Sydney but especially south of about Wollongong to Mystery Bay where a focus of low-level convergence into the Great Divide will see 150-250mm totals possible.

A couple of strong cold fronts will begin to stand up and cross into far SW WA before diving to the SE around the 10th and 11th but I can only see some isolated coastal showers at this stage around the 14th before more high pressure comes behind but instead of setting up and sitting in the Bight for so long the main blocking will now be more over the Tasman indicating the shift in the pattern but by no means a complete change. As I stress before, this will be a slow and grinding breakdown of one of probably the strongest autumn block I have seen. I remember 2005 being a very late break with a warm dry autumn and even the first 9 days of June ahead of a line of thunderstorms around the 10th. I don’t see the consistency in rain after like that year however but it certainly will improve with this period being the worst of the season.

Overall, for May, my forecast is for a below average month for most districts as I had targeted it for around average to potentially even above average. Nevertheless, the far NE Pastoral will see the best chance of above average rainfall due to the trough and weak upper low. Most other districts will be below average to well below average rainfall as patchy and sliding frontal systems make up that moisture. I will be able to give more clarity whether or not the longwave trough will make a dent in our region by the end of May in the mid-month update which also will be critical.

Climate Drivers: 

The Pacific has started to show a little more of that cold sub surface water reaching the surface with a nice little tongue evident on the latest anomaly chart over the Central Eastern tropical Pacific as per Figure 7:

Figure 7: Current SST anomalies –source Tropical Tidbits https://www.tropicaltidbits.com/analysis/ocean/cdas-sflux_ssta_global_1.png

This means that the La Nina is slowly developing as we expect. The Humboldt Current is also showing further upwelling signs off Peru. On a sidenote the Atlantic is incredibly warm atm and something I am going to look into for downstream complication of the global oceanic circulations given it all starts in the Atlantic. An active hurricane season beckons.

Moving on to the Indian we have a lot of warmer than average water, especially over the Western basin, the SE Indian and right up into the NE towards Thailand, Singapore and in the Bay of Bengal. This may increase chances of a Tropical Cyclone here which could in turn upturn colder water from the depths and enhance a Positive IOD.

Lots of agencies now are forecasting a Positive IOD and given the readings for the index have been positive for 7 weeks now, an 8th week for an official declaration is a formality. This is extremely early to be declaring one and, in many cases, has been solely driven by the warmer waters in the western IOD box and slightly cooler (but still above average!) waters in the eastern IOD box which is near Sumatra and Java.

Just to quickly recap for those a little confused by what exactly a Positive and Negative IOD is – essentially a Positive IOD is marked by warmer than normal sea surface temperatures in the western Indian Ocean near the horn of Africa and cooler than average in the Eastern Indian Ocean near Sumatra and Java. This causes rising motion and rainfall near Africa and sinking motion and drier air over Indonesia and in turn Australia causing a reduction in growing season rainfall.

The opposite can be said for a Negative IOD with warmer than normal SST’s over the Eastern Indian off Sumatra and Java with cooler than average off the horn of Africa. This causes sinking air and drought conditions in Eastern Africa and rising air and wetter conditions over Indonesia and in turn, more moisture feeding into Australia with NW cloudbands. Cut off lows increase during Negative IOD years over Australia due to the increased interactions between tropical moist air and colder frontal air.

Now back to the current events and as mentioned I expect a Positive IOD to be declared which is extremely early and non-traditional compared to the norm. Usually, these events build over winter and show their hand in August with a peak in September to October before easing back in November. Very strong events can take till December to ease. With this current pending Positive, values from early March will be used to declare this official which is made possible by the lingering monsoon in the Eastern Indian. This disturbed the water causing a below average SST area for many weeks and whilst it has recovered to above average, the Western Indian has simply held it for longer. Thus, it is very important not to be caught up in the numbers and this pending early declaration. To me it is essentially going to be a false positive at this point in time. It is hard to put all this into words so one day I think I might do a video update or two if it helps explain things a little better and save you reading endless lines of text. Please let your contacts know if you wouldn’t mind that format every now and again. However having said all of that, the current dominant high pressure and persistent SE-E flow under Java and in that red circle I highlighted earlier is going to cause that area to cool through July I suspect. I think it will be a weak affair before it warms again into spring which is when positive events normally increase not decrease. The result of all this is a patchy growing season, highlighted by this late start. June should rebound as we finally see fronts, but I can see more high pressure back in July – but hope this time it is not as significant as it has been of late. This is a highly critical watch point going forward and will determine how the rest of the season plays out. If it is weaker than expected I think the water warms up quickly through spring and we have a crack at a negative IOD (never seen a positive and negative in the same season but it is almost possible this year). I think the problem will be the African side as I think broadly the Indian stays warm. The most likely result is a return to neutral conditions through spring but with a negative tendency the deeper we go into spring which will increase rainfall potential through that period. August, September and October present better opportunities for rainfall, but especially O

The SAM has been strongly positive in the last 2 weeks and certainly is right now with easterlies across the mainland. It is quite surprising to me to see agencies saying the SAM is currently strongly negative, which would see cold fronts and the westerly belt in general further north. There is no way it has been or will be negative – only positive and well above neutral at that. As this dominant high-pressure period breaks down into June, then we will see a return to neutral conditions. Extended negative conditions are not expected for some time until well into June and even then, I see a mix of neutral for the most part as fronts come and go with high pressure in between and even some positive values. A real mixed bag ahead.

The MJO becomes less important as we go into winter, but it is currently moderate to strong across the top side of Australia and is marked by that cloud in the black box as per figure 6. It will continue eastwards at a solid strength pushing into phase 7 then 8 before weakening.

Districts:

NE Pastoral

Large 1040hPa blocking high pressure dominating the state with SE-NE flow. Weak upper low will allow a weak surface trough to develop over the the region on the 8th with scattered showers and isolated late thunderstorms developing. This activity will increase to areas of rain early on the 9th and remain chiefly confined to the NE and far eastern portions of the district before mostly clearing by the 10th. Isolated moderate falls are likely under the main rainband however activity will be light and patchy elsewhere. Conditions becoming fine for the remainder of the period till around the 13-14th or so as a ridge stabilises conditions. Upper trough approaching from the west around the 14-15th period with increasing mid-level cloud but at this stage remaining dry. Rainfall for the period 8-15mm within the rainband. Scattered 20mm totals with isolated local 25mm falls. Remainder less than 2-5mm with no rainfall closer to the NW Pastoral District.

Eyre Peninsula

Large 1040hPa blocking high pressure dominating the state with a SE-NE airstream prevailing. Weak upper low moving over aloft causing weak surface trough to form in the NE corner of the state but remaining generally fine apart from some middle and high-level cloud. Weak front clipping coast on 12th with possibly an isolated shower or two but generally no rainfall. Ridge dominating till just before mid-month ahead of a broad upper trough with increasing middle and high cloud though generally remaining dry at this stage apart from some possible light shower activity. Rainfall for the period essentially nil however some southern coastal regions may see 0.2-0.4mm.

Upper North

Large 1040hPa blocking high pressure dominating the state with a SE-NE airstream prevailing. Weak upper low moving over aloft causing weak surface trough to form in the NE corner of the state but remaining generally fine apart from potentially a few isolated afternoon showers about the Flinders and elevated terrain in the far east of the district on the 8th and moreso 9th with some surface heating. Ridge dominating till just before mid-month ahead of a broad upper trough with increasing middle and high cloud though generally remaining dry at this stage apart from some possible light shower activity. Rainfall for the period essentially nil however area that receive shower activity on 8th and 9th may see very local totals of 1-5mm.

Lower North

Large 1040hPa blocking high pressure dominating the state with a SE-NE airstream prevailing. Weak upper low moving over aloft causing weak surface trough to form in the NE corner of the state but remaining generally fine apart from potentially a very isolated afternoon shower about elevated terrain in the far east of the district on the 8th and moreso 9th with some surface heating. Ridge dominating till just before mid-month ahead of a broad upper trough with increasing middle and high cloud though generally remaining dry at this stage apart from some possible light shower activity. Rainfall for the period essentially nil however area that receive shower activity on 8th and 9th may see very local totals of 1-2mm.

Yorke Peninsula

Large 1040hPa blocking high pressure dominating the state with a SE-NE airstream prevailing. Weak upper low moving over aloft causing weak surface trough to form in the NE corner of the state but remaining generally fine. Weak front clipping the foot with a possible light shower or two on 12th. Ridge dominating till just before mid-month ahead of a broad upper trough with increasing middle and high cloud though generally remaining dry at this stage apart from some possible light shower activity. Rainfall for the period essentially nil however the foot may see 0.2-0.4mm from the weak front if any at all.

Kangaroo Island

Large 1040hPa blocking high pressure dominating the state with a SE-NE airstream prevailing. Weak upper low moving over aloft causing weak surface trough to form in the NE corner of the state but remaining generally fine. Weak front clipping the island with a possible light shower or two on 12th. Ridge dominating till just before mid-month ahead of a broad upper trough with increasing middle and high cloud though generally remaining dry at this stage apart from some possible light shower activity. Rainfall for the period essentially nil however some localised areas may see 0.2-0.6mm from the weak front.

Adelaide Plains

Large 1040hPa blocking high pressure dominating the state with a SE-NE airstream prevailing. Weak upper low moving over aloft causing weak surface trough to form in the NE corner of the state but remaining generally fine. Weak front clipping the southern suburbs with a possible light shower or two on 12th but generally just partly cloudy. Ridge dominating till just before mid-month ahead of a broad upper trough with increasing middle and high cloud though generally remaining dry at this stage apart from some possible light shower activity. Rainfall for the period essentially nil however the outer southern suburbs may see 0.2mm from the weak front if any at all.

Mt.Lofty Ranges

Large 1040hPa blocking high pressure dominating the state with a SE-NE airstream prevailing. Weak upper low moving over aloft causing weak surface trough to form in the NE corner of the state but remaining generally fine. Weak front clipping the bottom of the Fleurieu with a possible light shower or two on 12th but generally just partly cloudy. Ridge dominating till just before mid-month ahead of a broad upper trough with increasing middle and high cloud though generally remaining dry at this stage apart from some possible light shower activity. Rainfall for the period essentially nil however the southern coasts and ranges of the Fleurieu may see 0.2-0.6mm from the weak front if any at all.

Riverland/Mallee

Large 1040hPa blocking high pressure dominating the state with a SE-NE airstream prevailing. Weak upper low moving over aloft causing weak surface trough to form in the NE corner of the state but remaining generally fine apart from potentially an isolated afternoon shower or two about the far east of the Riverland and possibly the Northern Mallee on the 8th and moreso 9th with some surface heating. Ridge dominating till just before mid-month ahead of a broad upper trough with increasing middle and high cloud though generally remaining dry at this stage apart from some possible light shower activity. Rainfall for the period essentially nil however area that receive shower activity on 8th and 9th may see very local totals of 1-3mm.

Upper SE

Large 1040hPa blocking high pressure dominating the state with a SE-NE airstream prevailing. Weak upper low moving over aloft causing weak surface trough to form in the NE corner of the state but remaining generally fine although there is the slightest risk of a brief afternoon shower on the 9th and possibly again on the 10th. Ridge dominating till just before mid-month ahead of a broad upper trough with increasing middle and high cloud though generally remaining dry at this stage apart from some possible light shower activity. Rainfall for the period essentially nil however area that receive shower activity on the 9th may see very local totals of 0.6-2mm.

Lower SE

Large 1040hPa blocking high pressure dominating the state with a SE-NE airstream prevailing. Weak upper low moving over aloft causing weak surface trough to form in the NE corner of the state but remaining generally fine although there is the slightest risk of a brief afternoon shower on the 9th and possibly 10th in the far north/NE. Weak front crossing on 12th with some isolated showers but not a lot in it. Ridge dominating till just before mid-month ahead of a broad upper trough with increasing middle and high cloud though generally remaining dry at this stage apart from some possible light shower activity. Rainfall for the period essentially nil however area that receive shower activity on the 9th and with the weak front may see local totals of 0.6-2mm.

Western VIC

Large 1040hPa blocking high pressure dominating the state with a SE-NE airstream prevailing. Weak upper low moving over aloft causing weak surface trough to form over the NE corner of SA. Isolated to scattered afternoon showers and a possible thunderstorm are likely over the SW District, Wimmera and possibly the southern Mallee on the 9th and 10th as moisture feeds into the trough from high moisture levels over NSW. Weak front sliding away on 12th but may deliver some light coastal showers to the SW District. Ridge dominating till just before mid-month ahead of a broad upper trough with increasing middle and high cloud though generally remaining dry at this stage apart from some possible light shower activity. Rainfall for the period quite patchy however 2-5mm seems plausible for those affected by shower activity Local totals with thunderstorms of 10-15mm, more likely with eastern extent towards Central VIC near the ranges and elevated terrain there.

Final Notes

• Relentless blocking pattern causing dry spell to continue into late May with weak attempts only to break the pattern down
• Pattern slowly breaks down through second half of May as upper troughs and sliding fronts begin to move the high-pressure eastwards
• June appears much better for rainfall as fronts and longwave trough should finally remove the block
• Weak ‘false’ Positive IOD forming early to be declared in May but weakening late winter
• Neutral to possibly even weak negative IOD by spring in complex battle of wind patterns in Indian
• Return to drier conditions by July before August to October performs better, chiefly as we move further into spring
• In any case, slow and frustrating break for many with extreme dry conditions across SA and western VIC and indeed the wheatbelt of WA
• Not a huge fan of ‘analogue years but research on years that show some similarities before next mid-month update include 1914 & 2007.

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Happy Farming and Cheers from The Weatherman

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