March update 2024 SA

March update 2024 SA

*Forecast prepared between March 18th-21st 2024

Review:

G’Day everyone, it is now that time of the month again, and back on track after a later March outlook which I apologise for one again. I had my sinus surgery on the 13th and now has nasal splints in there until the 26th. I certainly don’t recommend them putting it mildly, 1-2hours of sleep a night so far in the last 4 days. Hopefully we can improve on that going forward. But it gives me some time to have a better look at the weather for you all as I have taken a week off work to help aid the recovery – it was a little ambitious going back to nightshift on the 16th I think.

So recent happenings have gone fairly much to plan with respect to the March forecast, some patchy rain areas with the jetstream squeeze before some isolated thunderstorms, chiefly in the east but certainly yesterday on the 17th, we saw one long-lived supercell thunderstorm for across the Lower North and track NNE across the range, passing just SE of Burra before running almost due north up the eastern side of the Barrier Highway. This thunderstorm was very well organised and balanced with a good amount of low level in flow due to the surface trough residing to the west keeping NE winds fuelling the updraft, but the strong upper jet vented the storm complex very well. From radar analysis and individual photographs of the storm, I can see that this underwent multiple cyclic phases of strong rotation with a gustnado being photographed just south of Burra by one of my good friends Peter, whom I’m sure many of you receive this update from. Localised very heavy rainfall of 30-40mm in 10-15minutes occurred from this storm along with some hail in the 2-3cm range, with perhaps the odd stone nearing 3.5cm which is severe. I have seen footage of water rushing down the sides of the road as well as across paddocks in and arond Burra. A couple more severe thunderstorms then developed immediately to the W/NW of the first cell and delivering 20-30mm in rapid time to areas south of Booborowie or so.

More isolated shower and thunderstorm activity also developed over the back side of the Mt.Lofty Ranges generating localised heavy falls near Harrogate and also towards Palmer/Mannum Falls area with slack steering resulting in slow moving cells with 20-30mm totals also occurring here. Many other areas however missed out completely due to the convective nature of the precipitation. The falls across the Lower SE from the patchy rain were also less due to the band being a bit further north than expected.

Meteorological Discussion:

Onto the current events and the main one is Severe Tropical Cyclone Megan which has crossed the coast ENE of Borroloola as a category 3 system. The very warm waters of the Gulf of Carpentaria combined with the low wind shear made for ideal intensification of the system. Rainfall totals in the order of 300-500mm have occurred closer to the centre of this system with widespread flooding. The main issue now is the ex-TC will continue to move inland across the Territory delivering widespread moderate to heavy rainfall over the next week essentially. This is a high Precipitable Water loading system with values exceeding 70mm located over the middle of the desert with assistance no doubt from the moderate to strong MJO pulse moving across the Top End at the moment. These deep monsoon lows or ex TC systems are classical in El Nino breakdown with March a typical month for these huge heavy inland rainfalls. I expect the Stuart Highway (and other routes) to be cut in multiple locations for a number of days to even weeks as this system moves through with widespread 100-200mm totals, scattered 200-300mm falls and some isolated 300-500mm totals over the next week. Whilst not unusual, falls of this magnitude in the desert only occur once every 7-10 years, not every season. Already this season we have seen large falls across the western interior and Channel Country in QLD as well as further N/NW in the NT with the VRD (Victoria River Downs) District receiving enormous rainfall. And just recently as mentioned earlier, inland WA across the Southern Interior and Eucla especially with that persistent convergence and widespread flooding cutting off the Eyre Highway. In this case though to aid in tracking the rain, it would be nice if the Bureau still had Tennant Creek radar operational. Some of the decisions over the years really do boggle my mind in terms of cost cutting.

Now a lot of this moisture will end up dumping over the NT but a reasonable amount will still flow SE and E into a deep trough over QLD and parts of N NSW delivering areas of rain, showers and isolated thunderstorms including areas of SW QLD that have received good falls already this wet season but also including some areas that largely missed out last time. The MJO shifting to phase 6 will assist in this focus.

Back to SA and we will continue to be influenced by high pressure for much of the remainder of the month or at least until about the 28th or 29th with a moderate to fresh SE flow with only the far N/NE portions of the state being impacted by the tropical moisture from EX TC Megan. This should allow areas of mid-level rain and isolated thunderstorms with light to moderate falls to occur, mostly in the order of 10-30mm. Further south, the ridging and more stable air should keep us generally dry with some cool to even cold nights possible in the Lower and Upper SE Districts around the 21st or 22nd mornings with a very slight risk of a light frost. However, there is a little bit of upper troughing present due to some cooler air aloft from the negative SAM and this may allow the inland surface trough to deepen southwards a little more and bring that moisture down further – something I will watch and certainly if needed will issue a special update. However, all in all this may be an opportunity missed down south for a big rain with that moisture not coming down hard enough however it will also not be the last opportunity either. The uptick in Tropical Cyclones has gone to plan but shouldn’t cease here either. Further cyclogenesis is likely well into April before the

longwaves start to stand up taller and pull moisture down. I should make a note that Severe Tropical Cyclone Neville has rapidly intensified to a category 4 but is moving steadily west and away from the mainland due to strong ridging.

Climate Drivers:

The Pacific has shown a continued push towards La Nina but of course there is a long way to go. Some isolated colder patches have also begun to upwell off Peru right on the coastline to go with the cold blob near the equator a tad further to the NW. Keep a close eye on this area, but I expect the upwelling to continue with the colder sub-surface water rising slowly over the coming months. Trade wind wise, they continue to blow strongly and already we have consistent warmer than average water around the Eastern seaboard and general Coral and Tasman Sea’s. The Tasman in particular is very warm in some patches which will fuel onshore flow and heavy rainfall into NSW in the coming months, especially the coastal side of the ranges.

The Indian in terms of the IOD is irrelevant as we know during the monsoon season, but we have noted some strong cooling on the 7-day change chart in Figure 2. But don’t be alarmed, this is not a sudden positive IOD forming despite the official value shooting up to +0.86C. The culprit here is the strong MJO passage and increased cloudiness/churning of the water due to the monsoon here causing colder water to mix up from below to the surface. Similar action happens in Tropical Cyclones but with a broad MJO and monsoon flow, this covers a greater area and the remnant mark left on the anomaly chart is very clear. This will quickly warm back up to where it was in the coming 2-3 weeks. Looking ahead we are still expecting a negative IOD through the growing season, chiefly later in the season when the impacts are the greatest.

Figure 1: Current SST anomalies – source Tropical Tidbits https://www.tropicaltidbits.com/analysis/ocean/cdas-sflux_ssta_global_1.png

Figure 2: Current SST anomaly 7-day change – source Tropical Tidbits https://www.tropicaltidbits.com/analysis/ocean/cdas-sflux_ssta7diff_global_1.png

The MJO Is currently strong and in phase 6, pushing into phase 7 very soon. The large area of convection north of QLD across PNG and beyond as well as the heavy cloud across the NT can be attributed to its influence. The MJO will continue to push east and weaken as it encounters the strong trade winds across the Pacific.

The SAM has trended slightly negative in the last day or two thanks to some larger lows throwing up colder air from the south. There is still a lack of inland penetration and I suspect this will be a short lived negative burst before we go back to a neutral SAM before a slightly positive tendency into April once again but not to the levels we saw late in 2023.

Districts:

Eyre Peninsula

Strong ridge in Bight from 21st through till 28th at least directing a moderate to at times fresh SE flow across the district with stiff seabreezes. Slight chance of a light morning shower on 23rd about the Lower Eyre Peninsula. Mild near the coast grading to warm to hot inland. Deepening trough potentially in last couple of days of month ahead of another extending from the west with a risk of some showers and isolated thunderstorms, most likely in the far north and west of the district at this stage. Rainfall for period less than 1mm in the far south of the Lower Eyre Peninsula. Possible 5-10mm falls in north and west if shower and thunderstorm activity develops.

Upper North

Strong ridge in Bight from 21st through till 28th at least directing a moderate to at times fresh SE flow across the district with stiff seabreezes and fresh to strong gully winds on western slopes at night. Mild near the gulf coast grading to warm to hot inland and in the north towards the Flinders. Deepening trough potentially in last couple of days of month ahead of another extending from the west with a risk of some showers and isolated thunderstorms, most likely about the Flinders Ranges. Rainfall for period generally nil although 5-15mm falls in north if shower and thunderstorm activity develops.

Lower North

Strong ridge in Bight from 21st through till 28th at least directing a moderate to at times fresh SE flow across the district with stiff seabreezes and fresh to strong gully winds on western slopes at night. Mild near the gulf coast grading to warm to hot inland and in the north/NE. Deepening trough potentially in last couple of days of month ahead of another extending from the west with a risk of some shower and thunderstorm activity, chiefly about the ranges/elevated terrain. Rainfall for period generally nil although 5-10mm falls in north if shower and thunderstorm activity develops.

Yorke Peninsula

Strong ridge in Bight from 21st through till 28th at least directing a moderate to at times fresh SE flow across the district with stiff seabreezes during the afternoons, especially about the foot. Slight chance of a morning shower or two on 23rd near the foot. Mild near the foot grading to warm to hot in the far north. Deepening trough potentially in last couple of days of month ahead of another extending from the west with a risk of some shower and thunderstorm activity, chiefly about the far north of the district only. Rainfall for period generally nil although 4-8mm falls in north if shower and thunderstorm activity develops.

Kangaroo Island

Strong ridge in Bight from 21st through till 28th at least directing a moderate to at times fresh SE flow across the district with stiff seabreezes during the afternoons. Slight chance of a morning shower or two on 23rd in moist onshore flow with new ridge. Mild at first grading to warm later in the period. Deepening trough potentially in last couple of days of month ahead of another extending from the west with a risk of some shower and thunderstorm activity but most likely too far north for the island. Rainfall for period generally less than 1mm with coastal shower activity.

Adelaide Plains

Strong ridge in Bight from 21st through till 28th at least directing a moderate to at times fresh SE flow across the district with stiff seabreezes and fresh to strong gully winds on western slopes at night. Mild near the gulf coast and southern suburbs at first grading to warm to hot towards the outer Northern plains later in the period. Deepening trough potentially in last couple of days of month ahead of another extending from the west with a risk of some shower and thunderstorm activity, chiefly about the ranges/elevated terrain and most likely north of the district. Rainfall for period generally nil although 2-6mm falls in far north/NE if shower and thunderstorm activity develops.

Mt.Lofty Ranges

Strong ridge in Bight from 21st through till 28th at least directing a moderate to at times fresh SE flow across the district with stiff seabreezes and fresh to strong gully winds on western slopes at night. Chance of some light morning showers early 23rd about the Southern coasts and ranges. Mild near the southern coasts and ranges at first grading to warm to hot towards the outer Northern ranges later in the period. Deepening trough potentially in last couple of days of month ahead of another extending from the west with a risk of some shower and thunderstorm activity about the far northern ranges. Rainfall for period generally nil although 2-6mm falls in far north/NE if shower and thunderstorm activity develops.

Riverland/Mallee

Strong ridge in Bight from 21st through till 28th at least directing a moderate to at times fresh SE flow across the district with stiff seabreezes about the SW parts of the Mallee near the Lakes. Mild near the lakes grading to warm to hot inland about both the Mallee and especially Riverland. Deepening trough potentially in last couple of days of month ahead of another extending from the west with a risk of some showers and isolated thunderstorms, most likely about the northern Riverland. Rainfall for period generally nil although 5-10mm falls in northern Riverland if shower and thunderstorm activity develops.

Upper SE

Strong ridge in Bight from 21st through till 28th at least directing a moderate to at times fresh SE flow across the district with stiff seabreezes about the Coorong coast. Mild near the coast grading to warm to hot inland, especially in the north towards the Mallee. Deepening trough potentially in last couple of days of month ahead of another extending from the west with a risk of some showers and isolated thunderstorms, however at this stage this looks likely to be too far north. Rainfall for period generally nil.

Lower SE

Strong ridge in Bight from 21st through till 28th at least directing a moderate to at times fresh SE flow across the district with stiff seabreezes about the SW/S coasts. A few light showers likely through the morning and evenings of the 23rd. Mild near the coast grading to warm inland, especially in the north towards the Upper SE District. Deepening trough potentially in last couple of days of month ahead of another extending from the west with a risk of some showers and isolated thunderstorms, however at this stage this looks likely to be much too far north. Rainfall for period generally less than 1-2mm.

Western VIC

Strong ridge in Bight from 21st through till 28th at least directing a moderate to at times fresh SE flow across the district with stiff seabreezes about the SW District coastline. Light coastal showers during 23rd as onshore flow persists. Mild near the coast grading to warm to hot inland, especially in the north towards the Wimmera and certainly the Mallee District. Deepening trough potentially in last couple of days of month ahead of another extending from the west with a risk of some showers and isolated thunderstorms towards the NSW border. Rainfall generally 1-3mm about the SW District coast, grading to nil for the remainder. Slight chance of 3-6mm falls for shower or thunderstorm activity towards NSW border.

Final Notes

• Dry period for most of south continues as ridging mostly holds moisture north
• Heavy rain and flooding across NT and parts of QLD with significant routes cut off across the NT for potentially a week or two
• Infeed of moisture likely to attempt to push south into weak but deepening trough later in month
• La Nina and Negative on track still and a general wetter growing season

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Happy Farming and Cheers from The Weatherman

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