April Outlook 2024 SA

April outlook 2024 SA

*Forecast prepared between April 1st-5th 2024

Review:

G’Day everyone, April is now here and certainly a time when most of you are looking to the heavens for that opening rainfall, something that has been devoid for much of the agricultural area in contrast to the wet inland and interior. Below I’ll post a few charts up to highlight the contrast of summer so far how a wet December and January turned to a very dry Feb and a mixed March (very wet inland very dry south).

In short March was a very stable and dry month for the agricultural area with numerous hot days and low-end heatwaves – nothing like 2008 of course but we saw the overall average temperature come in quite a bit above average to make it one of our hotter March’s in recent times. Heavy tropical moisture pumped into the interior consistently, but the entire lot barely made a scratch into the agricultural areas. The exceptions were a couple of thunderstorms with locally intense falls across the Lower North on the 17th where a long-tracked supercell produced numerous gustnadoes, hail, flash flooding and possible even a tornado briefly. But zooming out, this day was an anomaly amongst the broader dry with a vast majority struggling to crack 5mm for the month.

In the last 24hrs some good rainfall from another strong tropical feed occurred as expected through VIC, especially Central VIC where 30-60mm falls were quite widespread as thunderstorms amalgamated into a solid rainband, lifted by an upper low from the west. Back in SA falls were generally a lot lighter and more spasmodic due to being largely west of the best moisture however some isolated training thunderstorms did generate locally heavy rainfall totals bear Loxton with a report of 50mm and a few others in the 15-20mm range. Most however were less than 10mm from this activity with plenty of holes due to the convective nature of the event. Further south and west, the colder air influence from the upper low generated some showery bands and isolated thunderstorms, weaker than those further east, with most of the thundery activity being over the southern Spencer Gulf and later, the Lower SE District. Cooler air with some stream showers in the onshore flow followed with much cooler and rather fresh SW winds but meaningful totals from this activity were sadly not to be.

So, what did March look like as a whole? Refer to Figure 1 for the tell-tale answer, a month of contrast across the state.

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

Figure 2: Feb+Mar rainfall deciles showing very dry conditions over agricultural areas – source BoM http://www.bom.gov.au/climate/

Figure 3: Year to date (Jan-Mar) rainfall deciles – source BoM http://www.bom.gov.au/climate/

Figure 4: March 2024 rainfall percentages – source BoM http://www.bom.gov.au/climate/

Figure 5: March 2024 rainfall anomalies – source BoM http://www.bom.gov.au/climate/

In a nutshell we can clearly see the very dry conditions in the south trending to wetter over the interior, especially the far NW and NE corners where the tropical influence was the greatest. The further south away from these one ventured, the drier things became thanks to persistent ridging. I was hopeful we would get at least some moisture down to us in the months leading up to March but by the time the month came close that did look like it was only going to stay inland and alas that is what happened. A good tropical soak here would have taken the pressure off a little bit.

Now if we add February to the picture, things look quite drastic. It was no surprise February was dry here but adding it to March really shows how bad things were across the agricultural areas with Figure 2 being a sea of red, with some lowest on record in the south. Adding January to the picture as shown in Figure 3 makes things a little better as it wasn’t a bad month and for a vast majority, that is where the last decent rainfall occurred over agricultural areas. So in terms of year-to-date rainfall, there is a lot of catching up to be zone across the Lower SE,

Kangaroo Island and Lower North and these areas especially will be looking for those opening rains with extra interest for the coming growing season.

Meteorological Discussion:

Currently we are getting towards the tail end of the wet season as such for Northern Australia with the final rains now occurring, predominantly across QLD and some parts of the Top End as trade winds continue to push in heavy moisture. However, there are signs now that ridges are pushing back north with longer clearances in moisture and instability for both the Kimberley and the Top End with most activity being confined to the northern coastline.

Right now, a strong upper low is combining with a trough in the east to generate areas of rain and isolated thunderstorms, with a strong focus east of the Great Divide, though some good falls are wrapping under the upper low into SE NSW. Flooding is likely in and around Sydney as heavy convergence sets up but also potentially in some locations all the way up the Mid North coastline. However the further west one goes, the more stable things become with ridging dominating both SA and WA and partially VIC in the days and weeks going forward through April. This will be what I call the mid-autumn lull where there is a direct pattern shift between the wet season tropical moisture and the winter fronts where neither dominate and steady and stable days with temperatures in the 20’s dominate. Now of course some of you may be wondering what on earth is this lull when it has barely rained a drop in 2 months and that is a fair question as it has been very dry in the south but it is merely the period where there is no dominating pattern as we transit between the northern wet season and the southern wet season where frontal rainfall dominates. One is ending but the other has no begun and I don’t see the fronts properly standing up with the associated longwave troughs until very late April or more likely May as NW moisture feeds down ahead of them into a more volatile period in the westerly belt.

Now back to the current conditions and once this upper low and trough has finished with the eastern states it will allow a cold front and stronger upper trough to rapidly stand up from the south and swipe through the SE of the state and into VIC on monday 8th with a band of showers followed by a pool of cold air, with the cold air mostly being aimed at southern VIC, Bass Strait and Tasmania but the top edge of it will push through giving some scattered cold air showers to most of the southern agricultural area, with more isolated activity in the northern areas. A new large and strong high will ridge in behind the front and keep a relatively moist onshore flow going for much of the week with isolated to scattered light showers at times with quite cool conditions in most districts – there will certainly be a little chill in the air and this does pose a very slight light frost risk across prone spots in the agricultural areas, but mostly just the SE/E portions of the state. Partly cloudy conditions in the onshore flow with light showers at times will mitigate the frost potential to some

degree however, but that small window near sunrise may allow some brief pockets to form in cold holes/low lying areas. All in all though, only a very slight risk.

As we move into next weekend from the 13th and beyond, we should see the large high move eastwards with conditions warming up again into the mid 20’s generally, with fairly pleasant days and light winds. A cut off low is likely to develop to the west, mostly likely south of Perth at this early stage and deliver some showers there but not a lot. Lows that have already cut off over WA generally have no significance for us unless multiple cold air surges aloft are expected in a broad dumbelling longwave. We will have to wait for those as ridging and sinking with dominate for a while as we transit the seasons. Very late in the month is when I expect stronger longwaves to begin to stand up to our west and make an attempt on bringing some moisture down from the NW, however the bulk of this including a break is most likely in May. June through to October as a collective should see an above average rainfall trend for much of the state. The main months of interest are May, June and September for the best chance of getting average to above average rainfall.

Overall, my outlook for April however is for broadly below average rainfall for much of the state. The greatest chance of below average falls are likely over the interior as well as the Eyre Peninsula, Upper North and Riverland. Some onshore flow about far southern coastal and Lower SE Districts may take the edge off things however it may not be enough to reach average still.

Climate Drivers:

The Pacific is showing clear signs that a La Nina is waiting in the sub-surface though so far only small blobs have reached the surface, primarily in the far east near Peru as the Humboldt Current begins to stir once more. The surface still primarily is above average in temperature as the lingering heat from El Nino is yet to be fully removed, but this will occur as the year goes by, with some of that going west into the Indian Ocean through the Indonesian Throughflow Current, something I have discussed on here before. The trade winds are already up and piling moisture into Eastern Australia and now that we will see some upper lows interacting with troughs across the Eastern Seaboard, expect a lot of that moisture to be uplifted and brought back down as heavy rain, especially east of the Great Divide, something I mentioned in the last video but the signal for this is very strong at the moment, with flooding likely in many locations in the coming months.

Moving on to the Indian, we see broadly the basin is warmer than average and this is what I expect for much of the year. Now remember the last update we had a colder slice through the areas off Java, thanks to the monsoon. I re-assured this was just a temporary anomaly and now it is clear in just 2 weeks that the recovery has been significant and expect it to continue. It is also quite warm west/SW of Perth over the SE Indian Ocean, which will likely serve as a secondary source of fuel for

frontal systems and troughs that rear up in this region and assist in low pressure formation but more importantly, moisture for rainfall when the season properly breaks. But it is not ready to break just yet as we are still well and truly entering that in between phase from summer pattern to a late autumn one. I expect a Negative IOD through the later growing season this year.

Figure 6: Current SST anomalies – source Tropical TIdbits https://www.tropicaltidbits.com/analysis/ocean/cdas-sflux_ssta_global_1.png

The MJO has now moved rapidly back into the continent window with it becoming more active in phase 3 before being expected to strengthen in phase 4 to 6. This shows the main focus of uplift is north and NE of the mainland whilst the main down lift/sinking air is located west and SW, which clearly shows by the lack of rainfall as we struggle to get a change in the upper-level patterns. I expect once we push back to phase 6-7 that at least some upper troughing cold air will move up, primarily focused into the central to eastern parts of Australia and not at great strength. This MJO focusing in our window will enhance late season Tropical Cyclone potential in the coming week or so as the interaction between ridging to the south and the westerly flow to the north will cause cyclogenesis in more than one location – one off N WA and potentially another off the Top End and we may see a category 1 system get up but I do not expect anything significant. Heavy rainfall will dominate the eastern seaboard as upper troughing/MJO phase and trade wind onshore flow all combine to maximse rainfall.

The SAM has been ineffectual for a while now but there have been some brief negative tendencies of late with this colder air from upper troughing expected to continue for another week or two before a positive trend takes over with strong highs sitting south of WA and the mainland in general. This in turn keeps the easterlies across the mainland and pumps moisture into the eastern seaboard especially.

Districts:

Eyre Peninsula

Fine as a weak ridge resides over the district. Cold front with a band of showers due early afternoon on 8th with most activity about the Lower Eyre Peninsula as the band dries out with northern extent. Colder air to follow with fresh onshores and isolated showers, primarily about the Lower Eyre Peninsula and Eastern Eyre Peninsula coastlines off the Spencer Gulf on and off for much of the week as a new strong ridge anchors to the west. Light shower activity largely clearing by the 12th/13th as the ridge moves east and winds turn E/NE and temperatures rise with mid to high 20’s by mid-month likely, especially inland. Cut off low to develop over WA by this stage with potentially increasing high cloud. Rainfall for the period 2-5mm over Lower Eyre Peninsula, increasing to 5-10mm about the Gulf coast of Eastern Eyre Peninsula with onshore southerly flows into elevated terrain near Cleve. Totals decreasing to less than 2mm for the remainder of the Peninsula, especially in the north/NW.

Upper North

Fine as a weak ridge resides over the district. Cold front with a band of showers due early evening on 8th with most activity about the southern half of the district as the band dries out with northern extent towards the Flinders and off the back side of the ranges. Colder air to follow with fresh onshores and isolated showers in the south and about elevated terrain on and off for much of the week as a new strong ridge anchors to the west. There may be some light drizzle periods at times if local streams set up. Light shower activity largely clearing by the 12th/13th as the ridge moves east and winds turn E/NE and temperatures rise with mid to high 20’s by mid-month likely, especially towards Port Augusta. Cut off low to develop over WA by this stage with potentially increasing high cloud. Rainfall for the period 1-3mm generally, increasing to 3-5mm in the south near elevated terrain/drizzle stream areas. Totals decreasing to less than 1mm for the remainder further north with next to no rainfall.

Lower North

Fine as a weak ridge resides over the district. Cold front with a band of showers due late afternoon on 8th chiefly about the central to southern half of the district with the band drying out a little with northern and eastern extent, especially off the back side of the ranges. Colder air to follow with fresh onshores and isolated to scattered showers in the south and about elevated terrain on and off for much of the week as a new strong ridge anchors to the west. There may be some light drizzle periods at times if local streams set up. Light shower activity largely clearing by the 12th/13th as the ridge moves east and winds turn E/NE and temperatures rise with mid 20’s expected. Cut off low to develop over WA by this stage with potentially increasing high cloud. Rainfall for the period 2-5mm generally, increasing to 5-12mm in the south near elevated terrain/drizzle stream areas. Local totals to 15mm possible. Totals decreasing to less than 2mm in the NE of the district.

Yorke Peninsula

Fine as a weak ridge resides over the district. Cold front with a band of showers due early to mid-afternoon on 8th. Colder air to follow with fresh onshores and isolated to scattered showers, on and off for much of the week as a new strong ridge anchors to the west. Streams may set up into the Hummocks area off Gulf St Vincent in the southerly flow. Light shower activity largely clearing by the 12th/13th as the ridge moves east and winds turn E/NE and temperatures rise with mid 20’s by mid-month likely, especially in the north. Cut off low to develop over WA by this stage with potentially increasing high cloud. Rainfall for the period 2-5mm over the foot, increasing to 5-7mm towards the centre and inner north. Localised totals to 10mm near the Hummocks potentially. Totals decreasing to less than 2mm for the remainder of the Peninsula, chiefly in the N/NW areas.

Kangaroo Island

Fine as a weak ridge resides over the district. Cold front with a band of showers due early to mid-morning on 8th. Colder air to follow with fresh onshores bringing scattered showers, becoming isolated later on the 8th. Shower activity on and off for much of the week as a new strong ridge anchors to the west. Light shower activity largely clearing by the 12th/13th as the ridge moves east and winds turn E/NE and temperatures rise with low to mid 20’s by mid-month likely. Cut off low to develop over WA by this stage with potentially increasing high cloud. Rainfall for the period 4-6mm generally with localised falls up to 10-12mm.

Adelaide Plains

Fine as a weak ridge resides over the district. Cold front with a band of showers due mid to late afternoon on 8th. Colder air to follow with fresh onshores and isolated to scattered showers on and off for much of the week as a new strong ridge anchors to the west. Localised streams may set up at times. Light shower activity largely clearing by the 12th/13th as the ridge moves east and winds turn E/NE and temperatures rise with mid 20’s expected. Cut off low to develop over WA by this stage with potentially increasing high cloud. Rainfall for the period 3-5mm generally, increasing to 5-10mm potentially closer to the foothills or near streams. Totals decreasing to less than 3mm about the gulf coastline.

Mt.Lofty Ranges

Fine as a weak ridge resides over the district. Cold front with a band of showers due mid to late afternoon on 8th. Colder air to follow with fresh onshores and isolated to scattered showers on and off for much of the week as a new strong ridge anchors to the west. Localised streams may set up at times, especially about southern coasts and ranges where drizzle periods may also setup. Light shower activity largely clearing by the 12th/13th as the ridge moves east and winds turn E/NE and temperatures rise with mid 20’s expected. Cut off low to develop over WA by this stage with potentially increasing high cloud. Rainfall for the period 4-7mm generally, increasing to 7-12mm over known wet spots and about southern coasts and ranges/near streams. Isolated local 15-18mm falls possible. Totals decreasing to less than 4mm towards the northern ranges and rear slopes.

Riverland/Mallee

Fine as a weak ridge resides over the district. Cold front with a band of showers due early afternoon about the Mallee and early evening in the Riverland on 8th with most activity about the southern half of the Mallee as the band dries out with northern and eastern extent towards the Riverland where showers are likely to be a little lighter and more isolated. Colder air to follow with fresh onshores and isolated showers in the S/SW half of the Mallee mostly on and off for much of the week as a new strong ridge anchors to the west. Slight chance of some local streams off the Lakes. The Riverland will see only very isolated shower activity. Light shower activity largely clearing by the 12th/13th as the ridge moves east and winds turn E/NE and temperatures rise with mid to high 20’s by mid-month likely, especially towards the north of the Riverland. Cut off low to develop over WA by this stage with potentially increasing high cloud. Rainfall for the period 2-4mm generally about the Mallee increasing to 4-6mm in the SW near the Lakes. Totals decreasing to less than 1-2mm for the Riverland.

Upper SE

Fine as a weak ridge resides over the district. Cold front with a band of showers due early to mid-afternoon on 8th. Colder air to follow with fresh onshores and isolated to scattered showers (chiefly closer to the Coorong coast) on and off for much of the week as a new strong ridge anchors to the west. Light shower activity largely clearing by the 12th/13th as the ridge moves east and winds turn E/NE and temperatures rise with mid 20’s by mid-month likely, especially in the north. Cut off low to develop over WA by this stage with potentially increasing high cloud. Rainfall for the period 3-5mm, increasing to 5-7mm towards the Coorong coastline and NW towards the Lakes. Totals decreasing to less than 3mm closer to the VIC border.

Lower SE

Fine as a weak ridge resides over the district. Cold front with a band of showers due early to mid-morning on 8th. Colder air to follow with fresh onshores and isolated to scattered showers (locally heavier closer to the far southern coastline) on and off for much of the week as a new strong ridge anchors to the west. Light shower activity largely clearing by the 12th/13th as the ridge moves east and winds turn E/NE and temperatures rise with low to mid 20’s by mid-month likely, especially in the north. Cut off low to develop over WA by this stage with potentially increasing high cloud. Rainfall for the period 3-5mm, increasing to 5-8mm towards the Southern coastline. Totals decreasing to less than 3mm in the north/NE.

Western VIC

Cut off low over NE of state delivering light patchy rain to the NE of the Mallee District but not a lot. Residual moisture from east causing further light showers, scattered in the afternoon on the 7th, chiefly about the Wimmera and SW District, before clearing after sundown. Cold front due on 8th with a band of showers, reaching the SW District early morning before reaching the Mallee in the evening with the band drying out with NE extent slowly. Onshore flow with large new ridge to west driving isolated shower activity on and off till 13th, chiefly about the SW District but some activity still penetrating to the Mallee. New ridge moving east thereafter with winds turning more E/NE and temperatures rising to low to mid 20’s, especially over the Mallee for the latter. Rainfall for the period 4-8mm about the SW District, with isolated 10-12mm falls near the coast. Totals decreasing with northern extent down to 2-4mm generally however lingering patchy rain in east of the Mallee may add an extra 2-4mm there.

Final Notes

• Overall lull in weather through April as we enter transition between northern wet season and southern wet season
• Likely uptick in longwave troughs from the SW late April but especially through May which could be active as moisture returns from the NW ahead of these longwaves and should cause a break in the season
• La Nina and Negative IOD combination still on track for this year with the main hands being shown later winter and especially through spring as per tradition time slots, especially for the IOD.
• Above average rainfall for May-September period likely across agricultural areas
• Watch for worldwide uptick in earthquake/volcanic activity in and around the Solar eclipse time period, the earths magnetosphere is weakened at the present time which can cause instability

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Happy Farming and Cheers from The Weatherman

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