April update 2024 SA

April update 2024 SA

*Forecast prepared between April 18th-21st 2024

Review:

G’Day everyone, mid-April is here and time to have another look at how things are progressing weather wise – to be honest not a great deal to talk about so far but we will have a look through exactly how long this dry spell will last and what we can expect in the coming months ahead.

Firstly, though a quick look back at how April is going, and one word sums it up – dry. The overlying pattern this month so far is seeing significant downwelling in the wake of the strong MJO phase that repeatedly sat across the top end of Australia with an open channel down the eastern part of the mainland. Now we see more widespread sinking affect 80% of the mainland as the bulk of the activity shifts offshore from the eastern states though some still remains.

A quick tidbit on the WA thunderstorms that absolutely smashed the northern suburbs of Perth (primarily). We saw a sharp upper trough combine with remnant moisture form Ex TC Olga in slack wind shear so any thunderstorms that did develop barely moved. This sort of rainfall is not uncommon in these scenarios but typically happens inland away from the coastline more often than not. Appearing over suburbia gave it the coverage that it got and certainly given the concrete/roads/housing etc, the runoff was a lot more significant as well as the pooling in low lying areas. Blocked drains from debris building up over time from a long dry spell further contributed to the impacts. The heaviest I saw on a home weather station was about 131mm in just under one hour which is very intense rainfall anywhere in the world. The unfortunate aspect of this little system was it did not give much inland to the farming areas, and even some suburbs missed out. Overall, the very dry conditions in WA persist and will continue to do so for the remainder of April and early May before we start to see the Longwave Trough begin to appear at that longitude.

Meteorological Discussion:

Now looking at things currently it is safe to say the dry April is in full swing with minimal light rainfall over Southern Agricultural areas, mostly thanks to a weak front sliding up the eastern side of yet another strong high-pressure system moving into the Bight from WA however most of this activity has cleared by early on the 19th. Totals have largely been under 5mm gradient to less than 1-2mm further inland.

The current pattern as of the 19th shows a dominant high-pressure system moving across the state with cool to mild and mostly dry conditions apart from some light drizzly shower activity about the Lower SE Coast and the Southern coasts and ranges of the Fleurieu. Cold mornings are likely on both the 21st and 22nd with a slight frost risk mainly about the Lower and Upper SE but also parts of the Mallee, Lower and Upper North and Riverland as well. This large high will move eastwards in the coming few days before a weak upper shortwave and associated cold front move up from the SW on the 23rd delivering some isolated showers, tending scattered about the Lower SE, Kangaroo Island and the southern Fleurieu. Further inland, activity will lack penetration to be very isolated and light with minimal to no rainfall at all before a new second large high-pressure system moves in.

In the final few days of the month, it looks like a strong Ridge of high pressure will move in from the West clearing out any lot shower activity from the southern coasts and ranges. This should move steadily eastwards resulting in cool to cold nights, especially around the 26th to 27th where another slight frost risk presents, chiefly again over the Upper and Lower SE but some patches are likely further north in the Mallee, Riverland and Lower North. Days will again be cool to mild but mostly sunny with very minimal change in the weather as this settled period continues.

A very weak change is once again possible as another very strong high upwards of potentially 1040hPa moves over the waters south of WA, increasing the easterly flow across the mainland, keeping the eastern seaboard moist but largely keeping our state dry. Now I will note however when we get a period of strong high pressure like this, we often see colder air aloft wandering up, especially on the eastern flanks. With a repeat of this, there can be on occasion a rogue little low pressure system form within the high if the response to the cold air aloft is strong enough to be reflected down at the surface. It is a possibility and the window for this would be around the last day of the month or early May but certainly don’t hold your breath on it too much as the high pressure at the surface is quite strong and overwhelming right now. If we did see something out of this it would be some patchy light rain areas and convective showers, mostly through afternoon land heating in very slack steering.

Climate Drivers:

The Bureau has finally declared the El Nino is over (by the numbers) however the influence has long gone. The current dry spell over us right now is not driven by that. Looking at Figure 1 we can see renewed coastal upwelling off Peru as the Humboldt Current begins to pulse once more. The Central

Western Pacific and infeed much of the water off the east coast of Australia is above average. The Indonesian and general SE Asian area is above average as well. There isn’t much that isn’t, only the very coastal fringes caused by offshore winds and small-scale upwelling. Long term, we will see much colder water (5C below average as per Figure 2) in the central and eastern pacific sub-surface slowly up well over the coming few months to establish our La Nina. This will be a slow grind, it won’t just rock up and go bang, there will be a lot of ebbing and flowing before the signal becomes clear. Trade winds however are still average to above average keeping warmer water and moisture closer to the longitudes of the Eastern coast of Australia to about the Solomon Islands.

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

Figure 2: Sub-surface temperatures in the Pacific showing colder anomalies to 5C below average around 100-150m in depth yet to surface – source BoM http://www.bom.gov.au/climate/

Moving on to the Indian and I have seen a lot of chit chat about a drought and Positive IOD going around of late. It is still April, and that index has no bearing or meaning at this time of the year as the wind patterns that set up a proper IOD (negative or positive) have not formed yet. What we are seeing is very warm SST’s in the western IOD box off the horn of Africa skewing the index – the latest coming in at +0.57C. This warmth in the western Indian has certainly helped provide the moisture needed for the big falls and flooding we saw in Dubai, however, is not associated with an actual Positive IOD event. As we can see in Figure 1, the entire Indian is essentially above average apart for an area south of Madagascar. Going forward, I expect these values to potentially get even higher before they get lower again as we move back a neutral IOD through winter and then into negative territory in spring. I don’t see a strong event, it is probably going to be towards the weaker end of negative but there is a lot of warm water to push west from the Indonesian through-flow Current but not anywhere to the magnitude that 2016 had given 2015 was a strong El Nino with even warmer water pushing west. On the flip side however, the oceans broadly are warmer over the Indian than at the same time in 2016. In terms of a year that may be closer to the mark, patterns are a little like 2020 however that year April was wet whilst this year has been very dry so take that with a grain of salt as no two years are quite the same. Moisture continues to hold off Sumatra but due to a complete lack of long-waves standing up due to the broad-scale sinking, none of that has been captured and sent SE. May is the target month for that to change, with further improvement through June.

The SAM has been positive of late but with moisture contracting north, that is not ideal for rainfall for anywhere but the east coast. High pressure has dominated with frontal systems further south. The long-wave trough has been mostly peaking around the Tasmania to New Zealand areas, so we have seen some cooler SW-SE winds at times with light showers but honestly it is pitiful rainfall. Looking forward we see the SAM gradually move to neutral levels, but I’d be surprised if we get to negative values like some forecasts are suggesting. There will be plenty of strong high pressure to round out the month before things gradually improve through May with long waves starting to stand up, more-so under WA first.

The MJO is now weakening after a couple of moderate to strong pulses and has been responsible for good rainfall into QLD and associated islands in the Pacific. Now that the MJO has moved further out into the Pacific, we will see main tropical action begin to decay as we approach the winter season With the MJR having less and less influence on our weather until the return of the tropical season from about October. I should note that there is still some strong areas of convergence over the Gulf of Carpentaria which is resulting in heavy rainfall offshore from Arnhem Land and this may clip the coastal areas there in the coming days before ridging from the continent begins to bring out cooler and drier air from the interior clearing a bulk of the tropical moisture from the region.

Districts:

Eyre Peninsula

Large high moving steadily eastwards through till 22nd. Fast moving upper shortwave and weak cold front early on 23rd bringing a cooler change and isolated coastal showers, but minimal penetration inland. New strong high moving in from west thereafter but keeping a cooler SW-S onshore flow through till about the 26th with perhaps some light drizzly showers at times about the Lower Eyre Peninsula coast and some of the Eastern Eyre Peninsula coast roughly south of about Cowell. Mild then until the high moves east and another weak front waffles up from the south around 29-30th with further coastal light showers. Very strong high of 1040hPa+ then likely south of the Bight to round out the month and start May as easterly regime continues to dominate though weak upper low very end of month or early May could deliver some patchy light rain. Rainfall for the period 1-3mm about the aforementioned coastlines, with possible local totals to 5mm or so. Totals less than 1mm further inland to nil over the north.

Upper North

Large high moving steadily eastwards through till 22nd. Fast moving upper shortwave and weak cold front later on 23rd bringing a cooler change and isolated light showers in the far S/SW but minimal penetration further inland. New strong high moving in from west thereafter but keeping a cooler SW-S onshore flow through till about the 26th with perhaps some light drizzly showers at times about the windward slopes in the far south. Mild then until the high moves east and another weak front waffles up from the south around 29-30th with further light drizzly shower activity in the far S/SW. Very strong high of 1040hPa+ then likely south of the Bight to round out the month and start May as easterly regime continues to dominate though weak upper low very end of month or early May could deliver some patchy light rain. Rainfall for the period 1-2mm about the south and windward slopes, with possible local totals to 3mm or so. Totals less than 1mm further inland to nil over the north and NE.

Lower North

Large high moving steadily eastwards through till 22nd. Fast moving upper shortwave and weak cold front later on 23rd bringing a cooler change and isolated light showers, chiefly in the southern half of the district with minimal penetration further inland. New strong high moving in from west thereafter but keeping a cooler SW-S onshore flow through till about the 26th with perhaps some light drizzly showers at times about the windward slopes/elevated terrain. Mild then until the high moves east and another weak front waffles up from the south around 29-30th with further light drizzly shower activity. Very strong high of 1040hPa+ then likely south of the Bight to round out the month and start May as easterly regime continues to dominate though weak upper low very end of month or early May could deliver some patchy light rain. Rainfall for the period 1-3mm about the south and windward slopes, with possible local totals to 4mm or so. Totals less than 1mm further inland to nil over the north and NE.

Yorke Peninsula

Large high moving steadily eastwards through till 22nd. Fast moving upper shortwave and weak cold front early afternoon on 23rd bringing a cooler change and isolated light showers, chiefly about the foot. New strong high moving in from west thereafter but keeping a cooler SW-S onshore flow through till about the 26th with perhaps some light drizzly showers at times but not a lot in it and once again, chiefly about the foot. Mild then until the high moves east and another weak front waffles up from the south around 29-30th with further light drizzly shower activity. Very strong high of 1040hPa+ then likely south of the Bight to round out the month and start May as easterly regime continues to dominate though weak upper low very end of month or early May could deliver some patchy light rain. Rainfall for the period 1-3mm about the foot, with possible local totals to 4mm or so. Totals less than 1mm towards the north.

Kangaroo Island

Large high moving steadily eastwards through till 22nd. Fast moving upper shortwave and weak cold front around lunch on 23rd bringing a cooler change and isolated light showers. New strong high moving in from west thereafter but keeping a cooler SW-S onshore flow through till about the 26th with perhaps some light drizzly showers at times but not a lot in it and once again. Mild then until the high moves east and another weak front waffles up from the south around 29-30th with further shower activity. Very strong high of 1040hPa+ then likely south of the Bight to round out the month and start May as easterly regime continues to dominate though weak upper low very end of month or early May could deliver some patchy light rain. Rainfall for the period 1-4mm with possible local totals to 5mm or so.

Adelaide Plains

Large high moving steadily eastwards through till 22nd. Fast moving upper shortwave and weak cold front later on 23rd bringing a cooler change and isolated light showers, chiefly about the foothills and southern suburbs. New strong high moving in from west thereafter but keeping a cooler SW-S onshore flow through till about the 26th with perhaps some light drizzly showers at times but not a lot in it and once again, chiefly towards the foothills and southern suburbs. Mild then until the high moves east and another weak front waffles up from the south around 29-30th with further light drizzly shower activity. Very strong high of 1040hPa+ then likely south of the Bight to round out the month and start May as easterly regime continues to dominate though weak upper low very end of month or early May could deliver some patchy light rain. Rainfall for the period 1-3mm, with possible local totals to 4mm or so. Totals less than 1mm towards the northern outer plains.

Mt.Lofty Ranges

Large high moving steadily eastwards through till 22nd. Fast moving upper shortwave and weak cold front later on 23rd bringing a cooler change and isolated light showers, chiefly about the southern coasts and ranges of the Fleurieu and also known wet spots. New strong high moving in from west thereafter but keeping a cooler SW-S onshore flow through till about the 26th with perhaps some light drizzly showers at times, generally about those exposed southern coasts and ranges where lifting topography will keep the showers coming at times. Mild then until the high moves east and another weak front waffles up from the south around 29-30th with further light drizzly shower activity. Very strong high of 1040hPa+ then likely south of the Bight to round out the month and start May as easterly regime continues to dominate though weak upper low very end of month or early May could deliver some patchy light rain. Rainfall for the period 1-5mm, with possible local totals to 8mm or so. Totals less than 1-2mm towards the northern ranges.

Riverland/Mallee

Large high moving steadily eastwards through till 22nd. Fast moving upper shortwave and weak cold front much later on 23rd (especially Riverland) bringing a cooler change and isolated light showers in the far S/SW of the Mallee but minimal penetration to the northern parts and the Riverland. New strong high moving in from west thereafter but keeping a cooler SW-S onshore flow through till about the 26th with perhaps some light drizzly showers at times, mostly in the south of the Mallee. Mild then until the high moves east and another weak front waffles up from the south around 29-30th with further light drizzly shower activity once again mostly confined to the south of the Mallee. Very strong high of 1040hPa+ then likely south of the Bight to round out the month and start May as easterly regime continues to dominate though weak upper low very end of month or early May could deliver some patchy light rain. Slight frost risk morning of 22nd and again on 27th. Rainfall for the period 1-2mm over the south of the Mallee with possible local totals to 3mm or so. Totals less than 1mm further north grading to nil over most of the Riverland.

Upper SE

Large high moving steadily eastwards through till 22nd. Fast moving upper shortwave and weak cold front later on 23rd bringing a cooler change and isolated light showers, more likely closer to the Coorong coastline and Lower SE border. New strong high moving in from west thereafter but keeping a cooler SW-S onshore flow through till about the 26th with perhaps some light drizzly showers at times but not a lot in it and once again. Mild then until the high moves east and another weak front waffles up from the south around 29-30th with further light drizzly shower activity. Very strong high of 1040hPa+ then likely south of the Bight to round out the month and start May as easterly regime continues to dominate though weak upper low very end of month or early May could deliver some patchy light rain. Slight frost risk morning of 22nd and 27th. Rainfall for the period 1-3mm, with possible local totals to 4mm or so. Totals less than 1mm towards the NE and E of the district.

Lower SE

Large high moving steadily eastwards through till 22nd. Fast moving upper shortwave and weak cold front later on 23rd bringing a cooler change and a band of showers, more likely closer to the SW coastline. New strong high moving in from west thereafter but keeping a cooler SW-S onshore flow through till about the 26th with light drizzly showers at times. Cool at first until the high moves east and another weak front waffles up from the south around 29-30th with further light drizzly shower activity. Very strong high of 1040hPa+ then likely south of the Bight to round out the month and start May as easterly regime continues to dominate though weak upper low very end of month or early May could deliver some patchy light rain. Slight frost risk morning of 22nd and 27th. Rainfall for the period 1-4mm, with possible local totals to 5-6mm or so in the south. Totals less than 1-2mm towards the NE and E of the district.

Western VIC

Large high moving steadily eastwards through till 22nd. Fast moving upper shortwave and weak cold front much later on 23rd bringing a cooler change and a band of showers, mostly about the SW District only with activity much more isolated north of here and certainly by the Mallee District. New strong high moving in from west thereafter but keeping a cooler SW-S onshore flow through till about the 26th with light drizzly showers at times. Cool at first until the high moves east and another weak front waffles up from the south around 29-30th with further light drizzly shower activity. Very strong high of 1040hPa+ then likely south of the Bight to round out the month and start May as easterly regime continues to dominate though weak upper low very end of month or early May could deliver some patchy light rain towards SA or over the Mallee District with a weak surface trough also deepening. Slight frost risk morning of 22nd and 27th inland away from the coast. Rainfall for the period 1-4mm, with possible local totals to 5-6mm or so in the south of the SW District Totals less than 1mm towards over the Wimmera and basically nil over the Mallee.

Final Notes

• Dry spell dominating as broadscale downwelling maintains hold over mainland
• Longwave trough will continue to peak over the Tasman and New Zealand longitudes in the coming 2 weeks before another one begins to form off WA
• Strong high pressure at surface leaves upper levels susceptible to cold air aloft which in time will allow surface troughing to form
• Possible upper low/low scale rain event early May however break in season probably from mid-month at the earliest and certainly more established by June
• Still expecting much better period once break occurs however watch out for some drying in July potentially before August-October sees a slightly wetter signal once again
• Frost risk for this season is about average, perhaps ever so slightly above
• More thorough idea of patterns for winter likely by May outlook

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Happy Farming and Cheers from The Weatherman

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