April 2022 Update

April 2022 update SA

*Forecast prepared between Apr 17th-18th 2022

Review:

G’Day everyone, hope you have had a great Easter so far spent with your families, it sure has been a busy one here between work and social commitments, I think a workout or two will be needed considering the food and chocolate consumed! The last couple of weeks has seen predominantly dry conditions dominated by high pressure systems however there was one moderate upper trough with a band of rain that traversed chiefly south-Central districts of the state with some light falls. Better rainfall was recorded in the southern Adelaide area and about the ranges in the order of 8-10mm. A secondary trough that was the remannts of a strong cut off low over WA (which gave some tremendous falls) delivered a decayed patchy rainband to mostly the West Coast and Eyre Peninsula. This would have been handy for that region with 7-15mm falls recorded, which will keep the soil in good condition given the huge summer rainfall that occurred. Strong ridging largely eroded the remainder but some light falls to 2-3mm snuck through further east at least.

Meteorological Discussion:

Currently as I type this it is late Easter Sunday evening and an upper trough with scattered mid-level showers and isolated thunderstorms in lines is steadily moving in from the west squeezing the flow aloft and increasing upper-level divergence as the lee side of the upper wave accelerates along the jet. The large amount of high pressure recently combined with the wildly active sun has given rise to significant meandering in the jetstream, something which I touched on in the last outlook where this activity could lead to a rogue event amongst the broader drier scheme we have been in and this is going to ring true which will definitely help some, but not all areas get a handy drop of rain.

This upper trough will move eastwards on the 18th with areas of patchy rain, scattered showers and the odd isolated thunderstorm due to the colder air aloft. Given the stronger ridging at the surface, this upper feature will move fairly slowly and allow some locations to receive longer trains of convective rainfall throwing some random nice dumps amongst many smaller totals. The greatest

likelihood for this is over scattered sections of the Upper & Lower North, parts of the Riverland and Mallee and possible the northern portions of the Upper SE but I stress that not all will receive these lines. Currently a nice line is over the Northern Yorke Peninsula just north of Moonta giving a nice training shower line with some heavier cores embedded so it will be interesting to hear of totals here by the morning. Then through the later afternoon and evening due to lower-level convergence, training showers will move up the Spencer Gulf with some of these moving into the coastal portions of Eastern Eyre Pen and the Upper North with potentially some localised heavy falls. Areas of rain and scattered showers associated with the head of the upper trough will also traverse across the West Coast and up across Northern Eyre Pen into the Pastorals and Flinders with a mixture of light to moderate totals although quite patchy once again.

Beyond this we will see a very strong and dominant high-pressure system, far stronger than normal even for this time of the year which will cause a few things to happen. Firstly, the shape of the high looks to be a lot more elongated west to east with the ridge orientation so we will see further strengthening of the trades and easterlies across the top side of the high which will drive moisture across the north of the country as well as into the QLD coastline with heavy falls developing there. Secondly, it will once again allow for further significant jetstream meandering and kinking with a greater chance if cut off upper lows forming, the position and strength will remain the question. It does signal a classic setup of tropical moisture ready to be grabbed by a trigger to the south, usually I the form of a strong front or upper system attached. Gove has had a huge increase in rain lately, one of my favourite signs to look out for that things are moving into position. Will explain a little more again in the climate driver's section. The window for this system is from around April 25th-28th and some may say that is a traditional break in the season time wise but that varies significantly from district to district and from year to year. I will still be wary of a false break given the likelihood of more high pressure afterward though not to the strength of what we have seen and will see in the coming week. When this season does break, I expect it to be a fairly solid one. Once the longwave trough rises and the SAM become a little more negative, we will likely see more significant rainband and shower activity combined together.

Climate Drivers

The Pacific continues to remain in a La Nina state with minimal change from 2 weeks ago see Figure 1. This is an impressive feat with the reluctance to break down as normal La Nina’s do which keeps the warmer seas closer to Australia. This will only help going into winter the longer it remains this way. Trade winds have been consistent once again with moisture now piling across the top end around the top of some very strong ridging. One key indicator I look for is rainfall in Gove in April and May. So far this month Gove has recorded 405.2mm with 350mm in the last 2 days. That is very impressive and shows the strong easterly flow that is transporting tropical moisture westwards across the top of our highs whilst we are in this downwelling or drier phase. It is about moving that moisture into position so it can be then captured by systems from the west should they have a high

enough amplitude. I expect minimal change in the next 2 weeks again but a slow weakening trend through the remainder of autumn and winter. I should also note that I Figure 2 there has even been a slight increase in cooler water in the sub-surface in March compared to Feb showing the La Nina is not done in the oceans just yet.

Figure 1: Current SST anomalies showing persistent La Nina – source Tropical Tidbits https://www.tropicaltidbits.com/analysis/ocean/

Figure 2: Monthly sub-surface anomalies in Pacific Ocean – note resurgence of cooler anomalies in March from Feb – source BoM http://www.bom.gov.au/climate/enso/#tabs=Pacific-Ocean

Moving on to the Indian and no real changes there, nice and warm and still giving much potential to allowing moisture to creep SE when the Long wave trough stands up around the Perth longitude. In pole position to a weakish negative IOD once again but with La Nina having a greater influence on the mainland last year, this too could have a greater influence through the cooler months than last years. I don’t see a 2016 type of year however, that was the strongest negative IOD on record.

The MJO is weak and in phase 7 to 8, so largely the opposite side of the globe giving us the broad downwelling over the continent. This will however swing around back to the Indian by the end of the

month and increase in response to the big easterlies we have seen so a broad setup is starting to take place for the next moisture influx out of the tropics. A late season Tropical Cyclone west of the mainland chiefly cannot be ruled out given the warmth in the Indian when the next MJO phase roles by.

The SAM is neutral to positive and will increase further to positive levels in the coming week as a large high dominates. I expect a shift back to neutral to borderline negative territory briefly during early May but not long-lived.

Districts:

Eyre Peninsula

Areas of rain and isolated brief thunderstorms in the north late on 18th clearing overnight into 19th. Scattered coastal showers in onshore southerly flow enhanced by cooler air aloft from upper trough, especially about coast of Eastern Eyre Peninsula in early hours of 19th through nocturnal gulf convergence and streams where locally heavier showers are possible. Weak cold front extending from the SW early on 20th with a band of showers before a stronger high-pressure system moves below the continent and dominates with an easterly flow. Isolated coastal showers are still likely from 21st-22nd in that flow. Compex upper and surface trough interaction from 25th potentially generating more scattered showers out of the north, tending to areas of rain however system highly subject to change. Rainfall for period 5-10mm in north and about Eastern EP coast, with isolated 20mm localised dumps in the latter possible in streams on early 19th. Otherwise, 2-5mm for most other areas. End of month event has potential for up to 15-20mm.

Upper North

Scattered showers and areas of patchy rain late 18th gradually weakening and contracting eastwards through early 19th. A brief isolated thunderstorm also likely. Localised slow-moving showers in vicinity of Spencer Gulf early 19th in streams with upper trough assistance. Weak cold front extending from the SW on 20th with band of scattered showers before a stronger high-pressure system moves below the continent and dominates with an easterly flow till at least the 25th. Gully winds likely at night. Complex surface and upper trough feature likely developing inland from 25th-28th generating showers and areas of rain out of the north however this is highly subject to change. Rainfall for the period 3-8mm nearing 10mm about higher terrain. Potential for an extra 10-15mm with end of month event.

Lower North

Scattered showers late 18th gradually weakening and contracting eastwards through early 19th. Localised slow-moving showers in vicinity of Spencer Gulf early 19th in streams with upper trough assistance. Weak cold front extending from the SW on 20th with band of scattered showers before a stronger high-pressure system moves below the continent and dominates with an easterly flow till at least the 25th. Gully winds likely at night. Complex surface and upper trough feature likely developing inland from 25th-28th generating showers and areas of rain out of the north however this is highly subject to change. Rainfall for the period 5-10mm nearing 10-12mm about higher terrain. Potential for an extra 10-15mm with end of month event.

Yorke Peninsula

Isolated showers late 18th to early 19th chiefly about the north and also tip of Gulf St. Vincent in convergent southerly flow in streams. Weak cold front extending from the SW on 20th with band of scattered showers before a stronger high-pressure system moves below the continent and dominates with an easterly flow till at least the 25th. Complex surface and upper trough feature likely developing inland from 25th-28th generating showers and areas of rain out of the north however this is highly subject to change. Rainfall for the period 3-8mm. Potential for an extra 10-15mm with end of month event

Kangaroo Island

Isolated showers late 18th to 19th in onshore southerly flow. Weak cold front extending from the SW early on 20th with band of scattered showers before a stronger high-pressure system moves below the continent and dominates with an easterly flow till at least the 25th. Complex surface and upper trough feature likely developing inland from 25th-28th generating showers and areas of rain out of the north however this is highly subject to change and may well be too far north/west for here. Rainfall for the period 3-8mm. Potential for an extra 5-10mm with end of month event.

Adelaide Plains

Isolated showers late 18th in south gradually weakening and contracting eastwards through early 19th. Localised slow-moving showers in vicinity of Gulf St. Vincent early 19th in streams with upper trough assistance. Weak cold front extending from the SW on 20th with band of scattered showers before a stronger high-pressure system moves below the continent and dominates with an easterly flow till at least the 25th. Gully winds likely at night. Complex surface and upper trough feature likely developing inland from 25th-28th generating showers and areas of rain out of the north however this is highly subject to change. Rainfall for the period 3-6mm nearing 8-10mm about foothills. Potential for an extra 10-15mm with end of month event.

Mt.Lofty Ranges

Isolated showers late 18th in south gradually weakening and contracting eastwards through early 19th. Weak cold front extending from the SW on 20th with band of scattered showers before a stronger high-pressure system moves below the continent and dominates with an easterly flow till at least the 25th. Gully winds likely about western slopes at night. Complex surface and upper trough feature likely developing inland from 25th-28th generating showers and areas of rain out of the north however this is highly subject to change. Rainfall for the period 3-6mm nearing 8-12mm about higher terrain/wet spots. Potential for an extra 15-20mm with end of month event.

Riverland/Mallee

Areas of rain about the eastern border late 18th gradually weakening and contracting eastwards through early 19th. Weak cold front extending from the SW on 20th with band of scattered showers, more isolated about the Riverland before a stronger high-pressure system moves below the continent and dominates with an easterly flow till at least the 25th. Complex surface and upper trough feature likely developing inland from 25th-28th generating showers and areas of rain out of the north however this is highly subject to change. Rainfall for the period 7-15mm nearing 20mm near the eastern border although quite patchy due to narrow lines. Potential for an extra 10-15mm with end of month event.

Upper SE

Isolated showers more likely about the coast and far eastern border late 18th gradually weakening and contracting eastwards through early 19th. Weak cold front extending from the SW on 20th with a band of scattered showers, before a stronger high-pressure system moves below the continent and dominates with an easterly flow till at least the 25th. Complex surface and upper trough feature likely developing inland from 25th-28th generating showers and areas of rain out of the north however this is highly subject to change. Rainfall for the period 2-5mm mostly with weak front. Potential for an extra 5-10mm with end of month event though this is less likely for the SE than other districts at this stage.

Lower SE

Isolated showers more likely about the coast and far eastern border late 18th gradually weakening and contracting eastwards through early 19th. Weak cold front extending from the SW on 20th with a band of scattered showers, before a stronger high-pressure system moves below the continent and dominates with an easterly flow till at least the 25th. Lingering morning showers into 21st as ridge becomes established. Complex surface and upper trough feature likely developing inland from 25th-28th generating showers and areas of rain out of the north however this is highly subject to change. Rainfall for the period 2-5mm mostly with weak front. Potential for an extra 5-10mm with end of month event though this is less likely for the SE than other districts at this stage.

Finals Notes:

· Strong feed of moisture into Far North QLD and the Top End to filter SW over coming week setting up for potential end of month event, but lot of water to go under bridge to turn it from an inland event to an agricultural event as well.

· Long dry spell we have seen with Indian Summer like conditions having one last gasp after 20th

· Significant upper troughing developing due to combination of an active sun once again, and surface blocking from enormous high pressure.

· Potential for more ridging into May but gradually reducing in length as long wave trough starts to make an impact.

· Special update likely needed to monitor end of month event to see how it forms

· Seasonal break not necessarily from this potential event but will occur with first major longwave trough

· Winter still looking around average for rainfall but potential for slightly above is increasing with a weak negative IOD

· May outlook will be issued early as I will be away on a fishing trip for the first week of May out over the West Coast.

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Happy Farming and Cheers from The Weatherman

*Note – For interest only, all thoughts are of The Weatherman and may not be reproduced without my consent.

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