September update 2024 SA
September update 2024 SA
*Forecast prepared between Sept 14th-16th 2024
Review:
G’Day everyone, now before I get into the weather side of things I just want to mention that I really hope you are doing ok, all things considering. I understand that things right now are extremely tough for many of you, and I just want to acknowledge that. I hope that each and every one of you reading this can see some relief ASAP as the current situation of this drought is no doubt crippling beyond the understanding of many outside of regional centres and those that are working on the land. Let’s hope we can go onwards and upwards from here extremely soon.
Now looking back so far at September and it is safe to say that the dry conditions have gripped us completely with numerous failed systems struggling to connect at all to the tropics. Fast moving mid-level westerlies have ripped across the state whilst the low levels see significant lag which results in all potential sources of rainfall being out of sync with poor structure. I mentioned in the September outlook that modelling was not to be trusted and was completely unreliable and I maintain that at the current point in time until we see the lower to mid-levels get back in sync with one another which at least is showing signs of starting now.
The main talking point so far this month aside from the lack of rainfall has been the frost over the last 3-4 nights (with another 2 cold mornings to come on the 16th and 17th.). Now some areas have dodged and weaved the frost in this period but most areas have at least received one slight to moderate frost, especially over the Upper SE, Mallee and Riverland. The Lower and Upper North are not far behind either. The Upper SE especially has copped it with moderate to even potentially heavy frost conditions with Keith West recording back to back nights of –4.6C (a record on the 15th and –4.5C on the 16th). Having said that I have my suspicions that the BoM equipment is not reading correctly there but numerous –2C to –3C temperatures have been noted nearby on Personal Weather Stations. Many other centres have recorded at least 1 night below zero. Significant rainfall is needed now, and we look to the second half of the month for that. I think at least there will finally be some relief but not without frost battles thrown in amongst it.
I did make a point the other day of looking back at the current year versus all other years back to 1900. The closest match I found was 1967 and that no doubt would bring up bad memories for those farming then, one of the benchmark dry years of the last 100 years. This year currently is running behind that year for many areas in the growing season, for those that did not get good rain last November and December (east moreso), it is especially difficult and the only word that fits the bill is a drought, a word I don’t use lightly but it is a true reality of where things are right now.
Figure 2: 1967 Jan-Aug rainfall deciles showing focused drought over SE SA and SW VIC – source BoM
Figure 3: 2024 Feb-Aug rainfall deciles showing extremely focused drought over SE SA and SW VIC – source BoM
Figure 4: 1967 Feb-Aug rainfall deciles showing focused drought over SE SA, SW VIC & S NSW – source BoM
Now I thought I’d throw out some comparisons to 1967 and it is quite remarkable how similar the wetter and drier areas are. It also shows how focused the drought has been right over SE SA in both years, especially so since February when the taps essentially turned right off. Take out our small slice of drought and the rest of the nation is not that bad at all, not even closely resembling most bad dry years which affect much greater portions of the country such as 2019,
2002, 1994 for examples sake. This extreme focused drought over SE SA/SW VIC shows a distinct lack of SW-NE onshore flow into that region which shows a lack of fronts and the general longwave trough in our region and SE of. The one time we did get the longwave trough was when it was extremely powerful keeping a very flat westerly flow that brought more wind than anything else and only good showers to the south of the Lower SE District. After the year is done, I will attempt to do a more thorough review on this year, what went wrong forecast wise and what we need to look for in the future. It is a good opportunity for learning, even if the challenge presents is greater than usual. One can always improve their understanding going forward.
Meteorological Discussion:
Taking a look at current proceedings and we have a very large and slow-moving high-pressure system with a central pressure of 1044hPa to the SW of the state. This will slowly weaken in the coming days but unfortunately will be taking the worst possible path and that is to the NE right over us which will assist in bring down the very cold and dry air from the mid-levels across the state over the coming two mornings, the 16th and more so especially the 17th where frost potential will be the highest and most widespread. By the morning of the 17th, the centre of the high will be over the Lower North to Riverland and few areas will escape a frost except coastal fringes and the Lower SE which should have cloud. Perhaps the very dry nature of the air may limit external coverage, but prolonged low temperatures won’t do vines, fresh sappy growth or husks/flowers much good at all.
The good news is the high will weaken as we move forward replaced by a series of cold fronts, about 3 I think over the few days after in a moderate to fresh WSW airstream. These fronts will largely only clip the SE corner of the state, increasing showers near the front and just after, but chiefly over the Lower SE District, the southern and western portions of the Upper SE, Southern Fleurieu, Kangaroo Island, the foot of Yorke Pen and the Lower Eyre Pen. Showers will be very isolated north of here with not much getting past about Clare with rain shadowing hurting a lot of the northern Mallee and Riverland.
Of greater interest is the potential finally for a better system with tropical influence around the 24th-26th period. Right now, we have a significant area of moisture travelling from west to east across the top end, likely partially driven by an equatorial Rossby Wave. Where this moisture ends up will be crucial but most likely it will slow down and turn inland over WA before getting caught up in a passing frontal system. Right now, the speed of the mid-level westerlies is too great to allow a nice consistent hookup/connection of moisture to give us the rain we desperately need however they are slackening off and will keep weakening slowly in the coming weeks. I do expect some form of system this time are more boxes are being ticked than crossed but it looks partially convective with warmth ahead of it and reasonable speed plus a lack of blocking to the east. The most likely outcome is broken bands of showers, tending to patchy rain areas (and possibly some general areas of rain) and isolated thunderstorms with spasmodic falls – i.e. plenty of holes but mixed with useful to potentially heavier totals. Nailing this event will only be possible much closer to the date and I will provide an update on the expected system. I want to see a little more meridional action from the polar jet which would capture the moisture much better. The best totals at this stage are likely to be over the Pastoral Districts, especially the NW Pastoral towards the WA border where early indications are that 30-50mm is possible in some locations.
Beyond this, we are likely to see another high pressure, the nature of our region is to favour high pressure even still, so I expect another frost after the rain, but unlikely to the level of the 16th-17th. This should carry through to the end of the month however I look to October to see an uptick in thundery troughs, especially for the far west of the state.
Climate Drivers:
The Pacific has made slow but steady progress in the last 2 nights in terms of moving towards La Nina, but it is not there yet. The signature is no doubt there and has been developing for a while now after plenty of stalls. One thing to note and no doubt many of you will be aware but La Nina does not necessarily mean wet, especially so when weak. The year most similar to this, 1967 which I alluded to further up was also a La Nina year albeit a weak one. That also came with a Positive IOD. This year it is more neutral. It is usually only the moderate to strong ones that have a meaningful impact on us in SA rainfall wise, with the weaker ones generally influencing NSW and QLD and even then things are patchy. I expect one to be declared late spring or very early summer, but it won’t be much more than a weak one.
Now moving onto the Indian and greater changes have been noted here since the September outlook. The craziest year I have ever seen with this index continues but these swings have been expected. The dryness being so long-lived out of it however has not been. Initially we entered the Positive phase which I labelled false due to the inclusion of monsoon influenced oceanic water which as we know, the IOD has nothing to do with. It’s like putting petrol into a diesel engine, it just doesn’t work at all. Next, we charged back to a negative value, well beyond the cutoff threshold of –0.4C reaching almost –1C. However, that was too short-lived to be official, so we came flying back to a weak positive around +0.4C almost or just under. I expected this in July; however, it happened in August with the dryness returning with a vengeance after a reasonable June. Now we are swinging back towards negative once more as you can see the cold spot off Java and Sumatra has rapidly diminished whilst the Gulf of Aden is spilling cooler water back into the western IOD measurement area. I expect a peak in this from mid-October to mid-November before it all washes out. October to December should see an uptick in rainfall but moreso from thundery means as heat gets into the systems, it will help rain come but it will be patchy so please keep that in mind. Traditional SW streams are completely non-existent this year and that has been a major reason why so many areas have failed, especially the Mallee which relies on streams off Lake Alexandrina/Albert and off the general Coorong area.
Figure 5: Current SST anomalies – source Tropical Tidbits https://www.tropicaltidbits.com/analysis/ocean/cdas-sflux_ssta_global_1.png
Now the SAM has been fluctuating around a fair bit since the major negative phase in August and early September. We are now neutral to positive with the large highs sitting well south, but I expect things to drop back to neutral or just under in the next week. Generally, we should see a slow settling of the index with a slight trend towards more positive values much later in spring to summer.
The MJO has been passing over the north of Australia in the past week assisting in cloud building over the region with an increase in easterlies pulling in ahead of it. There is also likely a westward moving Rossby Wave here and this is very critical to positioning moisture ahead of our potential rain/storm system within the next week or so. Something to watch closely. Showers and the odd isolated thunderstorm will develop across the top end with some isolated heavy dumps.
Districts:
Eyre Peninsula
Large 1044hPa high moving NE from Bight with mild days and cold nights. Slight to moderate frost risk 16th and 17th mornings. Series of weak fronts clipping Lower Eyre Peninsula 19th to 21st, with isolated showers, but scattered near and immediately behind the fronts on 20th and 21st. Shower activity very isolated further north and east and remaining generally dry although partly cloudy. Weak ridge to follow across north of state ahead of a more substantial and complex low-pressure system deepening from the NW from about the 24th. Showers and thunderstorms are likely to develop in the Far West of the state tending to areas of rain at times and extend across the Peninsula through till the 26th or 27th before the flow tends onshore with further stream showers over more coastal areas. Rainfall for period 10-15mm generally, increasing to 15-25mm about Lower Eyre Peninsula. Isolated falls to 30mm possible. Totals decreasing over the eastern portions down to 5-10mm. If the system slows down, then expect a potential doubling of totals.
Upper North
Large 1044hPa high moving NE from Bight with mild days and cold nights. Slight to moderate frost risk 16th and 17th mornings. Series of weak fronts clipping the coastline 19th to 21st, with isolated showers in the south, chiefly on 20th and 21st. Shower activity very isolated further north and east and remaining generally dry although partly cloudy. Weak ridge to follow across north of state ahead of a more substantial and complex low-pressure system deepening from the NW from about the 24th. Showers and thunderstorms are likely to develop in the Far West of the state tending to patchy rain at times and extend SE across the district through till the 26th or 27th before the flow tends onshore with further isolated stream showers over more coastal and southern/windward slope areas. Rainfall for period 8-15mm generally, increasing to 15-25mm in the south. Totals decreasing towards the eastern slopes down to 6-8mm. If the system slows down, then expect a potential doubling of totals.
Lower North
Large 1044hPa high moving NE from Bight with mild days and cold nights. Slight to moderate frost risk 16th and especially 17th mornings. Series of weak fronts clipping the coastline 19th to 21st, with isolated showers, scattered in the south chiefly on 20th and 21st. Shower activity becoming isolated further north and east with some areas remaining dry. Weak ridge to follow across north of state ahead of a more substantial and complex low-pressure system deepening from the NW from about the 24th. Showers and thunderstorms are likely to develop in the Far West of the state tending to patchy rain at times and extend SE across the district through till the 26th or 27th before the flow tends onshore with further isolated stream showers over more coastal and southern/windward slope areas. Rainfall for period 12-18mm generally, increasing to 18-28mm in the south and about windward slopes. Totals decreasing towards the eastern slopes down to 7-12mm. If the system slows down, then expect a potential doubling of totals.
Yorke Peninsula
Large 1044hPa high moving NE from Bight with mild days and cold nights. Slight to moderate frost risk 16th and especially 17th mornings, chiefly the northern half of the Peninsula. Series of weak fronts clipping the coastline 19th to 21st, with isolated showers, scattered about the foot chiefly on 20th and 21st. Weak ridge to follow across north of state ahead of a more substantial and complex low-pressure system deepening from the NW from about the 24th. Showers and thunderstorms are likely to develop in the Far West of the state tending to areas of rain at times and extend SE across the district through till the 26th or 27th before the flow tends onshore with further isolated to scattered stream showers. Rainfall for period 12-20mm generally, increasing to 20-30mm towards the foot. Totals decreasing slightly to the north around 10-12mm. If the system slows down, then expect a potential doubling of totals.
Kangaroo Island
Large 1044hPa high moving NE from Bight with mild days and cold nights. Slight frost risk 16th and 17th mornings near island centre. Series of weak fronts clipping the coastline 19th to 21st, with scattered showers, chiefly on 20th and 21st. Weak ridge to follow across north of state ahead of a more substantial and complex low-pressure system deepening from the NW from about the 24th. Showers and thunderstorms are likely to develop in the Far West of the state
tending to areas of rain at times and extend SE across the island through till the 26th or 27th before the flow tends onshore with further isolated to scattered stream showers. Rainfall for period 15-25mm generally, increasing to 25-30mm towards the western end. Totals decreasing slightly to the east around 12-15mm. If the system slows down, then expect a potential doubling of totals.
Adelaide Plains
Large 1044hPa high moving NE from Bight with mild days and cold nights. Slight frost risk 16th and especially 17th mornings, chiefly about the outer northern plains. Series of weak fronts clipping the coastline 19th to 21st, with isolated showers, scattered about the foothills and southern suburbs chiefly on 20th and 21st. Weak ridge to follow across north of state ahead of a more substantial and complex low-pressure system deepening from the NW from about the 24th. Showers and thunderstorms are likely to develop in the Far West of the state tending to areas of rain at times and extend SE across the district through till the 26th or 27th before the flow tends onshore with further isolated to scattered stream showers. Rainfall for period 15-25mm generally, increasing to 25-35mm towards the foothills. Totals decreasing slightly to the north around 12-15mm. If the system slows down, then expect a potential doubling of totals.
Mt.Lofty Ranges
Large 1044hPa high moving NE from Bight with mild days and cold nights. Slight to moderate frost risk 16th and especially 17th mornings, chiefly about the Barossa and adjacent low-lying valleys. Series of weak fronts clipping the coastline 19th to 21st, with isolated showers, scattered about the southern coasts and ranges chiefly on 20th and 21st. Weak ridge to follow across north of state ahead of a more substantial and complex low-pressure system deepening from the NW from about the 24th. Showers and thunderstorms are likely to develop in the Far West of the state tending to areas of rain at times and extend SE across the district through till the 26th or 27th before the flow tends onshore with scattered stream showers, more frequent about southern coasts and ranges. Rainfall for period 18-28mm generally, increasing to 28-40mm. Totals decreasing about the rain shadow areas of the eastern slopes down to around 12-18mm. If the system slows down, then expect a potential doubling of totals.
Riverland/Mallee
Large 1044hPa high moving NE from Bight with mild days and cold nights. Moderate frost risk 16th and especially 17th mornings. Series of weak fronts clipping the coastline 19th to 21st, with isolated showers in the south of the Mallee only, chiefly on 20th and 21st. Weak ridge to follow across north of state ahead of a more substantial and complex low-pressure system deepening from the NW from about the 24th. Showers and thunderstorms are likely to develop in the Far West of the state tending to patchy rain at times and extend SE across the district through till the 26th or 27th before the flow tends onshore with further isolated stream showers over the Mallee chiefly. Rainfall for period 7-10mm generally, increasing to 10-15mm in the south and east of the Mallee. Totals decreasing towards the backside of the ranges in the immediate rain shadow around 5-7mm. If the system slows down, then expect a potential doubling of totals.
Upper SE
Large 1044hPa high moving NE from Bight with mild days and cold nights. Moderate frost risk 16th and especially 17th mornings. Series of weak fronts clipping the coastline 19th to 21st, with isolated to scattered showers chiefly over the western and southern portions of the district on 20th and 21st. Weak ridge to follow across north of state ahead of a more substantial and complex low-pressure system deepening from the NW from about the 24th. Showers and thunderstorms are likely to develop in the Far West of the state tending to patchy rain at times and extend SE across the district through till the 26th or 27th before the flow tends onshore with further isolated to scattered stream showers. Rainfall for period 10-15mm generally, increasing to 15-20mm in the south and west of the district. Totals decreasing towards the north and east down to around 8-10mm. If the system slows down, then expect a potential doubling of totals.
Lower SE
Large 1044hPa high moving NE from Bight with mild days and cold nights. Slight to moderate frost risk 16th. Series of weak fronts clipping the coastline 19th to 21st, with scattered showers, more frequent near the frontal bands on 20th and 21st. Weak ridge to follow across north of state ahead of a more substantial and complex low-pressure system deepening from the NW from about the 24th. Showers and thunderstorms are likely to develop in the Far West of the state tending to patchy rain at times and extend SE across the district through till the 26th or 27th before the flow tends onshore with further isolated to scattered stream showers. Rainfall for period 12-18mm generally, increasing to 18-25mm. Localised heavier totals to 30mm.Totals decreasing towards the north and east down to around 10-12mm. If the system slows down, then expect a potential doubling of totals.
Western VIC
Large 1044hPa high moving NE from Bight with mild days and cold nights. Slight to moderate frost risk 16th and 17th, especially over the Wimmera and Mallee. Series of weak fronts clipping the coastline 19th to 21st, with scattered showers, more frequent near the frontal bands on 20th and 21st over the SW District and southern Wimmera. Weak ridge to follow across SA/NSW and the far north of the state ahead of a more substantial and complex low-pressure system deepening from the NW from about the 24th or more so 25th. Showers and isolated thunderstorms are likely to develop from SA tending to patchy rain at times and extend SE across the general western portions of the state through till the 26th or 27th before the flow tends onshore with further isolated to scattered stream showers. Rainfall for period 10-15mm generally, increasing to 15-25mm. Localised heavier totals to 30mm over the SW District and about the Grampians. Totals decreasing towards the Mallee down to around 5-10mm. If the system slows down, then expect a potential doubling of totals.
Final Notes
· Drought conditions severely hampering crop growth in most districts of SA and W VIC
· Relief on horizon with a potentially significant system however there will still likely be holes
· Slight relaxing of mid-level westerlies may allow better balance between low level winds and mid-level winds increasing deterioration shear and upslide
· Weak La Nina combined with neutral to weakly negative IOD to finish the year off which should arrest the dry we have seen thus far
· Good rainfall over NT/WA will temporarily mitigate the heat engine of northern Australia but also increase moisture levels ahead of future systems
· Multiple rainfall events needed, but an uptick in a thundery signature through spring will mean unreliable rainfall, albeit locally heavy at times
· Rain Shadow reducing but still present in many systems going forward
· Summer looking relatively standard but with increased rainfall over the inland of the nation and Northern Australia, exceeding the previous wet season
· Special update will be issued for the next system to nail down totals
Link to temporary blog website: https://farmweathersa.blogspot.com/
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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