September 2022 Update SA

                                                         September 2022 update SA

*Forecast prepared between September 17th-19th 2022

Review:

G’Day everyone, and already time for that Mid-month update, things are certainly happening with lots of active systems and rainfall, but that is a fantastic thing at this time of the year, and it is re-assuring that nearly everything is going to plan (with the exception of some nasty frost very early September). Crops will start to get big and thirsty now and demand more moisture, so regular rainfall will still be needed for many.

As mentioned, the early month frosts from the 3rd till 6th did not help things in some districts but the N Yorke Peninsula into the Lower North did cop some nasty hits with stem frost, especially those low-lying valleys. Hopefully this damage that I did see was not too widespread.

In terms of rainfall events there have been plenty with copious fronts and cut off the lows however the most significant even occurred on the afternoon and evening of the 7th getting good rainfall to places that don’t normally receive much from fronts. Places such as Yunta received 27.8mm, and Port Augusta with 39.6mm in a big thunderstorm that unfortunately had a lot of hail. This was a left moving supercell thunderstorm that took advantage of the low-level NE inflow and used it as a moisture tongue to sustain thew rotating updraft. Left moving (of the mean steering flow) thunderstorms are the most notorious and can travel vast distance causing damage. Hail damage covering the highway was noted at Stirling as well as into places like Hammond, smoking sadly some cropping areas in between. I truly hope the damage was not too bad but in that narrow path, it is likely some near wipeouts may have occurred. Other notable totals included 22.4mm at Renmark, 27.2mm at Loxton and 24.2mm at Karoonda. Generally, 20-30mm totals were well received right down from the Upper North down into the Riverland and Mallee before the rainband began to decay towards the SE Districts.

After this we saw frontal passages with good shower activity and heavy streams into many districts (southern Adelaide and Fleurieu were short-changed here) before a series of fronts from the 14th-18th delivered constant shower activity, especially for southern ag areas with a bit less northward penetration due to the flow being quite westerly. Overall, so far, the real winners this month have been the Mallee and Riverland, with many marginal areas getting a very nice drink and sitting above average already with another little top up drop to come early next week which I will discuss in the next section.

Meteorological Discussion:

Now looking ahead for the remainder of September, we won’t have to wait long before another decent system crosses the agricultural areas with a handy top up rainfall. We should see a fine day on the 19th for the most part but very late in the evening a trough will develop in the west of the state and deepen before moving SE. A low-pressure system will then develop within this trough on the 20th with support aloft from an upper trough generating isolated showers and thunderstorms in mid-level cloud out over the Nullarbor in the early hours. As the day progresses, the trough will deepen causing a general increase in showers and isolated thunderstorms over the Western agricultural areas before this activity gradually thickens to a band of patchy rain. Further east ahead of this over the Central/Northern agricultural and more eastern agricultural districts isolated showers and thunderstorms will develop in cleaner air. Thunderstorm activity is more likely towards the Flinders, NE Pastoral and perhaps the Riverland where more warmth and lift will be generated. All this activity will thicken and join up/fill in with the approaching band on the trough to the west as a small cut off low develops further to be centred somewhere over the Lower North district during the evening. The band will thicken as it moves east but once again due to a fairly convective signature, have holes with some fairly heavy areas closer to the leading edges of both the first and second band coming down out of the Flinders, Upper and Lower North with thunderstorm activity later in the day on 20th.

The whole band moves eastwards with the low clearing by 3-4am on 21st or so but around the back side of the low, a weakish wrap around band will form sending light rain areas and thick drizzly showers over most districts north and west of about Adelaide with particular orographic assistance into the ranges of the Lower and Upper North before easing mid to late morning. Renewed small convection across mostly land areas and elevated terrain, chiefly Lower and Upper North and N YP should keep shower activity going into the later afternoon before clearing as solar heating diminishes.

A clearance should occur for a few days after this with a weak ridge before a weak front quickly clips the southern agricultural areas and mainly just the Lower SE District generating a small band of showers on the 23rd. 

Following this another small bubble high quickly moves in and then out as the mobile pattern continues with a deepening low pressure system moving SE out of the WA interior ahead of a coldfront. This type of pattern is indicative of spring where the warming interior allows low pressure to form from the heat that has risen and these often then drop down as prefrontal troughs or lows and can provide a focus for thunderstorm activity. 

This low will slide ESE quickly on the 25th supported from behind by a weak upper trough with a frontal system over waters to the south. Combined these features will generate showers and isolated thunderstorms, tending to areas of rain at times further south. The overall structure of the system is yet to be determined but it should have moisture to work with a mobile pattern. 

Beyond this we should see a potential cut off low at end of month or into early October. I’m expecting a significant system to affect many states soon, but we just might need a little more heat into the middle of the country first. All in all, we are on track for our above average month for most districts though few holes may occur in the Central Pastoral areas. 

Key frost dates are around morning of 24th as the rapid bubble high moves in behind the weak front, with slight to moderate risk and again possible from 28th-30th depending how the end of month cut off low develops but I think moreso into October for that. 

I should note that as mentioned in previous updates in the past that widespread flooding will occur over the Eastern states, especially NSW over the floodplains as slow onset inundation occurs over a long period. Essentially, many areas are already beginning to see flooding, with rivers beginning to break banks. Combined with multiple significant rain bearing systems to come, the added intensity of thunderstorms through spring and early summer as well as the snow melt, this will only go one way and be historically significant with big flows coming into SA as well, likely well above 2016 and probably above 2011 so do take heed of slow onset inundation if you are near the Murray and associated tributaries.

Climate Drivers

The Pacific has now officially been declared as a La Nina once again by the BoM, but we know this has been the case for quite some time, it’s just that the BoM criteria is stricter and more rigid than anywhere else. If it looks like a duck and walks like a duck, then it sure as hell is a duck for mine! Trade winds are consistently strong, moisture keeps pushing west and the Coral Sea is as warm as it has ever been with slight warming in the last fortnight whilst in the east and Nino 3.4 areas, the cold pool below the surface has expanded once more (see Figure 1). In essence, I am sounding like a broken record when it comes to the Pacific and La Nina. Figure 2 shows the overall SST pattern, and you can clearly see the La Nina signature with colder Humboldt Current in the east and warm in the west (notably the Coral). 

I feel the Coral could unlock significant moisture over late spring to early, especially for the eastern states although would likely increase the risk of Tropical Cyclones through summer in general spawning in that region. It has been very quiet in the Coral Sea for some time, so it is due, and it has never been this warm. 

I will watch with a close eye here. Overall, though, whilst this La Nina is now official, I do not expect the same impacts as last year nor such late impacts in early autumn like we saw but more a peak in early to possibly mid-summer before it breaks down a little earlier than the most recent one.

Figure 1: Pacific Sub-surface anomalies showing cold pool 100-120m below surface – source BoM http://www.bom.gov.au/climate/enso/#tabs=Pacific-Ocean&pacific=Sea-sub%E2%80%93surface

Figure 2: Current SST anomalies – source Tropical Tidbits https://www.tropicaltidbits.com/analysis/ocean/

Now moving onto the Indian and we have seen a relatively consistent pattern over much of the last 4 weeks but a slight tendency in the last few days to more warming west of Sumatra and south of Sri Lanka. I’d like to see warming coming up from Java as well and there is a little bit just starting now so hopefully this expands in the next few days. These signatures usually peak from late September to mid-October on average. I expect the IOD index will dip down one more time for that peak around –1.1C to maybe –1.2C although the latter may be a stretch.

The MJO has weakened as it approached the Australian region with only minor impacts in giving some extra tropical moisture. The main moisture off the Indian is still coming from the strong convection south of about Sri Lanka and just SE whilst the Coral and Pacific trades are comfortably taking care of the eastern seaboard. The MJO will float around through phases 6-8 and back to 1 in the coming weeks with little impact.

The all-important SAM has certainly been a feature in the last month with a strong positive to finish August before making its way back to neutral values briefly in early September. This little dip back allowed a frontal response without being too significant but with a positive trend once more into mid-September. The result has been numerous cut off lows mixed with moderate strength cold fronts but no heaving cold blasts from the deep south to really clean us out and set up a nasty frost apart from that frost in early September. The balance has been pretty good, and it is this balance that I talk about as being so important to good rainfall with consistent systems.

I still expect a much more significant and dominant cut off low to either round out September or come in October with the way the patterns are at this stage. As we move closer to summer and warm up, a more positive SAM becomes increasingly favourable for humidity and high moisture which is not welcome at harvest time so it will be important to watch this index. As mentioned previously, I expect plenty of that moisture to filter in during mid to late spring and potentially early summer (thought December a little 50/50 at this stage on how much remains in SA) but definitely above average thunderstorm activity.

Districts:

Now due to less significant and complex systems than previous updates, as mentioned prior this update will revert back to the standard districts, but when greater variance and more discussion is expected, I will break them up into greater detail like the previous two.

Eyre Peninsula

A trough of low pressure will extend from the west on 20th causing NE/N winds to freshen ahead of it. Isolated showers and thunderstorms and patchy light rain will develop over the West Coast early before extending eastwards during the afternoon and thickening to rain areas. Rain areas will begin to transition back to patchy light rain and scattered showers closer to midnight. Fresh to strong SW winds behind the trough with isolated showers lingering into 21st. Weak front 23rd with a band of scattered light showers but not a lot in it. Another brief clearance on 24th ahead of a developing low-pressure system with scattered showers and isolated thunderstorms on 25th, possibly tending to patchy rain over Lower Eyre Peninsula with an increased thunderstorm risk further north. A lingering coastal shower or two possible till 27th. Chance of end of month low but more likely early October. Frost risk slight to moderate 24th and slight only from 28th-30th. Rainfall generally 12-20mm with isolated falls to 25-30mm over Lower Eyre Peninsula. Totals decreasing over NE EP down to 7-12mm.

Upper North

A trough of low pressure will extend from the west on 20th causing NE/N winds to freshen ahead of it. Isolated showers and thunderstorms will develop during the morning before extending eastwards during the afternoon and thickening to rain areas. Thunderstorm potential increases over the Flinders. Rain areas will gradually ease later in the evening at times but still maintain light areas with showers as the wrap around band pushes in with extra orographic assistance. Fresh to strong SW winds behind the trough with thick drizzly showers or light rain areas persisting, chiefly on and west of ranges on 21st. Weak front 23rd with a band of scattered light showers but not a lot in it. Another brief clearance on 24th ahead of a developing low-pressure system with scattered showers and isolated thunderstorms on 25th, possibly tending to patchy rain in the south. Chance of end of month low but more likely early October. Frost risk moderate 24th and slight only from 28th-30th. Rainfall generally 18-30mm with isolated falls to 40mm over elevated terrain and known wet spots. Totals decreasing over far east of ranges to 10-15mm. Local dumps also to 20-30mm in stronger thunderstorms over Flinders potentially.

Lower North

A trough of low pressure will extend from the west on 20th causing NE/N winds to freshen ahead of it. Isolated showers and thunderstorms will develop during the morning before extending eastwards during the afternoon and thickening to rain areas. Rain areas will gradually ease later in the evening at times but still maintain light areas with showers as the wrap around band pushes in with extra orographic assistance, especially about higher peaks. Fresh to strong SW winds behind the trough with thick drizzly showers or light rain areas persisting, chiefly on and west of ranges on 21st. Weak front 23rd with a band of scattered light showers but not a lot in it. Another brief clearance on 24th ahead of a developing low-pressure system with scattered showers and isolated thunderstorms on 25th, tending to patchy rain later. Chance of end of month low but more likely early October. Frost risk moderate 24th and slight only from 28th-30th. Rainfall generally 20-30mm with isolated falls to 40mm over elevated terrain and known wet spots. Totals decreasing over far east of ranges to 12-20mm.

Yorke Peninsula

A trough of low pressure will extend from the west on 20th causing NE/N winds to freshen ahead of it. Isolated showers and possible thunderstorms and patchy light rain about the foot will extend from the NW early before extending to the remainder gradually during the afternoon and thickening to rain areas. Rain areas will begin to transition back to patchy light rain and scattered showers later in the evening. Fresh to strong SW winds behind the trough with isolated showers lingering into 21st. Weak front 23rd with a band of scattered light showers but not a lot in it. Another brief clearance on 24th ahead of a developing low-pressure system with scattered showers and isolated thunderstorms on 25th, possibly tending to patchy rain once more. A lingering coastal shower or two possible near the foot till 27th. Chance of end of month low but more likely early October. Frost risk slight to moderate 24th chiefly in the N and slight only from 28th-30th again chiefly N. Rainfall generally 15-25mm with isolated falls to 30mm near the foot. Totals decreasing over N YP down to 10-15mm.

Kangaroo Island

A trough of low pressure will extend from the west on 20th causing NE/N winds to freshen ahead of it. Areas of rain will extend from the NW early before temporarily easing. Rain should resume later in the afternoon and thickening once more. Rain areas will begin to transition back to patchy light rain and scattered showers later in the evening. Fresh to strong SW winds behind the trough with isolated showers lingering into 21st. Weak front 23rd with a band of scattered light showers but not a lot in it. Another brief clearance on 24th ahead of a developing low-pressure system with scattered showers and isolated thunderstorms on 25th, tending to patchy rain once more. A lingering shower or two till 27th. Chance of end of month low but more likely early October. Frost risk slight on 24th. Rainfall generally 15-20mm though the western end may see closer to 25mm.

Adelaide Plains

A trough of low pressure will extend from the west on 20th causing NE/N winds to freshen ahead of it. Isolated showers and possible thunderstorms and patchy light rain will extend from the NW early before extending to the remainder gradually during the afternoon and thickening to rain areas. Rain areas will begin to transition back to patchy light rain and scattered showers later in the evening. Morning light rain or thick drizzly areas lingering before activity becomes more isolated later in the day. Fresh to strong SW winds behind the trough. Weak front 23rd with a band of scattered light showers but not a lot in it. Another brief clearance on 24th ahead of a developing low-pressure system with scattered showers and isolated thunderstorms on 25th, possibly tending to patchy rain once more. A lingering coastal shower or two possible closer to southern suburbs till 27th. Chance of end of month low but more likely early October. Frost risk slight to moderate 24th chiefly about outer plains and slight only from 28th-30th again chiefly outer plains. Rainfall generally 15-25mm with isolated falls to 30mm near the foothills. Totals decreasing towards coastal suburbs to 12-17mm.

Mt.Lofty Ranges

A trough of low pressure will extend from the west on 20th causing NE/N winds to freshen ahead of it. Isolated showers and possible thunderstorms and patchy light rain will extend from the NW early before extending to the remainder gradually during the afternoon and thickening to rain areas. Rain areas will begin to transition back to patchy light rain and scattered showers later in the evening. Morning light rain or thick drizzly areas lingering before activity becomes more isolated later in the day. Fresh to strong SW winds behind the trough. Weak front 23rd with a band of scattered light showers but not a lot in it. Another brief clearance on 24th ahead of a developing low-pressure system with scattered showers and isolated thunderstorms on 25th, possibly tending to patchy rain once more. A lingering shower or two possible closer to southern coasts and ranges till 27th. Chance of end of month low but more likely early October. Frost risk slight to moderate 24th chiefly about low-lying valleys and slight only from 28th-30th again chiefly low-lying valleys. Rainfall generally 25-40mm with isolated falls to 50mm near known wet spots. Totals decreasing towards northern ranges and eastern/SE slopes down to 15-20mm.

Riverland/Mallee

A trough of low pressure will extend from the west on 20th causing NE/N winds to freshen ahead of it. Isolated showers and thunderstorms will develop during the afternoon before thickening to rain areas and isolated thunderstorms during the evening. Thunderstorm potential increases over the far northern Riverland and much more so into the NE Pastoral north of there. Rain areas will gradually ease later in the evening and mostly clearing out in early hours of 21st. More holes and patchy lighter falls towards the Murray Valley. Fresh to strong SW winds behind the trough with thick drizzly showers or light rain areas persisting, chiefly closer to backs of ranges on 21st. Weak front 23rd with a band of scattered light showers but not a lot in it. Another brief clearance on 24th ahead of a developing low-pressure system with scattered showers and isolated thunderstorms on 25th, possibly tending to patchy rain in the south of the Mallee. Chance of end of month low but more likely early October. Frost risk moderate 24th and slight only from 28th-30th. Rainfall generally 10-20mm with isolated falls to 30mm over parts of the far eastern border and the Riverland. Also, possibly with any thunderstorms though the best/strongest of these may be too far north. Totals decreasing over Murray Valley to 8-12mm.

Upper SE

A trough of low pressure will extend from the west on 20th causing NE/N winds to freshen ahead of it. Patchy light rain will develop during the afternoon before thickening a little at times. Patchy rain will gradually ease later in the evening and mostly clearing out in early hours of 21st though weak wrap around band may keep patchy light rain or drizzly areas lingering into the afternoon. Fresh to strong SW winds behind the trough. Weak front 23rd with a band of scattered light showers but not a lot in it. Another brief clearance on 24th ahead of a developing low-pressure system with scattered showers and isolated thunderstorms on 25th, possibly tending to areas of rain. Chance of end of month low but more likely early October. Frost risk moderate 24th and slight only from 28th-30th. Rainfall generally 15-25mm with isolated falls to 30mm over parts of the far eastern border. Totals decreasing over towards the Coorong coastline nearing 10-15mm.

Lower SE

A trough of low pressure will extend from the west on 20th causing NE/N winds to freshen ahead of it. Patchy light rain will develop during the later afternoon, but rainfall totals won’t be too significant. Patchy rain will gradually ease later in the evening and mostly clearing out in early hours of 21st though weak wrap around band may keep patchy light rain or drizzly areas lingering into the morning. Fresh to strong SW winds behind the trough. Weak front 23rd with a band of scattered light showers but not a lot in it. Another brief clearance on 24th ahead of a developing low-pressure system with scattered showers and isolated thunderstorms on 25th, possibly tending to areas of rain. Chance of end of month low but more likely early October. Frost risk moderate 24th and slight only from 28th-30th. Rainfall generally 12-18mm with isolated falls to 20-22mm over parts of the far eastern border. Totals decreasing over towards the coastline as low as 8-12mm with most of this coming from the systems after.

Western VIC

A trough of low pressure will extend from the west later on 20th causing NE/N winds to freshen ahead of it. Patchy light rain will develop during the evening before thickening overnight with a possible isolated thunderstorm towards the northern Wimmera and Mallee. Rainfall much lighter and patchier towards the south. Areas of rain about the Mallee and Wimmera will gradually ease later in the evening and mostly clearing out in early hours of 21st though weak wrap around band may keep patchy light rain or drizzly areas lingering into the afternoon further south. Fresh to strong SW winds behind the trough. Weak front 23rd with a band of scattered light showers but not a lot in it. Another brief clearance on 24th ahead of a developing low-pressure system with areas of rain and possible isolated thunderstorms. Chance of end of month low but more likely early October. Frost risk moderate 24th and slight only from 28th-30th. Rainfall generally 15-25mm with isolated falls to 30-40mm over parts of the Eastern Mallee and the Wimmera, chiefly about the Grampians. Totals decreasing over towards the Southern coastline nearing 12-15mm.

Finals Notes:

• Many holes in this first system due to convective nature and lack of grunt to the south of the low leading to patchier falls towards southern Adelaide and the SE Districts, chiefly Lower SE.
• Regular systems to still be a feature in a mobile pattern but much heavier totals for eastern states as systems grab extras moisture from Pacific as trades stay strong
• October looking same as previous outlook with average to above average rainfall and a higher thunderstorm risk
• November much of the same with a greater thunderstorm risk again
• December may begin to taper off, but the eastern states will be extremely wet indeed causing slow onset flooding to be widespread and head downstream (although it has already started)

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.