October outlook 2023 SA
October outlook 2023 SA
*Forecast prepared between September 30th-October 2nd, 2023
Review:
G’Day everyone, I trust you are well! October is here and things are certainly warming up rapidly as the sun makes a move south once again. There is much to discuss in the next section but first we will recap September and see how the outlook performed and touch on the notable events.
The main rainfall event of the month was a strong cold front and associated rainband before a dumbelling cut off low with cold air aloft barrelling up from the SW which did bring a dusting of snow to Mt Bryan, Hallett and some very high portions of the Greater Mid North. The rainband on the cold front from late 6th and through part of the 7th delivered useful totals and without it, the month would have been a horror story. Falls ranged generally in the order of 10-20mm although some pockets of 20-25mm were recorded through the Mt.Lofty Ranges. Sadly, the rain shadow was strong and falls diminished east of the ranges somewhwat. Numerous wind gusts in the 70-80km/hr range were recorded from the back side of the dumbelling low with a maximum gust of 107km/hr at Neptune Island although this is nothing unusual for this location.
Area of slight to moderate frost were widespread on the morning of the 9th with numerous negative temperatures recorded and although this event didn’t quite come on as cold at it initially appeared (very thankful), some crop damage did occur in many locations due to the timing and especially longevity of the cold temperatures with a very rapid drop after sunset on the 8th setting up for quite some time near or just below 0C in the pre-dawn hours of the 9th.
The second half of the month saw a lot of heat move in very quickly which was expected with upper ridging starting to dominate allowing hot air to penetrate further SE ahead of frontal and trough systems extending from the west, many of which were devoid of moisture providing only mere wind changes and cooler air behind which is a hall mark of Positive IOD springs here in SA.
The main hot days were the 17th where many temperatures reached the high 30’s across the West Coast (Ceduna a monthly record with 39.8C) with low to mid 30’s across many other agricultural
areas. The 24th and especially 29th again saw many areas move into the low to mid 30’s with high 30’s across the West Coast areas on the 29th with Nullarbor reaching 40C.
Rainfall as a general whole for the second half of the month was pitiful as a pattern of strong high-pressure systems dominated the state and areas east with near stationary high-pressure systems over the Tasman Sea directing that warm NE to N airstream across the state.
Now lets take a gander at the rainfall charts to have a better idea of how the September outlook performed:
Figure 1: September 2023 rainfall deciles – source BoM http://www.bom.gov.au/climate/
Figure 3: September 2023 rainfall anomalies – source BoM http://www.bom.gov.au/climate/
In short, there is nothing pretty about those charts with a clear below to well below average month. There is even a small portion of the SW part of Kangaroo Island with its lowest on record for September which goes to show the lack of frontal activity and high-pressure dominance as this area is one of the more exposed to rainfall from them west and SW.
Very much below average rainfall presented across the general Eastern agricultural areas such as the Riverland, Mallee and Upper SE with another section across the NW Pastoral right into the NW corner of the state. This is including the useful rainband from the last 6th to 7th so without that, there would have been a lot more lowest on record areas across the state. A very dry month and one that has been targeted as a bad month for many months prior so at least that expectation (unfortunately) has come off as forecast. Here is the official September outlook:
“Overall, for September, I expect average to below average rainfall. Larger areas of below to very much below average rain are likely over the NW and NE Pastorals with a lack of infeeds. The broader agricultural areas will come in mostly just below average with most rain falling in the expected cold outbreak with only weaker systems likely till later in the month where some trough activity may bring small accumulations. The greatest risk for below to well below average rainfall
for the agricultural areas remains across the Eyre Peninsula and the Riverland and down into the Northern Mallee. Western VIC should grade from average towards the south to well below for the Mallee. The greatest chance for average or even slightly above if there is any is over the Fleurieu Peninsula and parts of the Mt.Lofty Ranges but this is slim. If this cold outbreak was not on the cards, then it could have been quite a horrible month indeed so at least it has dialled back a little on the extremity levels. October could well take that mantle now if the heat kicks in. Something to watch closely now. “
Across the board that performed fairly well with even the tiny slither across the Fleurieu Peninsula that got the closest to average rainfall but still fell short.
Meteorological Discussion:
The current situation is quite interesting as amongst a very dry period across much of the nation, we have a rogue and powerful system developing which will bring some significant rainfall to Central and Southern NSW and Eastern VIC, especially the Gippsland portion of Victoria where flooding is likely even with the current dry situation. The main area of rain will be too far east to affect SA however we will see some welcome and useful totals in some area, however due to the convective nature of the rainfall, these totals will be very sporadic in nature and plenty of locations will miss out on anything more than a few showers. In any case, currently we have a warm to hot and dry NE to N airstream across much of the state (very hot over the west and north) which is likely to set some new diurnal temperature records (that being from the morning minimum to the maximum temperature later in the afternoon). At the time of typing this in the early afternoon on the 2nd, we have Ceduna ranging from 6.3C to 39.2C, Nullarbor from 7.4C to 38.1C, Cummins from 3.3C to 32.3C, Wudinna 4.8C to 37.7C, Minnipa 6.9C to 36.7C, Pallamana 3C to 33.6C and Keith West 2C to 32.3C and all of these likely still rising. This hot air is advecting SE from the interior ahead of a significant upper trough and cold front.
Now due to that large and persistent Tasman high and some rising moisture levels across the Top End and the high amplitude of this upper trough, moisture is feeding down ahead of the system across the interior of SA resulting in some mid-level cloud development within a broad pre-frontal trough. As the cold front and upper trough close in, the upper-level lift will increase and in response we will see an expansion both horizontally and vertically of this mid-level cloudband causing showers and thunderstorms to develop, isolated initially but increasing through the evening of the 2nd and early hours of the 3rd. Most of this activity has just started to electrify so we should see an increase in high based (10,000ft bases approx) of mid-level showers and thunderstorms across the NW Pastoral, Eyre Peninsula through the remainder of the afternoon and early evening of the 2nd and then expanding across most remaining areas through the evening and overnight into the 3rd, especially north of a rough line from Port Neill, to Maitland to Adelaide to Keith although some activity is still likely south of this too. Thunderstorm activity could form into training NW-SE lines with the same location receiving multiple storms but due to the nature of the system, bases will remain high with no flooding risks. Damaging winds, short sharp bursts of large dropped heavy rainfall are likely as well as near ‘dry lightning’ (I kind of hate that term as it is extremely rare to get a storm without a drop of rain but more a case of trying to imply that the storm will contain very little in the way of rainfall and thus increasing the fire risk). I wouldn’t rule out a dash of hail either in strongest cells but nothing large.
The cold front will move eastwards on the 3rd bringing a cooler change across the state with some onshore showers although not much initially. The ongoing showers and thunderstorms across eastern districts will increase in coverage through the early hours, especially closer to sunrise as nocturnal cooling of cloud tops is maximised. As the strong upper trough with cold air aloft moves in from the west later on the 3rd and into the 4th we will see an increase in showers with greater instability aloft resulting in some better totals starting to develop. Scattered showers, very isolated brief thunderstorms and even some small hail are likely across the Central and Western agricultural areas (Eyre Peninsula, Yorke Peninsula) later on the 3rd before this activity translates chiefly to the Central and Eastern agricultural areas during the 4th before contracting eastwards during the 5th. Due to the development of a deep and intense surface low off the Gippsland coastline, a fresh and moist SW to southerly flow will establish across the state on the 4th causing showers to push well north and potentially form some streams up the gulfs at times should the winds convergence enough. This may enhance rainfall up the top of the Spencer gulf a little.
Now on the 5th we will see the upper trough and cold air aloft contract away with a strong new high will move in well south of the mainland but before this a weak front will slip up from the south and direct a broken band of showers, mostly for the SE Districts but also pushing north to most agricultural areas but weakening as it does so. The large and dominant high to 1040hPa will then move below the mainland directing a SE-NE flow across the state for many days until about the 10th when another trough system is likely to develop over the west of the state. Ahead of this trough, hot to very hot conditions will return, especially in the west and north of the state with temperatures nearing 40C once again. These 37-40C temperatures are likely to extend eastwards across the agricultural areas, especially the northern agricultural.
Beyond this till mid-month it is likely that a large dominant high pressure will once again move into the Bight or below the mainland with SE-E winds setting up across the state.
October as a whole is likely to come in with below average rainfall, with many areas of very much below average rainfall. The most likely area for this are the NW of the state and also Kangaroo Island, Eyre Peninsula and parts of the Lower SE. The Upper North may do a little better mostly from
this system but still fall largely short of average. The Central and Eastern Districts in general should see below average rainfall, with perhaps only the far eastern/SE corner of the NE Pastoral and some of the Riverland coming close to average with lingering moisture feeding into eastern state systems.
Climate Drivers:
The Pacific has not shown much change in the last 2 weeks at the surface however in the sub-surface across the far East, we see the warmer anomalies accumulate to a higher level at approx 100-125m in depth (see Figure 5). A traditional El Nino of significant strength greater than we currently have (weak) will see these surface so that is what needs to happen. They also need to move west a little but to bump up Nino 3.4 to warmer levels a little bit. Persistent warmer anomalies east of PNG are still unmoved and are matching Nino 3.4 which traditionally is not what we see so that gradient is not strong enough yet. Having said that, waters in general north of Australia are slightly cooler than average which is a testament to the downwelling that is present over this region of the globe and is one part of the double whammy that we are experiencing.
Part two of this double whammy mentioned above is the Indian Ocean which is currently showing a near fully fledged Positive IOD which can be clearly seen in Figure 4 with the colder water upwelling off the coast of Sumatra and Java. This is the most notable anomaly on earth at this time showing that the upwelling is in full swing. The dryness across much of Australia, but especially across SA and VIC can be largely attributed to this instead of El Nino as it is clearly having a more significant impact on our weather by strangling moisture out of that area and changing the behaviour of systems as they move into our state. I expect this to continue through October with a peak before a slow decline in November and then a greater decline in December. The current value as of September 26th was +1.45C and I still expect this to get closer to +1.6 to +1.8C making it a moderate to strong event.
The MJO is still largely irrelevant at this time of the year, but it still does have some influence and can enhance the El Nino in the Pacific. The current state at the moment is a very weak signal and it is located in phase 5 over the Maritime Continent but should move into the Pacific in the coming week and gain some strength but still overall be weak to moderate. A strong Westerly Wind Burst (WWB) to turbocharge this El Nino does not look likely and so this means that the very slow ambling nature of the El Nino will continue.
The SAM is neutral but almost positive and is expected to push positive into early October as some strong high-pressure systems move below the mainland. The reason for this location is the inland is
now heating up causing low pressure to form there from the surface, but we still have an overwhelming downwelling from aloft caused by the double whammy of El Nino and the Positive IOD. This downwelling drags very DRY air from the upper levels (including the stratosphere) and pulls it down to the mid and low levels of the atmosphere and the result is a very dry signal on any moisture chart you look at across Northern and Central Australia which is the powerful downwelling arm of the Hadley Cell. It is very easy to see this dry air on atmospheric sounding charts where the dry air is trapped above a surface or mid-level inversion. I might explain these charts in more detail as we move into severe thunderstorm season.
Figure 4: Current SST anomalies clearly showing Positive IOD off Sumatra coast – source Tropical Tidbits https://www.tropicaltidbits.com/analysis/ocean/cdas-sflux_ssta_global_1.png
Figure 5: Sub-surfce of the Pacific showing warming in east at 100-125m depth – source BoM http://www.bom.gov.au/climate/
Districts:
Eyre Peninsula
Hot to very hot on 2nd ahead of a pre-frontal trough with mid-level showers and thunderstorms extending from the NW later, chiefly across the northern half of the Peninsula. Isolated severe thunderstorms possible bringing damaging winds as well as dry lightning. Cold front due on 3rd with isolated showers ahead of strong upper trough with colder air aloft increasing showers with greater inland penetration during the afternoon and evening. Isolated cold air thunder and small hail also possible. Showers becoming isolated on 4th and largely clearing as large high moves in below Bight. Light shower or two about the tip of the Peninsula on 6th with weak front. Warm to hot conditions developing thereafter as large high dominates with SE-NE airstream. Hot to very hot conditions on 10th ahead of a trough on about 11th with mid-level showers and a cooler gusty change, with stream showers to follow, chiefly coastal. Very slight frost risk 5th. Rainfall for period generally 3-6mm.
Isolated falls of 10-15mm about the Lower Eyre Peninsula and Western coast of Eyre Peninsula. Lower totals towards the NW pastoral border of less than 2-3mm.
Upper North
Hot to very hot on 2nd ahead of a pre-frontal trough with mid-level showers and thunderstorms extending from the NW later. Isolated severe thunderstorms possible bringing damaging winds as well as dry lightning. Some hail to marble size possible in stronger thunderstorms. Cold front due on 3rd with isolated showers ahead of strong upper trough with colder air aloft increasing showers with greater inland penetration during the afternoon and evening. Possible patchy rain areas forming on leading edge, chiefly in the east. Isolated cold air thunder and small hail also possible. Showers becoming frequent on 4th with streams establishing off Spencer Gulf in the early hours with localised moderate falls possible near the Melrose/Mt.Remarkable area before becoming isolated in the afternoon and largely clearing by evening as large high moves in below Bight. Light shower or two in far south on 6th with weak front. Warm to hot conditions developing thereafter as large high dominates with SE-NE airstream. Hot conditions on 10th ahead of a trough on about 11th with mid-level showers and a cooler gusty change, with stream showers to follow, chiefly in the south. Slight frost risk 5th and again on 7th. Rainfall for period generally 8-15mm with scattered totals to 20mm. Isolated falls of 20-30mm about the Melrose and mountainous terrain near Mt.Remarkable. Lower totals towards the far S of the district down to 5-8mm.
Lower North
Hot on 2nd ahead of a pre-frontal trough with mid-level showers and thunderstorms extending from the NW later. Isolated severe thunderstorms possible bringing damaging winds as well as dry lightning. Some hail to marble sized possible in stronger thunderstorms. Cold front due on 3rd with isolated showers ahead of strong upper trough with colder air aloft increasing showers with greater inland penetration during the afternoon and evening. Possible patchy rain areas forming on leading edge, chiefly in the east or NE. Isolated cold air thunder and small hail also possible. Showers becoming frequent on 4th with streams establishing off Spencer Gulf and Gulf St. Vincent in the early hours, chiefly over the western half before becoming isolated in the afternoon and largely clearing by evening as large high moves in below Bight. Light shower or two on 6th with weak front. Warm to hot conditions developing thereafter as large high dominates with SE-NE airstream. Hot conditions on 10th are stream showers to follow. Slight frost risk 5th and again on 7th. Rainfall for period generally 7-10mm with scattered totals to 15mm. Isolated falls of 20-25mm about the western half and near elevated terrain closer to the Upper North. Lower totals towards the far SE of the district down to 5-7mm.
Yorke Peninsula
Hot on 2nd ahead of a pre-frontal trough with mid-level showers and isolated thunderstorms extending from the NW later, chiefly across the northern half of the Peninsula. Isolated severe thunderstorms possible bringing damaging winds as well as dry lightning. Cold front due on 3rd with isolated showers ahead of strong upper trough with colder air aloft increasing showers during the very late evening. Isolated cold air thunder and small hail also possible. Showers increasing early on 4th chiefly in the north before becoming isolated and largely clearing as large high moves in below Bight. Light shower or two 6th with weak front. Warm conditions developing thereafter as large high dominates with SE-NE airstream. Hot conditions on 10th ahead of a trough on about 11th with mid-level showers and a cooler gusty change, with stream showers to follow. Very slight frost risk 5th in the north. Rainfall for period generally 5-8mm. Isolated falls of 10-15mm in the far north. Lower totals towards the SE foot maybe only 4-5mm.
Kangaroo Island
Warm on 2nd ahead of a pre-frontal trough with isolated mid-level showers and thunderstorms extending from the NW later. Isolated severe thunderstorms possible bringing damaging winds as well as dry lightning. Cold front due on 3rd with scattered showers ahead of strong upper trough with colder air aloft with renewed showers during the evening. Isolated cold air thunder and small hail also possible. Showers increasing early on 4th chiefly in the north before becoming isolated and largely clearing as large high moves in below Bight. Showers re-developing from south late 5th and into 6th with weak front. Mild to warm conditions developing thereafter as large high dominates with SE-NE airstream. Warm to hot conditions on 10th ahead of a trough on about 11th with mid-level showers and a cooler gusty change, with stream showers to follow. Rainfall for period generally 6-8mm. Isolated falls of 10-12. Lower totals towards the NE coast down to 5-6mm.
Adelaide Plains
Hot on 2nd ahead of a pre-frontal trough with mid-level showers and isolated thunderstorms extending from the NW later, chiefly north of Adelaide. Isolated severe thunderstorms possible bringing damaging winds as well as dry lightning. Cold front due on 3rd with isolated showers ahead of strong upper trough with colder air aloft. Showers increasing early on 4th chiefly in the north before becoming isolated and largely clearing as large high moves in below Bight. Isolated cold air thunder and small hail also possible. Light shower or two late 5th and into 6th with weak front. Warm conditions developing thereafter as large high dominates with SE-NE airstream. Hot conditions on 10th ahead of a trough on about 11th with mid-level showers and a cooler gusty change, with stream showers to follow. Very slight frost risk 5th about outer plains. Rainfall for period generally 12-18mm. Isolated falls of 20-25mm near the foothills. Lower totals in the south closer to 7-12mm.
Mt.Lofty Ranges
Hot on 2nd ahead of a pre-frontal trough with mid-level showers and isolated thunderstorms extending from the NW later, chiefly north of about Mt.Lofty. Isolated severe thunderstorms possible bringing damaging winds as well as dry lightning. Cold front due on 3rd with isolated showers ahead of strong upper trough with colder air aloft. Showers increasing early on 4th chiefly in the north towards the Barossa before becoming isolated and largely clearing as large high moves in below Bight. Isolated cold air thunder and small hail also possible. Light showers later 5th and into 6th with weak front, more frequent about Southern coasts and ranges. Warm conditions developing thereafter as large high dominates with SE-NE airstream. Hot conditions on 10th ahead of a trough on about 11th with mid-level showers and a cooler gusty change, with stream showers to follow. Very slight frost risk 5th about outer plains. Rainfall for period generally 15-20mm. Isolated falls of 20-25mm near higher peaks and known wet spots. Lower totals in the south closer to 7-12mm and towards backs of ranges.
Riverland/Mallee
Hot to very hot on 2nd ahead of a pre-frontal trough with mid-level showers and thunderstorms extending from the NW later. Isolated severe thunderstorms possible bringing damaging winds as well as dry lightning. Showers and thunderstorms increasing in coverage after about 2am on 3rd ahead of a cold front due on 3rd with isolated showers. Some hail to marble size possible in stronger thunderstorms. Strong upper trough with colder air aloft with possible patchy rain areas forming on leading edge, chiefly about the Riverland later on 3rd. Scattered showers on 4th with Isolated cold air thunder and small hail also possible, largely clearing by evening as large high moves in below Bight. Light shower or two on 6th with weak front. Warm to hot conditions developing thereafter as large high dominates with SE-NE airstream. Hot conditions on 10th ahead of a trough on about 11th with mid-level showers and a cooler gusty change, with stream showers to follow, chiefly in the south. Slight frost risk 5th and again on 7th. Rainfall for period generally 5-10mm with scattered totals to 12mm. Isolated falls of 20mm about the Riverland and eastern border. Lower totals towards the far south of the Mallee and west of the districts down to 3-5mm.
Upper SE
Hot on 2nd ahead of a pre-frontal trough with mid-level showers and thunderstorms extending from the NW later. Isolated severe thunderstorms possible bringing damaging winds as well as dry lightning. Showers and thunderstorms increasing in coverage after about 2am on 3rd chiefly in the north and east ahead of a cold front due on 3rd with isolated showers. Some hail to marble size possible in stronger thunderstorms. Strong upper trough with colder air aloft to follow front. Scattered showers on 4th with Isolated cold air thunder and small hail also possible, largely clearing by evening as large high moves in below Bight. Light showers developing later 5th and into 6th with weak front. Warm conditions developing thereafter as large high dominates with SE-NE airstream. Hot conditions on 10th ahead of a trough on about 11th with mid-level showers and a cooler gusty change, with stream showers to follow. Slight frost risk 5th and again on 7th. Rainfall for period generally 6-12mm with scattered totals to 15mm. Isolated falls of 18-20mm near the NE corner of the district. Lower totals towards the coast down to 5-6mm.
Lower SE
Hot on 2nd ahead of a pre-frontal trough with mid-level showers and thunderstorms extending from the NW late evening. Isolated severe thunderstorms possible bringing damaging winds as well as dry lightning. Cold front due on 3rd with scattered showers. Strong upper trough with colder air aloft to follow front though most unstable air will pass north of the district. Scattered showers on 4th with isolated small hail possible, largely clearing by evening as large high moves in below Bight. Showers re-developing during 5th and into 6th with weak front. Warm conditions developing thereafter as large high dominates with SE-NE airstream. Hot conditions on 10th ahead of a trough on about 11th with mid-level showers and a cooler gusty change, with stream showers to follow, more frequent about the coast. Slight frost risk 5th and again on 7th. Rainfall for period generally 8-15mm with scattered totals to 18mm. Isolated falls of 25mm near the far southern coast. Lower totals the NW down to 7-8mm.
Western VIC
Hot on 2nd ahead of a pre-frontal trough with mid-level showers and thunderstorms extending from the NW later. Isolated severe thunderstorms possible bringing damaging winds as well as dry lightning. Showers and thunderstorms increasing in coverage with training lines from the NW after about 2-3am on 3rd chiefly about the Wimmera and Mallee ahead of a cold front due on 3rd with isolated showers. Some hail to marble size possible in stronger thunderstorms. Strong upper trough with patchy rain about the Mallee on leading edge later 3rd and early 4th with colder air aloft to follow front. Isolated to scattered showers on 4th chiefly during the afternoon and then largely clearing by evening as large high moves in below Bight. Light showers developing later 5th about the SW District before moving inland into 6th with weak front. Warm conditions developing thereafter as large high dominates with SE-NE airstream. Hot conditions on 10th ahead of a trough on about 11th with mid-level showers and a cooler gusty change, with stream showers to follow. Slight frost risk 5th and again on 7th. Rainfall for period generally 8-15mm with scattered totals to 20mm. Isolated falls of 25mm in multiple training thunderstorms, more likely towards the NE of the Wimmera or Mallee. Lower totals towards the SA border down to 6-8mm.
Final Notes
- Positive IOD at 80% strength and having a big impact on the weather nationwide, but especially over SA
- El Nino still weak with a reduced impact in comparison but together both basins are combining together to create broadscale downwelling over Australia
- Rogue upper trough for SE AUS in coming days to induce deep low off Gippsland generating heavy rain and flooding there with damaging winds. Local falls to 250mm likely
- Heating up into November with increasing showers and thunderstorms as inland Australia becomes more trough dominated with this activity fluxing SE ahead of active troughs and frontal systems, chiefly medium to high based in nature but also likely severe at times with hail in organised thunderstorms
- Hot summer still on the cards with rainfall closer to average before an increase in moisture as El Nino breaks down and warm SST’s feed that moisture across the country as we move into the break for 2024.
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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