May update 2024 SA
May update 2024 SA
*Forecast prepared between May 18th-21st 2024
Review:
G’Day everyone, we have gone past Mid-May and without change in the current pattern over the state, which is no doubt a concern to many of you. We’ll take a look back quickly at the last couple of weeks and also discuss the April pressure anomalies which have now been uploaded.
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| Figure 1: April 2024 500hPa (18,500ft) pressure anomalies – source NOAA |
Figure 1 above tells a very grave tale about the level of blocking that has been going on showing in red the areas of higher-than-normal pressure for April. The blue areas represent lower than normal pressure. We can see here that the stronger Longwave near New Zealand has caused quite a roadblock upstream with the large red high-pressure area south of about WA. However, there is a slightly larger high-pressure area below Africa and an even bigger one SW of South America. This pattern has been very persistent and has clearly strengthened since the March anomalies from the last outlook 2 weeks ago and has been the main culprit in why we are so dry. The pattern is not fluid at all and has only got stronger from March to May. I will explain further about what we need to see for this to break down in the next section.
So in short, not a lot to review in terms of actual weather, there have been some frosts when the high-pressure centre has been near above our state with multiple days of 1030-1034hPa being recorded. Notable frost nights have been the 7th, 9th, 18th & 19th where on the latter, –2.1C was recorded at Snowtown and –2C at Clare. Overall, though, it would have to be close to the most persistent high pressure I have seen, and it would be worth looking at average values for Adelaide to see where this ranks. Remember the image above is for April with the May image likely even stronger but a little further east as the whole pattern inches eastwards at a snail's pace.
The Mid-month uptick in cold fronts approaching the west coast of the mainland into SW WA did occur however that is as far as it got with all activity diving SE afterwards due to the blocking ahead. Some good totals were recorded in more coastal parts of WA however the inland struggled to see much benefit still. A tornado ripped through Bunbury on the 10th causing quite a bit of damage and this is normal for May in SW WA which is when cold fronts and rainfall normally or traditionally increases however apart from this front and a couple of weaker ones, nothing further has occurred which of course completely ruins any potential into SA and VIC if WA is only just getting smaller and less frequent impacts.
The last couple of days has seen a new strong high move in and strengthen in the Bight which has driven some colder air up from the south bringing with it some light shower activity, chiefly to the southern coasts and ranges but also running up the ranges and just behind with 10mm recorded at Monarto, 9mm at Macclesfield, and 8mm at Mt.Lofty. Numerous 3-7mm totals were also recorded into the Lower North which whilst still nowhere near season breaking, may have provided some slight relief just for the time being for those of you who have already seeded but really it isn’t significant at all. I cannot remember it being this brown this late since probably 2005, a year that rapidly changed fortunes from June 10th.
Meteorological Discussion:
Now onto current proceedings and to no surprise I am sure, the current pattern continues to show a well-entrenched block with the Longwave Trough stuck east of New Zealand, strong high pressure over mainland Australia (together with upper ridging) with the next Longwave Trough still well west of WA. There has been a shift east overall in where the sinking air is and high pressure, but of course given how wide high-pressure systems are, that still means dry weather.
Cold and frosty nights have occurred and will continue to occur on many occasions until the end of the month, but especially so on the morning of the 22nd and 23rd but at least isolated patches in most districts inland and away from the coast right up till the 27th. Over in VIC however that threat is a little greater and also begins around the 23rd or 24th and extends through till about the 27th with the slow-moving high pressure dominating.
The remainder of the month will be dominated by the current high pressure which is currently centred almost directly above the Mallee. This will slowly move eastwards and be re-enforced by another weaker high bubbling across the Bight before the combination of the two moves over NSW and eventually into the Tasman Sea, all while extending a firm ridge back NW across the mainland. This ridge will consistently deflect any shortwave troughs and cold fronts that will rotate around the next Longwave to the west and drive them SE below the mainland. Perhaps one may sneak through, which looks like around the 30th at this stage however relatively weak.
Now with this high pressure finally sitting in the Tasman it is at least a sign the block is moving on but as mentioned before it will be very slow process taking multiple rounds of systems to wear it down and bring proper rainfall in – long and frustrating and potentially on the cusp of too late for some. The key word (or phrase if you were) I want to use however for this update is Northwest flow. Finally, there are signs that it will begin to overspread the western half of the country, with a better signal across WA than SA and VIC however getting that NW flow aloft is critical to bringing Indian Ocean moisture into the mainland through the mid and upper levels which is where a lot of our rainbands form. The next step will be getting the triggers of those shortwaves rotating around the broader longwave to increase the jetstream speeds and squeeze that moisture out from dynamic lift. That all still needs to shift eastwards but at last I do see those signs, some signs which I could not see earlier this month but was hopeful by June these would present as these blocks rarely last 3 entire months like this one has. Often strongly surpressed atmosphere's rebound vigourously the other way once properly released but as highlighted in the review, this goes right around the globe, not just Australia. A broadscale expansion of the polar vortex will greatly assist in this and that is expected through June, especially later in the month which is my target time for colder air to have a much easier path northwards with a negative SAM phase.
So to summarise, I expect largely dry conditions for the remainder of May, though perhaps a weak front on the 30th or thereabouts. The blocking high pressure should be sitting roughly across eastern NSW to the Tasman by the end of the month with the next longwave getting closer to WA but still sitting west. The high being here at least will allow low-level moisture to push into the mainland from the NE and N. For meaningful rainfall, it is still going to be at least 1/3rd of the way through June as the pattern wears down with further follow ups throughout the remainder of the month. If signals change in terms of a specific breaking rain then I will be sure to let you know in advance.
The signals ahead show a drier July with renewed high pressure but that eases back in August and September with an increased rainfall signal as we move through Spring as the influence of a La Nina and an attempted negative IOD assist in that rainfall.
Climate Drivers:
The Pacific has continued to move towards a La Nina with further surfacing of colder water from the depths. There is still however much colder water again yet to surface which is up to 5-6C colder than normal and this is located at around 120-130m in depth.(see Figure 2)
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| Figure 2: Pacific Sub-surface temperature anomalies showing majority of cold water still yet to surface – source BoM http://www.bom.gov.au/climate/enso/#tabs=Pacific-Ocean&pacific=Sea-sub%E2%80%93surface |
The Humboldt Current, which is a big driver in cold water upwelling off Peru is starting to also get going to further assist in cooling that side of the Pacific. I will stress despite these indicators developing towards a La Nina and already showing up at the surface as per figure 3, we still have a long way to go both in terms of numbers, time and actual real-world influence. The main influences from the La Nina will be spring and summer as per normal, although the constant trade winds in the last few months that have drenched the eastern seaboard are a good example of more immediate impacts on a smaller scale. The effect on more areas of Australia including SA will come through the latter third of the year.
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| Figure 3: Current SST anomalies – source Tropical Tidbits https://www.tropicaltidbits.com/analysis/ocean/cdas-sflux_ssta_global_1.png |
Now to the Indian which has been the main talking point in the last few weeks with the constant positive values of the IOD sustaining for 7 consecutive weeks until last week of course when it dipped just below Positive thresholds thanks to some cooling in the Western Indian. Now I’ve said this will be a ‘false’ positive IOD if it is declared due to the reasons mentioned in the May outlook but just to re-cap, the early development of these positive values is not in the expected window for a Positive IOD which is more like August to October. Furthermore, values have been used since March to determine the Positive Index when we still had monsoonal influence! This cooled the waters due to the overturning of the surface and cold-water upwelling. Now the large high-pressure systems we have seen over Australia through especially April and May have caused strong trade windows to blow offshore, further contributing to some cold-water upwelling. As soon as these highs weaken then I expect that upwelling to reduce.
We still could see a Positive declared and then a negative later in the season which would be pretty left-field. It will be coming from a long way back to get to negative territory but I expect us to still be pushing that way come spring time as the warmer water feeding into the Indian from the Pacific as well as the broadscale warm Indian and re-enforcement across the Indonesian region from a La Nina formation to assist in that – it will be a fascinating battle. I maintain that the worst dry weather is right now with such significant blocking and sinking air unlikely to repeat for the duration we have seen. I do expect more big high pressure in July which needs to be watched but more of a standard winter episode at this stage.
Moving onto the SAM and last update I couldn’t believe the strong negative forecast and that certainly proved to be mis leading with strong high pressure dominating over the last 2 weeks with no fronts or westerly belt to be seen. Values have been strongly positive and will remain so until the end of the month however the good news is we should see a return to neutral conditions by early
June before moderate to even strongly negative values take over through mid-month as we see the longwave trough finally move into our longitude.
Now the MJO has weakened right off after the stronger push into phase 7 and 8 over the last fortnight. We see it lingering back in the Eastern Indian and towards Australia but with a very weak signal overall and given the time of the year now, the influence will continue to wane but keep an eye out for enhancements to tropical convection ahead of the rising and approaching Longwave Trough into June and they may line up.
Districts:
Eyre Peninsula
Strong high pressure dominating the weather across the district. Cold to very cold mornings with slight frost on the 22nd and 23rd chiefly. A lesser risk but still presenting in patches each morning till 27th. Dominant high pressure moving eastwards into NSW and Tasman from 28th until end of month with winds turning NE to N and temperatures rising into the mid 20’s, possibly higher towards the West Coast. Weakening front around 30th with a broken band of showers, isolated towards the north. Rainfall for period 1-4mm in that band, chiefly in the south.
Upper North
Strong high pressure dominating the weather across the district. Cold to very cold mornings with slight to moderate frost on the 22nd and 23rd chiefly. A lesser risk but still presenting in patches each morning till 27th. Dominant high pressure moving eastwards into NSW and Tasman from 28th until end of month with winds turning NE to N and temperatures rising into the mid 20’s. Weakening front around 30th with a broken band of showers, a little more isolated towards the Flinders. Rainfall for period 1-3mm in that band, chiefly in the south.
Lower North
Strong high pressure dominating the weather across the district. Cold to very cold mornings with slight to moderate frost on the 22nd and 23rd chiefly. A lesser risk but still presenting in patches each morning till 27th. Dominant high pressure moving eastwards into NSW and Tasman from 28th until end of month with winds turning NE to N and temperatures rising into the low to mid 20’s. Weakening front around 30th with a broken band of showers, though a little more consistent in the south. Rainfall for period 2-4mm in that band, chiefly in the south.
Yorke Peninsula
Strong high pressure dominating the weather across the district. Cold to very cold mornings, more so in the far north of the district with slight frost on the 22nd and 23rd chiefly. A lesser risk but still presenting in patches each morning till 27th, again chiefly towards the northern regions of the Peninsula but not totally confined to there. Dominant high pressure moving eastwards into NSW and Tasman from 28th until end of month with winds turning NE to N and temperatures rising into the low to mid 20’s. Weakening front around 30th with a broken band of showers, though a little more consistent with southern extent. Rainfall for period 2-5mm in that band, chiefly towards the foot.
Kangaroo Island
Strong high pressure dominating the weather across the island. Cold mornings with slight frost on the 22nd and 23rd chiefly towards the centre of the island. Dominant high pressure moving eastwards into NSW and Tasman from 28th until end of month with winds turning NE to N and temperatures rising to just over 20C. Weakening front around 30th with a broken band of showers and cooler conditions. Rainfall for period 2-6mm in that band.
Adelaide Plains
Strong high pressure dominating the weather across the district. Cold to very cold mornings, more so about the outer Northern Plains and suburbs with slight frost on the 22nd and 23rd chiefly. A lesser risk but still presenting in patches each morning till 27th, again chiefly towards the outer northern plains. Dominant high pressure moving eastwards into NSW and Tasman from 28th until end of month with winds turning NE to N and temperatures rising into the low to mid 20’s, especially north of about Parafield. Weakening front around 30th with a broken band of showers, though a little more consistent with southern extent. Rainfall for period 2-5mm in that band, chiefly about the foothills and moreso southern suburbs.
Mt.Lofty Ranges
Strong high pressure dominating the weather across the district. Cold to very cold mornings, more so about low-lying valleys and the Barossa with slight to moderate frost on the 22nd and 23rd chiefly. A lesser risk but still presenting in patches each morning till 27th, again chiefly again in low lying valleys and the Barossa. Dominant high pressure moving eastwards into NSW and Tasman from 28th until end of month with winds turning NE to N and temperatures rising into the low 20’s, especially towards the northern ranges generally. Weakening front around 30th with a broken band of showers, though a little more consistent with southern extent. Rainfall for period 2-7mm in that band, especially about known wet spots and southern coasts and ranges.
Riverland/Mallee
Strong high pressure dominating the weather across the district. Cold to very cold mornings with slight to moderate frost on the 22nd and 23rd chiefly. A lesser risk but still presenting in patches each morning till 28th, especially with southern extent down through the Mallee towards the Upper SE border. Dominant high pressure moving eastwards into NSW and Tasman from 28th until end of month with winds turning NE to N and temperatures rising into the mid 20’s. Weakening front around 30th with a weakening band of isolated showers, a little more consistent however with southern extent towards the southern Mallee. Rainfall for period 1-3mm in that band, chiefly in the south of the Mallee.
Upper SE
Strong high pressure dominating the weather across the district. Cold to very cold mornings with slight to moderate frost on the 22nd and 23rd chiefly. A lesser risk but still presenting significantly enough each morning till 28th, especially with southern and eastern extent down through towards the Lower SE border. Dominant high pressure moving eastwards into NSW and Tasman from 28th until end of month with winds turning NE to N and temperatures rising into the low 20’s. Weakening front around 30th with a broken band of showers. Rainfall for period 3-8mm in that band, chiefly in the south.
Lower SE
Strong high pressure dominating the weather across the district. Cold to very cold mornings with slight to moderate frost on the 22nd and 23rd chiefly. A slightly lesser risk but still presenting significantly enough each morning till 28th. Dominant high pressure moving eastwards into NSW and Tasman from 28th until end of month with winds turning NE to N and temperatures rising into the low 20’s. Weakening front around 30th with a broken band of showers. A follow up cold front potentially clipping coast on 31st also to keep a moist onshore flow briefly continuing. Rainfall for period 4-10mm in that band and with the potential secondary front albeit mostly coastal and with increasing southern extent.
Western VIC
Strong high pressure dominating the weather across the district. Cold to very cold mornings with slight to moderate frost on the 22nd and 23rd chiefly. A slightly lesser risk but still presenting significantly enough each morning till 28th, especially with southern extent from the 25th to 28th about the SW District. Dominant high pressure moving eastwards into NSW and Tasman from 28th until end of month with winds turning NE to N and temperatures rising into the low to mid 20’s about the Mallee near 20C towards the SW District. and Weakening front around 30th to 31st with a broken band of showers. A follow up cold front potentially clipping coast later on 31st also to keep a moist onshore flow briefly continuing. Rainfall for period 3-8mm over the southern portions of the SW District but decreasing with northern extent with 2-4mm about the Wimmera and then less than 1-2mm over the Mallee, if any at all in some locations further N and NE.
Final Notes
- Dominant high pressure controlling our weather for remainder of May with cold frosty nights and mild days
- Weak front spinning off broad longwave stuck well west of WA on about 30th with some shower activity but not a lot
- Multiple shortwaves sliding below mainland from longwave to west of WA as we enter June with increasing NW flow aloft and thus putting a squeeze on the jetstream where some cloudbands aloft may begin to form
- Proper break in season likely from 1/3rd of way into June where much better pattern presents after block breaks down, but it will take a while still
- Colder air outbreaks most likely later June
- Better June may be replaced again by drier July as high-pressure resumes
- Increased rainfall potential from August and moreso September to November as attempt at both negative IOD and La Nina increases moisture
- Positive IOD earlier in season may still become official but essentially will be a ‘false’ positive and not have the same impacts as a classical one such as 1994, 2006, 2019, 2023.
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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