Special Update SA Strong trough & cold front with severe thunderstorms June 6-7th 2023
Special Update SA Strong trough & cold front with severe thunderstorms June 6-7th 2023
G’Day one and all, hope you are keeping well on this beautiful sunny and mild Tuesday, a marked difference from what will be occurring this time tomorrow in some areas. Thought I’d whip this one out today to refine some thoughts on the system that is due. Whilst the broad gist of it has not changed, the position, timing and rainfall has, and I will explain all in the next section
Meteorological Discussion:
In short, a large powerful upper trough is advancing eastwards spun off from an active node of the Longwave Trough south of WA. Aloft this will cause the polar and subtropical jetstreams to meet over SA generating what is termed a ‘jet streak’ or ‘speed max’ over the interior where the strongest core winds of the jetstream is located. The squeeze of the upper trough pushing NE hard whilst over Cape York Peninsula we have a large equatorial upper-level ridge pushing back the other way. (Please see Figure 1 for a visual of what I mean here) The tight gradient between the two is simply accelerating the flow aloft and when this happens on the leading edge of an upper trough of this size we get severe weather, almost without fail. The current setup and scenario is a strong one, not the most dynamic or powerful I have seen by a longshot, but certainly would rate in the top 5 over the course of an average year and most definitely in terms of winter setups where the current pattern is resembling a mid-spring setup.
Ahead of this upper trough and on the SE side of that jet streak over the interior of SA is where the strongest upper divergence will occur and this is critical in giving forced ascent without the need for any heating or sunlight and is termed dynamic lift and will be responsible for a rapid development of showers and thunderstorms later this evening and overnight into Wednesday 7th morning. Initially we will see mid-level Altocumulus fields form in lines across the state and then with de-stabilisation and lift, mid-level showers and thunderstorms will erupt from these and rapidly increase in coverage and also intensity. I expect only isolated activity at first, chiefly over the Pastoral areas but this will become scattered quickly late evening and race down towards the Eyre Peninsula and gulf areas. More showers and thunderstorms may form further NE and E across the Upper North/NE Pastoral and Riverland well ahead of the other activity and race off SE as often lee mountain waves in strong NW-SE flow can generate inertial instability and kick off storms over the Riverland and Mallee. In any case, a general increase and expansion of these showers and thunderstorms should occur after about midnight and they should be moving SE very quickly, steered by strong mid-level winds. A low-level jetstream (LLJ) will also intensify overnight through the levels below about 5-6000ft and this will provide extra convergence and forcing down below to give further assistance to the already prominent lift ongoing aloft and thus with these two combined, severe thunderstorms are likely overnight, especially north and east of about Adelaide with the main area of interest being the Southern NE Pastoral, Riverland and Mallee/eastern border areas.
But all districts east of about Ceduna will have the potential for severe thunderstorms, it is just a lot less likely west of about Eastern Eyre Peninsula with more isolated activity. The Riverland for example may see repeat waves of thunderstorms, some severe move down until the system is over. Now as we get closer to sunrise on Wed 7th, there is a fairly strong chance that some of these showers and thunderstorms will have amalgamated to areas of rain or blobs of decaying thunderstorm activity/messy showers. This is most likely over the Riverland, Mallee and especially Upper and Lower SE. This first lot of activity, primarily nocturnally driven will begin to clear out of the eastern border of the state by late morning and generally be a band of rain and embedded thunderstorms as it passes over VIC with a much more consistent soak of rain as opposed to the wild hit and miss storm lotto we will see here (except the eastern border areas). The severe thunderstorms from this first band will most likely feature damaging winds as they will be a little higher based, but intense rainfall and large hail is also quite possible, the latter still may occur in these higher based storms with potential up to 2-3cm stones or so. Now further west a cold front at the surface will be advancing eastwards and giving further lift as it does so. With a combination of solar heating, frontal lift and still very sufficient upper-level lift we will see renewed showers and thunderstorms develop on and ahead of the cold front but with a specific focus near it. Any thunderstorms developing on or just ahead of the cold front will have the potential to be severe east of the ranges and also across the Upper and Lower SE with damaging winds, large hail, and very heavy rainfall, with a greater flash flood threat with these thunderstorms due to slower speeds and lower bases which means a high-water loading content. On a local scale, these thunderstorms will have the potential to veer low level winds to enhance their inflow and thus increasing low level helicity a little which will make for some rotating supercell like structures. There is a low risk of a tornado or two from the strongest storms of the day that can maintain clear air ahead of them with the very low cloud bases being of assistance to allowing this (less physical distance for rotating air to overcome). The hail size again is most likely to be in the 1-3cm area, but I would not rule out 4cm if a strong core can maintain. These severe thunderstorms will then cross the border into VIC and likely pose a similar level threat from later in the afternoon. Behind the front however, colder air will begin to slam into the Eyre Peninsula from mid arvo and see showers and isolated thunderstorms occur over mainly the western and southern coasts of Lower Eyre Peninsula and the West Coast District. This colder air shower and thunderstorm activity will then spread to Lower YP and KI and also the bottom of the Fleurieu by late evening and into the early hours of the 8th. These also have the potential to be severe, however nowhere near the level of the earlier activity with damaging wind the main threat but small hail is a threat in the stronger cells. Now I should point out that this system will be highly convective and as such should not be seen so much as a rain soaking event. Localised heavy falls will occur from thunderstorms, but these are more likely over the Riverland, Mallee and Upper and Lower SE and more isolated parts of the Upper and Lower North, Mt.Lofty Ranges and possibly the Adelaide Plains. I think whilst the Yorke Peninsula may see some heavy precipitation overnight with short intense bursts, heavy falls (accumulations) and less likely. But the main focus for the heaviest will be the eastern border areas (approximately within 50km-80km of the border give or take). Similarly, there will be holes and gaps like Swiss cheese across many districts but most definitely over the Eyre Peninsula with a slow grading to less holes over Yorke Peninsula, then less again over the Upper and Lower North and so on as we progress east. This can all be attributed to the style of the storm formation and their progression from isolated single cells to clusters to decayed storms and broader rain areas (miss less with the latter).
Rainfall:
Now the all-important rainfall and certainly there will be some good falls across the state, in particular the NW and W portions where moisture is the most plentiful but also the Mt.Lofty Ranges where orographic lift of both the rainband and aggressive convective showers, some in streams will result in isolated heavy falls.
The following is from the start of the event to about 9am Thursday 8th
NW Pastoral:
Minimal rainfall but perhaps 3-5mm closer to the east of the district and isolated 10mm dumps from thunderstorms.
NE Pastoral:
Generally minimal rainfall further north and west however 10-20mm likely in the Southern and SE portion of the district and at least 5-10mm east of about the Flinders with isolated 25mm falls from thunderstorms.
Eyre Peninsula:
Sparse and light totals for the most part with 1-4mm through northern to Central and parts of Eastern EP. Totals increasing with southern extent but chiefly over the western and southern coasts where 8-15mm is likely with isolated 15-25mm falls, mainly from the colder air behind the front. Expect better falls inland with systems to come through month.
Upper North:
General 5-10mm with scattered falls to 15mm. Isolated falls to 20-25mm in heavier thunderstorm activity and any training precipitation. Locally isolated drier holes of 3-5mm, most likely in the east.
Lower North:
General 8-15mm falls, with scattered 20mm totals. Isolated heavier falls to 30-35mm in heavier thunderstorm activity and training precipitation. Locally isolated drier holes of 3-8mm
Yorke Peninsula:
General 4-8mm falls, with scattered 10mm totals. Isolated heavier falls to 15-20mm possible if any heavier cores occur early enough, most likely in the far N. Locally isolated to scattered drier holes of 2-3mm, more likely about Central YP.
Kangaroo Island:
General 10-20mm falls. Isolated heavier falls to 30mm possible about the western end. Totals decreasing over far east down to around 7-10mm or so. Most precipitation occuring from colder air at night.
Adelaide Plains:
General 12-18mm falls, with scattered 20-25mm totals. Isolated heavier falls to 30mm with thunderstorms or repeat training of precipitation. Totals decreasing over far southern and western suburbs down to around 8-12mm or so.
Mt.Lofty Ranges:
General 20-30mm falls, with scattered 35mm totals. Isolated heavier falls to 40-45mm about known wet spots and in any stronger thunderstorms or training precipitation. Totals decreasing in immediate rain shadow off SE ranges down to 5-15mm and also parts of northern ranges down to 10-20mm
Riverland:
General 10-15mm falls with scattered totals to 20-25mm. Isolated totals to 40-45mm about the eastern border and in any intense showers and thunderstorms/training precipitation near the eastern border again. Totals decreasing towards the far west/SW of the district around 7-10mm.
Mallee:
General 10-18mm falls with scattered totals to 25-30mm. Isolated totals to 40-50mm about the eastern border and in any more intense showers and thunderstorms/training precipitation near the eastern border again. Totals decreasing towards the far west/SW of the district around 5-10mm.
Upper SE:
General 15-25mm falls with scattered 30mm totals. Isolated 40-50mm totals in more intense showers and thunderstorms and from general longer accumulations near the eastern border. Totals decreasing towards the Coorong coastline and far NW of the district down to 10-15mm
Lower SE:
General 20-30mm falls with scattered 35-40mm totals. Isolated 45-50mm totals in more intense showers and thunderstorms and from general longer accumulations near the eastern border. Totals decreasing slightly in the far SW of the district down to 15-20mm.
Western VIC:
General 20-30mm falls with scattered 40mm totals. Isolated 50-60mm totals in more intense showers and thunderstorms and from general longer accumulations. Totals decreasing towards the far southern/SE coast of the SW District but still around 15-20mm. Flash flooding and general overland flooding from thunderstorms most likely in Western VIC due to thunderstorm maturity, coverage and overall moisture content of the system being higher here.
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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