Quick Update Opening rains and cold front May 30-31st 2024

 Quick Update Opening rains/cold front May 30-31st 2024

G’Day one and all, hope you are doing well. It is finally time for that opening rain of the season, so I thought I’d pop out a quick little update on the expected progression of the system(s).

Meteorological Discussion

To the charts and currently we see a cloudband developing across WA bringing with it some moisture sourced from the Central Indian Ocean, the area I noted from the last update as we finally have a longwave trough with a high enough amplitude to grab some of it. A cold front is marked to the rear of the cloudband extending out of a deepening surface low south of the Bight. This low will continue to deepen rapidly in response to the strong forcing of the broad upper trough behind it. A secondary trough associated with the core of the cold air/cold pool has been slamming SW WA overnight with strong cold air thunderstorms and copious amounts of hail. The warm waters off the coast of WA have assisted in adding more fuel to this system with some good falls recorded across the SW of WA with some useful totals also penetrating the Wheatbelt. The break here is more pronounced and will continue to be so compared to further east.

Figure 1: Precipitable Waters for 30/05/24 @830CST from ECMWF showing moisture feeding in from the NW/N

Now for SA, this cloudband will continue to develop into a general rainband through today and overnight tonight with rain areas reaching the West Coast of Eyre Peninsula by about midnight tonight and into the Adelaide/Central areas just before sunrise. Ahead of this though today on the 29th we will see Cirrostratus thicken aloft, N winds freshen and raised dust develop across most districts thanks to the dry conditions and recently ploughed fields. Steady light rain of 4-5hours duration for most districts will be the main feature before the rainband contracts and clears eastwards during the 30th likely leaving sunny but cooler conditions in its wake with a fresh NW to W flow. The band will encounter increasing moisture as it gets closer to NSW and VIC so there is a chance for slightly better falls right near and especially so over the border. For the most part, 4-8mm is the expectation from this rainband, however some higher totals are likely closer to the ranges and near the eastern border. We could see a few scattered 10-12mm totals potentially in these locations and perhaps a lucky isolated 15mm fall. I should make a note there will be some lee shadowing giving the angle of the cloudband so some locations across the backside of the ranges and western half of the Mallee may see only 2-4mm from the rainband.

Beyond this we have the colder air with a trough feature behind and a bulk of that will spin around and move below the state however the top half will push across mostly the Southern Agricultural areas, reaching the Southern/Western Eyre Peninsula coastline later in the afternoon on the 30th with a band of showers which will likely peter out with eastern extent. Colder air behind with scattered showers will then move in across coastal districts with fresh to strong winds about exposed coastlines especially as the low passes to the south.

Now an interesting feature of this system will be the pinching off of a small shortwave on the 31st causing the mid-levels to compress and convergence broadly to increase in the low levels with some extra lift. A renewed band of showers, tending to rain will thicken up during the day on the 31st and move eastwards from the West Coast right to the Eastern border, primarily north of about Adelaide. This will bring a little extra boost in rainfall, in particular for the greater Mid North but it will be fairly narrow. An extra 5-10mm from this is likely generally speaking but areas south of about Adelaide and certainly the SE Districts will miss out on this secondary feature, instead getting just general scattered showers in the colder air.

By June 1st, much of this will have cleared as a ridge rapidly snaps in behind which could also lead to a frost. The issue with this little shortwave pinching off for the secondary little area of rain/showers is it forms into a broad upper low and this will likely induce an ECL off NSW. That is then going to block the next ridge/high pressure system right over us and thus will lead to 3 cold and frosty mornings bringing the cold air to the surface that moved over us aloft a couple of days prior. As mentioned just prior I expect a slight frost risk on the 1st across most prone districts, however that increases to moderate on both the 2nd and 3rd as the ridge intensifies briefly.

The other consequence of the ECL forming is the next front in the sequence will now have no choice but to pinch off another low and ride up and over us to the north.

Rainfall

The following rainfall totals are to about 9am on the 1st

NW Pastoral:

Generally 4-8mm from the rainband, isolated 10-15mm in the far NE.

NE Pastoral:

Generally 8-15mm, however 10-20mm more likely in the NE corner with isolated 25mm totals possible.

Eyre Peninsula:

Generally 10-15mm with isolated 20mm totals.

Upper North:

Generally 8-15mm, isolated 20mm possible near elevated terrain. Lower pockets of 5-8mm still possible in lee of ranges.

Lower North:

Generally 10-20mm, isolated 25mm possible near elevated terrain. Local pockets down to 7-10mm in lee of ranges.

Yorke Peninsula:

Generally 10-15mm, with isolated 15-18mm falls more likely with northern extent. Local pockets down to 7-10mm near the foot.

Kangaroo Island:

Generally 6-12mm, isolated 15mm towards the Western end of the island. Local pockets of 4-6mm in the far east potentially.

Adelaide Plains:

Generally 12-18mm, isolated totals up to 22mm, most likely closer to the foothills. Local pockets and lower totals towards the coast of 8-12mm.

Mt.Lofty Ranges:

General 15-25mm, isolated 30mm falls possible about known wet spots and higher peaks. Totals decreasing markedly towards lee of ranges down to 10-15mm and maybe some unlucky spots as low as 5mm.

Riverland:

Generally 5-10mm over the western half increasing to 8-15mm over the eastern half. Isolated 20mm falls near the border. Locally lower holes in the SW-W of 3-5mm.

Mallee:

Generally 4-8mm however isolated totals towards the border around 8-15mm. Locally lower holes of 3-5mm closer to the western Mallee and near lee of ranges.

Upper SE:

Generally 6-10mm, isolated 12-15mm closer to the VIC border. Locally lower totals in the SW possibly of 4-5mm.

Lower SE:

Generally 6-12mm, isolated 15mm totals possible right near the southern/SW coastline and in the far NE corner. Locally lower totals towards the N/NW of 4-6mm.

Western VIC:

Generally 15-25mm about the Mallee and Wimmera and 8-15mm about the SW District. Totals increasing with eastern extent with 30mm falls possible near the eastern portions of all 3 districts but especially the Mallee. Totals decreasing towards the SA border where 8-15mm is more likely.

Central VIC

Generally good rainfall, especially over the Central Northern portions of the state and about the NW/N slopes of the ranges. Totals of 20-30mm will be common, scattered 35mm and isolated 40mm not without as chance. South of about Ballarat however, they do drop away with more general 10-20mm likely and even holes down to 5mm closer to the coastline west of Geelong. An excellent breaking rainfall in general for Central VIC, and also into NE VIC and a swath of Central NSW with 20-30mm totals. Rainfall into the Alpine area of VIC is likely to ramp right up to 50mm or so on those NW slopes and higher peaks with the orographic ascent.

So, in summary a patchy break for SA, not a complete one and overall not the greatest system with some winners but unfortunately still plenty of losers, chiefly immediately east of the ranges and the SE Districts. Much better with eastern extent through VIC and NSW with a good soaking rain due to extra Pacific moisture residing 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.