Special Update SA Trough and upper low with showers

Special Update SA Trough and upper low with showers, rain and thunderstorms Nov 22nd-25th 2023

***Forecast prepared evening of Tues 21st and morning of 22nd.

G’Day everyone, thought I would put out that update as promised for this event, and certainly it is a complex one with some implications for those still harvesting, especially in the east of the state over locations chiefly east of the ranges. In terms of changes from the forecasts mid-month, no major ones but there is a slight westward movement of the activity as a whole thanks to the upper low pushing NW a bit more which will allow more rainfall across the agricultural areas as a whole with a higher bottom line (hinted in forecast as my gut feel told me this will be the case...always trust the gut...always!)

Meteorological Discussion:

For this update, I thought the best way is to break the event down into separate days, so it is nice and clear for all to read and understand. We are still a couple of days out from the main action, but things do kick off tomorrow further north and NE with some expected afternoon activity.

Figure 1: ECMWF Precipitable Waters showing moisture feed from NE for 6AM AEDT 24/11/23 - source Meteologix

Wednesday 22nd

A surface trough will slowly begin to move west and SW during the day and be located across the NE Pastoral District primarily with a moist and increasingly humid airmass lying to its east. Further west that strong high-pressure system that I mentioned in the previous mid-month update will continue to do its best to muscle a ridge back up into the Bight driving fresh SE winds across much of the agricultural area with stiff seabreezes and gully winds at night. Aloft, an upper low that has now begun to pinch off from the westerly belt will begin to move N to NW across the agricultural area and will likely split into two centres and be located closer to the Eyre Peninsula during the evening.

With this de-stabilisation occurring, we should see isolated afternoon and early evening showers and thunderstorms develop with convection across NE Pastoral District, Flinders and down into the Upper North, chiefly near or along the ranges. The elevated terrain towards the north and NE of the Lower North may also see some activity. The thunderstorm activity won’t be too strong although the stronger cores are likely to produce locally heavy dumps of rainfall and possibly small to medium sized hail. But widespread rainfall is not expected here on Wednesday.

Across VIC, there shouldn’t be much if any activity on Wednesday, perhaps a brief shower or rumble in the far NW corner of the state.

Thursday 23rd

The upper low will begin to strengthen and churn in position, slowing down it’s N to NW movement. With dynamical lift from aloft as well as some likely low-level convergence from the trough to the east, isolated showers and thunderstorms are likely to re-develop in the early hours of the morning to the east of the upper low. This activity will roughly be north and east of a line from Kimba to Kadina to Gawler to Renmark although much more isolated activity is still possible south of this.

As the day progresses, with solar heating once again we should see more activity develop north of this line especially across the Southern NE Pastoral down towards the Riverland and certainly over the border into NSW and NW VIC. Also, coverage should increase across the Eyre Peninsula near the head of the upper low with activity possible out to Ceduna and down to about Cummins. Towards the evening, the upper low will start to move back S/SE which will steepen the lapse rates across the area to the east of it causing an increase in shower and thunderstorm activity. Isolated severe thunderstorms are likely during the late afternoon and evening, especially across the Riverland, Flinders and Southern NE Pastoral but also potentially across the Mallee extending from over the border where activity will be scattered. Locally heavy rainfall causing flash flooding, hail (1-2cm) can be the main two hazards expected but also potentially some brief localised pockets of damaging wind from wet microburst activity.

As the evening progresses, we should see the activity begin to spread out and start to wrap under the upper low located to the west as deeper moisture begins to be pulled in from NSW and VIC. Ongoing showers and thunderstorms will tending to areas of rain and more isolated/embedded thunderstorms as they wrap under the upper low although the cold air aloft and dynamical lift will still keep things relatively unstable, even closer to Adelaide although much less than further east and NE. This period through the early hours from midnight to dawn is probably the best chance of a thunderstorm across metropolitan Adelaide and will be coming out of the NE to E which is quite rare, but I doubt anything too spectacular, maybe a few rumbles here and there. The Area of rain will push west across Yorke Peninsula and moreso Kangaroo Island where the pivot below the upper low should be roughly.

Now across VIC, isolated showers and thunderstorms will develop potentially as early as mid to late morning across the NW corner of the Mallee District, but as the day wears on, this activity will slowly get a little stronger. Through the evening, showers and thunderstorms will become scattered over the Mallee with isolated activity extending over the Wimmera also. Further east nothing much is expected as of yet due to being fairly removed from the surface trough and also the upper low.

Friday 24th

This period through the early hours from midnight to dawn is probably the best chance of a thunderstorm across metropolitan Adelaide and will be coming out of the NE to E which is quite rare, but I doubt anything too spectacular, maybe a few rumbles here and there. The upper low will continue to move SE with the areas of rain and isolated embedded thunderstorms wrapping underneath it, gradually extending south across the remaining districts like the Upper and Lower SE as dawn approaches. This will see a more soaking and steady rain area, moderate at times as well. At the same time the surface trough will also be deepening across the Riverland and Mallee with a small low forming in the trough. The rainband will also act to pre-moisten the low levels with resultant dewpoints through the morning across the SE Districts and Mallee in particular being a little higher than other districts.

As the morning progresses, showers will rapidly convect in the unstable air north of the rainband with steering now switching back to the NW as the upper low continues to move SE. With a combination of lower convective cloud bases, greater amounts of CAPE (Convective Available Potential Energy) and lift from the upper low, we should see fairly rapid late morning and afternoon showers and thunderstorms develop off the back side of the ranges chiefly, extending E/SE across the Riverland, Mallee and both SE Districts. Thunderstorms over the SE Districts are especially likely to be severe (but possibly the Mallee too) and organise into a line before gathering further organisation and strength as they cross the border into VIC. Intense rainfall, large hail (2-3cm) and damaging winds are likely with these thunderstorms or this line of thunderstorms.

For the remaining districts, the activity will mostly be some messy showers and left-over mid-level rain areas, gradually thinning out and decaying through the day as the sources of lift move further away into VIC as the ridge in the Bight starts to fight back.

For Victoria, this day will be the one with the greatest amount of activity. Well before sunrise, thanks to deepening surface trough and approaching upper low over SA, showers and thunderstorms will become scattered (tending to rain at times) and extend southwards right across the western half of the state including the Northern and Central districts though thunderstorm activity will be fresher and stronger closer one goes to the NSW border. East of halfway, it will be mostly rain areas and chiefly later in the day and through the evening.

Through the afternoon, there should be a clearance across the western half with solar heating creating a greater depth of instability. A line of thunderstorms, many of them severe will advance into the state from SA during the late afternoon. Heavy rainfall causing flash flooding, large hail (2-3cm) and damaging winds are likely from any severe thunderstorms. Towards the northern end of the line, the thunderstorms are likely to be a little longer lived with some left moving characteristics possible which includes areas across the border into Southern NSW. These supercell thunderstorms will likely be the strongest of the system with a chance of hail to 4cm as well as the aforementioned other hazards. The thunderstorm activity will gradually peter out and spread to decaying rain areas from midnight or just after as it gets to the halfway point of the state with nothing severe expected in the eastern half.

Saturday 25th

Now the main upper low did split into two sections with a weaker segment lingering over the West Coast and Eyre Peninsula. This will begin to pull back SE through the day but also speed up and strengthen as it becomes re-absorbed into the westerly belt. With a weak surface low lingering over the SE corner of the state, some wrap around showers or light rain areas/drizzle periods coming back up from the SW are likely in a much cooler and cloudy day with fresh SW winds across the southern agricultural areas. Most activity will be confined to the gulf areas immediately east. Isolated afternoon thunderstorms are still possible anywhere east of the ranges but chiefly across the Lower and Upper SE. These will lack the punch from the previous afternoon however locally intense rainfall is still possible under stronger cores as steering down here will be slacker and moisture levels still high.

Given that the upper low will be strengthening a little, there is still a possibility of some isolated afternoon showers and thunderstorms developing on the lead edge across the Riverland or Southern NE Pastoral.

Over VIC, most activity will be confined to the western and SW half of the state with scattered showers and isolated thunderstorms, tending scattered potentially in the SW during the afternoon with isolated severe thunderstorms possible but not to the level of the day before.

Lingering showers will continue to affect the central and eastern parts of the state with a persistent low-pressure trough through the 26th-30th with isolated afternoon thunderstorms likely in the NE portions, especially at this stage on the 28th.

Rainfall

Now over to expected rainfall...

The following is rainfall to approx 9am on the 26th

NW Pastoral:

Generally nil for much of the district however 3-8mm totals are likely in the far SE. Isolated 15mm dumps from thunderstorms.

NE Pastoral:

Generally nil for the western and northern half but the S-SE portion of the district should see general 8-15mm with scattered falls to 20mm. Isolated totals of 30-50mm are possible in thunderstorms.

Eyre Peninsula:

Generally, less than 1-2mm over the western parts grading to 5-10mm over the east, Scattered totals to 15mm possible. Isolated 20-25mm dumps in thunderstorm activity in the far NE.

Upper North:

General 15-25mm, scattered totals to 30mm. Isolated totals of 40-50mm in thunderstorms. Isolated lower pockets still of 8-15mm.

Lower North:

General 15-25mm, scattered totals to 35mm. Isolated totals of 40-50mm in thunderstorms. Isolated lower pockets still of 8-15mm.

Yorke Peninsula:

General 5-10mm, scattered totals to 15-20mm with isolated 30mm in possible thunderstorms or lingering rain areas.

Kangaroo Island:

General 20-30mm, scattered falls to 35mm. Isolated 40mm possible in the south.

Adelaide Plains:

General 15-25mm scattered totals to 30mm, isolated 35mm possible. Isolated lower pockets still of 8-15mm

Mt.Lofty Ranges:

General 18-28mm scattered totals to 35mm, isolated 40mm possible. Isolated lower pockets still of 10-18mm

Riverland:

General 15-25mm scattered totals to 35mm, isolated 40-70mm dumps possible in thunderstorms. Isolated lower pockets still of 8-15mm.

Mallee:

General 15-25mm scattered totals to 30mm, isolated 40-60mm dumps possible in thunderstorms. Isolated lower pockets still of 8-15mm.

Upper SE:

General 18-28mm scattered totals to 35mm, isolated 40-50mm dumps possible in thunderstorms. Isolated lower pockets still of 10-18mm.

Lower SE:

General 15-25mm scattered totals to 30mm, isolated 40-50mm dumps possible in thunderstorms. Isolated lower pockets still of 8-15mm.

Western VIC:

General 20-30mm scattered totals to 40mm, isolated 40-70mm dumps possible in thunderstorms, chiefly over the Mallee. Isolated lower pockets still of 5-15mm, chiefly in the far south of the SW District.

Central VIC

General 15-25mm, scattered totals to 30mm, isolated dumps in thunderstorms to 40-50mm. Totals decreasing with southern extent with pockets of 5-15mm towards Melbourne on the lee side of the Macedon's.

Link to temporary blog website: https://farmweathersa.blogspot.com/

Happy Farming and Cheers from The Weatherman

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