Special Update SA cold front and low with rain, showers and isolated thunderstorms June 21st-22nd 2023
Special Update SA cold front and low with rain, showers and isolated thunderstorms June 21st-22nd 2023
G’Day everyone, thought I’d whip out a special update as promised for this next system as it is still looking significant with a two pronged event which should give very handy totals to most districts and that is before the stronger front due a few days after (I’ll issue another update for that as it is quite dynamic atm and hard to pin down the synoptic shape)
Meteorological Discussion:
Cutting to the chase here and currently we have a strong upper-level trough standing up over the Bight which is squeezing the jetstream to the north of it. This jetstream has been actively feeding down mid-level moisture for a number of days now and is responsible for the cloudband that we are seeing over the inland of the country. At the surface a cold front is also providing a focal point for lift and that will be a focus for most of the precipitation on Thursday 22nd.
In figure 1 above we can see the classic signature of a strong jet streak (purple area) over the interior which will act to intensify the system and generate areas of rain, showers and isolated thunderstorms across the state. Dynamical lift will be a key feature once again with severe weather possible however a key difference between this system and the last system is that the low-level moisture content is lower this time and the surface temperatures are also lower, limiting the raw potential for severe thunderstorms in such vast amounts which we saw last time. Nevertheless, thunderstorm activity will still occur with the most likely focus being along the cold front which can be seen being marked by a line of thunderstorms over the Western Bight in Figure 2.
This system will come in two main parts. The first will be a mid-level patchy rainband generated primarily from the inland cloudband shooting down along the jetstream atm. With the cold front closing the gap, the lift will increase, N winds will freshen and areas of light patchy rain will gradually develop across the NW and western parts of the state and extend SE, thickening as they do so. The rain should remain fairly light to moderate with slow accumulations only at first before they increase towards the rear edge of the rainband. The rainband will roughly be due into the Eyre Peninsula by about 5-530am on the 21st and into the Central districts including Adelaide by about 11am and
finally the Mallee by early afternoon. Isolated weak/brief thunderstorms may occur along the leading-edge dependent on the overall lift.
Now the second part of the system involves the cold front, and this will gradually start to slow down and almost stall later 21st and into the 22nd with more of a sliding band than the 21st rainband. What should happen is a low level-jetstream of weak to moderate strength will kick up behind the first rainband overnight on the 21st and especially so in the early hours of the 22nd and this should see winds become fresh to strong at times out of the N. Aloft we will still have that ever-powerful jetstream over the interior of the state giving lift but now added convergence down near the surface ahead of the sliding cold front. Combining all these factors, should see renewed showers and isolated to scattered thunderstorms develop across the NW Pastoral and West Coast/Eyre Peninsula later 21st and into the early hours of the 22nd, chiefly on and ahead of the cold front. This activity will then gradually thicken out to areas of rain with the odd isolated thunderstorm, chiefly towards the back edge over the Eyre Peninsula, KI and waters below the mainland with more steady soaking rainfall over the Eastern Districts than the first rainband. The entire system will slide and then pivot on itself as it begins to dive to the SE, which will allow colder air to spring up the western flank over the Bight increasing lapse rates and instability aloft. This may then cause renewed afternoon showers and isolated thunderstorms from convection in the wake of the rainband on the 22nd utilising the moistened low levels to create more instability. There is a chance of small hail with any stronger cells. Activity petering out steadily overnight and into the early hours of the 23rd.
Beyond this I will cover in a separate update but in short, the next burst of more vigourous fronts will be rocketing up across the waters south of the Bight due to impact the more southern coastal portions of the state especially from late 23rd to the 25th. This series of fronts will result in a lot of strong to gale force NW to W winds and no doubt severe weather warnings will go out for damaging winds.
Rainfall
Now over to expected rainfall...
The following is from the start of the event to about 9am 23rd June
NW Pastoral:
Light 4-8mm totals generally under the mid-level cloudband, increasing with SE extent to 15-25mm towards the Flinders Ranges and N Eyre Pen.
NE Pastoral:
Generally minimal rainfall less than 5mm further north and east however totals increasing with western and southern extent to about 15-25mm near the Flinders Ranges.
Eyre Peninsula
General 10-18mm with scattered falls to 20-25mm towards the Western Coast and also Lower Eyre Peninsula and potentially also in the far N. Isolated localised totals to 30-35mm possible near the Western and Lower EP coastline.
Upper North:
General 12-20mm with scattered falls to 25mm. Isolated falls to 30mm near higher terrain and about the Flinders. Locally isolated drier holes of 5-10mm towards the far east of the district off the backs of the ranges.
Lower North:
General 15-20mm falls, with scattered 25-30mm totals. Isolated heavier falls to 30-35mm possible near higher terrain. Totals decreasing off the backs of the ranges down to 7-10mm.
Yorke Peninsula:
General 18-25mm falls, with scattered 30mm totals about the foot. Isolated localised heavier falls to 40mm possible also near the western tip of the foot. Totals marginally decreasing about the eastern coast but still around 15-18mm.
Kangaroo Island:
General 20-30mm falls. Isolated heavier falls to 40-50mm possible about the western end. Totals decreasing slightly towards the east but still closer to 18-20mm or so.
Adelaide Plains:
General 20-30mm falls, with scattered 35mm totals. Isolated heavier falls to 40mm near the foothills. Totals decreasing over the outer northern plains and also western suburbs down to 15-20mm.
Mt.Lofty Ranges:
General 30-40mm falls, with scattered 45-50mm totals. Isolated heavier falls to 60-75mm about known wet spots. Totals decreasing in immediate rain shadow off SE ranges down 15-20mm and also parts of northern ranges down to 15-25mm
Riverland:
General 8-15mm falls with scattered totals to 15-18mm. Isolated totals to 20mm in the far south. Isolated drier holes in the far N down to around 5-8mm.
Mallee:
General 12-18mm falls with scattered totals to 20mm. Isolated totals to 25mm in the far south to SE. Isolated small drier holes near the Murray Valley rain shadow around 8-12mm.
Upper SE:
General 15-25mm falls with scattered 30mm totals. Isolated 35mm totals in the far SW-S. Totals decreasing slightly towards the north/NW near the Lakes but still around 12-15mm.
Lower SE:
General 15-20mm falls with scattered 25mm totals. Isolated 35mm totals towards the western/nw coastline. Totals decreasing slightly in the far S/SE of the district around 12-15mm.
Western VIC:
General 15-25mm falls across the northern part of the SW District and Wimmera, decreasing again towards the far Southern coastline down to 8-15mm and the Mallee around 5-10mm as well as the far eastern sections of all districts where the bulk of rainfall will be held out by ridging further east. Localised higher totals to 30mm near the Grampians and higher terrains.
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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