Special Update SA Complex Upper low with rain
***Forecast prepared evening of Friday 8th.
Meteorological Discussion:
G’Day everyone, as promised a little update on this major rain event. I was planning a longer update but got a second call into work in as many nights! Like last time I will break it down into the days. The main changes is the shift west in the entire event, and whilst it looked significant for the NE EP and W Upper and Lower North last time, the focus now shifts to a large bulk of the EP and up into the NW Pastorals with a reduced rainfall impact east of the gulfs over the Upper and Lower North which in many ways is a sigh of relief for those still harvesting. There will still be some rain, however. Expansion below.
Saturday 9th
Strong upper trough beginning to push up from the SW causing strong dynamical ascent/lift to the eastern side of it. Mid-level showers and isolated brief thunderstorms will develop across a broad area from the Bight into the southern agricultural areas before sunrise. As daybreak emerges, this activity will gradually thicken and spread into a band of mid-level rain, running quickly SE along the Jetstream in a narrow band extending right across into the southern half of the Mallee and Upper SE and then into Western VIC (and eventually Melbourne!). The rainband will gradually shift NE during the day as the strong upper trough to the west gradually rises up from the deep south. Due to being positively tilted (the top leans further west than the bottom), it will become cut off and form into an upper low just east of the WA/SA border later in the evening. Areas of rain with moderate falls within that band will continue across the state in that band that will stretch from the NW Pastoral down across the northern, southern and parts of the eastern agricultural areas although the band will clear rainfall from the southern coastline temporarily in the late afternoon or evening. Isolated thunderstorms are most likely across the Far North of the state towards the top of the rainband.
Sunday 10th
The upper low will begin to rotate on itself and activate a new, heavier rainband across the NW Pastorals from the top section of the original rainband and begin to pull it down into the Eyre Peninsula which will form part of the main baroclinic leaf, an area of maximised upper lift and divergence. The entire band will pivot on itself before the rain begins to pull back SW slowly through the day. Areas of heavy continuous rain will extend southwards from the NW Pastoral (especially the southern portions north of the Eyre Peninsula) into the Eyre Peninsula with heavy falls and flooding. Isolated thunderstorms, tending scattered will form along a new band just in front of the main band and begin to feed south as well, adding to the thicker old band with more heavy falls, especially focusing on areas roughly south along a line from Woomera to Kimba or there abouts, this will be where low-level convergence is maximised. Further flooding is likely as this training effect of thunderstorms into rain occurs. Some rainfall totals here will be record breaking for December with a number of individual records for sites likely to be set for 24hr rainfall totals for December and potentially all time. It is not unreasonable to see the potential for scattered totals of 100-150mm and isolated falls up to around 200mm, potentially even higher.
Now the original band that extended SE the day before will begin to get pulled back SW as well as the upper low rotation captures the steering flow and pulls it back towards itself. Rain areas across areas east of the gulfs with moderate falls will begin to ease, especially from areas east of the ranges quite early but then eventually most districts by mid-afternoon. There is a chance of a brief isolated thunderstorm on the top/NE edge of the band although not a lot here really.
Monday 11th
Monday sees things become quite complex. At the low levels a strong low level jetstream will establish beneath a deepening surface trough which will go onto to form a surface low pressure system. Fresh to potentially strong downslope gully winds are likely of the ranges down the western slopes in the early hours of monday morning. This will in turn feed the system from the east with mid and upper-level winds being NE-N. With this renewed feed and low-level moisture advection along a developing warm frontal zone, showers and thunderstorms are likely to develop in this zone of ascent across Eyre Peninsula, the Spencer Gulf and also to a lesser degree the Yorke Peninsula, Gulf St Vincent and adjacent areas east of here such as the Adelaide Plains, Mt.Lofty Ranges and Lower North. A hot to very hot and increasingly humid east of the surface trough after the warm front has moved to the south with humid air increasing from the east from Victoria. Dewpoints are likely to be around 15-20C which is getting quite high for this part of the world. This will pose a conditional instability risk should sufficient heating and lift overcome the low-level capping. Nevertheless, elevated thunderstorm activity is still likely along a broken band running roughly NNE-SSW through those aforementioned areas though severe thunderstorms are most likely over the Eastern Eyre Peninsula and up into the Pastoral areas. Further east across the Riverland, NE Pastoral east of the Flinders and Mallee areas especially, a very hot day is likely with temperatures reaching the low 40’s as the heat with much drier air from NSW gets pulled back west into the developing low. Thunderstorm activity will continue into the evening.
Granted the uncertainty for monday (and tuesday coming up) I may do a quick update the night before if there are any changes to the above.
Tuesday 12th
The upper low will now begin to start sagging southwards after churning around on itself for the past few days. Early morning isolated showers and thunderstorms will again be possible along a weakening trough line which will begin to shift eastwards affecting areas chiefly east of the Eyre Peninsula but west of the Mt.Lofty Ranges. Warm to hot and humid, especially across districts east of the gulfs with a specific focus near Adelaide and the Mt.Lofty Ranges. Conditional instability once again will be in place with low level capping to overcome. Should this occur, isolated afternoon showers and thunderstorms are likely, especially on the ranges but also now over the Mallee and parts of the Riverland, all extending southwards. Localised heavy rainfall is the main threat in these cells.
Rainfall
Now over to expected rainfall...
The following is rainfall to approx 9am on the 13th
NW Pastoral:
Generally 10-20mm for most areas except for the far NE and SW of the district. Within the main band through the Central to Southern areas, 40-80mm should be expected. Scattered falls of 100-150mm and isolated totals of 150-175mm towards the Eyre Peninsula. Slight chance of localised extreme totals to 200-225mm in the far S/SE.
NE Pastoral:
Generally nil for the NE corner and areas east of the Flinders but perhaps 1-5mm in scattered areas with isolated thundery sections to 20mm. Totals increasing towards the west and SW with 10-20mm in broken areas, with scattered totals to 30mm. Isolated 40-50mm dumps in training thunderstorms.
Eyre Peninsula:
Generally 50-100mm with scattered falls of 100-150mm in the north. Isolated 150-175mm falls again in the far north. Localised extreme totals of 200-225mm where trained thunderstorms add to areas that have received heavy continuous rain. Totals decreasing about the Lower Eyre Peninsula down to 30-50mm although the risk of local totals to 70mm is still a distinct possibility here.
Upper North:
General 10-20mm western half up to Port Augusta with scattered totals to 30mm. Isolated totals of 40-50mm near the far west/SW Spencer Gulf areas. Eastern half totals diminish quickly down to 5-10mm and less than 5mm east of the Barrier Hwy. Flinders portion in the north mostly less than 10-15mm however isolated thunderstorms may throw a quick 20-30mm in some areas.
Lower North:
General 20-30mm, with scattered falls to 40mm in the far SW. Isolated 60mm totals in the W/SW and with those areas that receive repeat thunderstorms, most likely in the SW or along the ranges to the south. Totals decreasing with NE extent down to 10-15mm in the far E/NE of the district.
Yorke Peninsula:
General 30-40mm, with scattered falls to 60mm in the north Isolated 70-75mm totals possible in the N and with thunderstorms that train over the same area. Totals decreasing towards the foot but still 20-30mm.
Kangaroo Island:
General 10-20mm, scattered totals to 25mm. Isolated 30-35mm totals should thunderstorms train over the island from the N, more likely monday morning.
Adelaide Plains:
General 30-40mm, scattered totals to 45mm Isolated totals to 50mm in the outer north and also with any areas that receive repeat thunderstorms. Totals decreasing towards the far southern suburbs down to 20-30mm.
Mt.Lofty Ranges:
General 35-45mm, scattered totals to 50mm. Isolated 55-60mm totals about known wet spots and areas where thunderstorms may train.
Riverland:
Western half, general 8-15mm with scattered totals to 20mm. Isolated 25-30mm near the ranges where thunderstorms may train. Totals decreasing towards eastern half border down to 5-8mm. Eastern half less than 5mm for most grading to 1-2mm near the northern portions. Totals potentially a little better in the far south towards the Mallee but still only 5-10mm. Local dumps possible in thunderstorms anywhere from 5-20mm on mon or tues.
Mallee:
General 20-30mm in narrow band mostly on Saturday. Scattered 35mm falls and isolated 40mm totals in the far west towards the ranges and where thunderstorms may train. Totals decreasing with NE extent towards the Riverland border down to 10-20mm.
Upper SE:
General 20-30mm in narrow band mostly in the northern half. Scattered totals to 35mm and isolated 40mm falls in the far NW and where thunderstorms may train. Totals decreasing in the south and SW down to 10-20mm.
Lower SE:
General 5-10mm, with isolated 15mm falls possibly if any thunderstorms develop mon or tues.
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 20-30mm in narrow band through chiefly the Wimmera. Totals decreasing markedly towards the Mallee with 5-15mm in the southern portions and less than 1-2mm or nil in the far northern portions. The SW District should see generally 10-20mm increasing to 30-40mm in the far NE against higher terrain. Totals decreasing towards the SA border and far S/SW down to 7-10mm.
Quick note for those of you following Tropical Cyclone Jasper – it is currently category 4 however weakening for the time being though the stronger mid-level ridge and further west movement of the upper low has allowed a more northern track, this time into and north of the Cairns area. Some phenomenal rainfall is expected on crossing with 1000mm totals not out of the question in some of the known wet spots around Cairns.
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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