August 2022 Update SA

August 2022 Update SA

*Forecast prepared between August 16th-18th 2022

Review:

G’Day everyone, the middle of the month is here and that means time for another little update on proceedings. So far, the month has gone to plan with a broad shift in the patterns from the blocking and high pressure of July to more mobile systems and increased moisture out of the NW. But we have only just started, more significant systems are yet to come through either later this month or spring generally once higher Precipitable Water (PWAT) moisture can feed in.

The main change has been the Longwave Trough, which has stood up very strongly over the WA longitude, giving them a significant burst of good falls to start August. This large Longwave resulted in a period of windy conditions over SA with N to NW winds over many days, strong and gusty at times with raised dust in very dry parts, in particular on the 2nd ahead of the first sliding front. Multiple fronts and troughs then rotated around a complex low over the period of 3-4 days however a lot of rainfall was absorbed by the Eyre Peninsula, resulting in far lighter falls over areas to the east across the Lower and Upper North and Northern Yorke Peninsula in particular. The West Coast of EP, Lower Yorke Pen, KI and parts of the foot of the Fleurieu all did very well in the dominant NW/W flow but the inability for a sustained SW flow hurt areas north of about Gawler.

Fortunately, due to the shifting pattern, this system was not a one off with a follow up slow moving cut off low traversing the state from the 10th through to the 14th with cold air aloft allowing slow moving heavy showers to form in streams, especially so during the afternoons with daytime heating. This favoured areas such as Northern Yorke Peninsula and broadly the Lower and parts of the Upper North generating some excellent falls with up to 70mm in the Sevenhill region and numerous 30-50mm totals over slightly elevated terrain. Local flooding occurred near Georgetown and also Sevenhill. Isolated thunderstorms were also reported during the afternoons in the same regions as well as some across the Mallee and Riverland though activity was a little less widespread with lower but still useful totals to 10mm or so occurring.

Meteorological Discussion:

Now currently as I type this on the evening of the 17th, we have a weakening NW cloudband moving across the state generating areas of very light patchy rain. Rainfall totals under this mid-level deck are minimal and mostly less than 0.6mm to a mere trace. On the back edge however, the jetstream is getting squeezed aloft ahead of a faster moving shortwave trough over WA. As a result, we are seeing a delta striped cloudband with a very spring like generation of high based showers and thunderstorms. Whilst these aren’t in any way particularly strong or active, it is a small sign that we are translating into spring soon and this sort of thundery activity will see quite an uptick as more heat is introduced. This band will progress eastwards during the early hours of the 18th with a band of scattered showers and isolated thunderstorms, the latter likely weakening to just a thin band of patchy rain.

During the daylight hours however on the 18th, the shortwave over WA will move swiftly into SA and cause a re-thickening of mid-level cloud ahead of it. Upper divergence will be sufficient to increase areas of rain on and ahead of the band with the thickening at a maximum as the rainband traverses across the Central parts of the state, chiefly over the Upper North, Flinders and Southern Pastoral areas. The band will gradually weaken as it moves further east as the upper trough support immediately behind will gradually start to weaken and sag back south. But some handy top up rainfall for the agricultural areas, but moreso the northern areas this time in the 7-12mm range for the most part, then decreasing with southern and eastern extent. Behind this some stream showers will occur however there won’t be a great deal in it however a weak cold front will advance in from the SW during the 19th reaching the coastline of Eyre Peninsula by about lunchtime and through Adelaide by about 3:30pm or so. This will be accompanied by a band of showers, tending to rain areas about the Lower SE with some heavier showers on the back edge near the colder air however rainfall elsewhere will be much less significant than the one on 18th and inland penetration less as well.

On the 20th we should see some diurnal daytime heating give some convective showers over the landareas chiefly across Yorke Pen, and the Lower/Upper North higher terrain areas and the Adelaide Hills but again light totals.

A mostly fine sunday 21st leading into a slightly better front with more SW-NE punch on the 22nd. This will get an extra kick out of the SW to S as the high behind ridges in with a little more vigour. A band of showers will accompany the front, possible heavy on the leading edge frontal zone. Colder air behind will drive stream showers inland with possible small hail south of about Gawler in showers however this won’t last too long with activity becoming quite isolated but not totally clear as onshore SW moisture with an embedded trough or two will see isolated to occasionally drizzly showers move in from the SW through till about the 26th when it all should finally clear. There is likely to be a period of high pressure towards the end of the month to end the run of fronts and troughs with some good sunshine to assist crops that would likely be fairly damp in most (but perhaps not all) areas. Some for of renewed cut off low type of system looks to be setting up for early September at this stage but thats a bit far out. One thing I do expect is plenty of cut off lows through September as the inland begins to warm, the tropical to polar interactions will be a little more significant. Overall, the initial monthly outlook rainfall wise is well on track to be reasonably close to expectations with average to above average for many districts, perhaps only some portions of the Southern Pastorals, western interior and northern Mallee to east of the Flinders may be closer to below average but not by much.

Climate Drivers:

The Pacific has been in motion over the last few weeks with a trend back towards La Nina. Constant cooling through the mid to eastern Pacific as well as in the sub-surface has seen the Nino 3.4 index take a dive with a clear La Nina modoki style setup taking shape. This occurs when the coldest anomalies appear more in the central Pacific than the east however, I eventually expect that to extend a little further east to be closer to the standard La Nina. I don’t expect it to be strong but no doubt enough to count. NOAA have already declared one and the BoM have finally moved to a La Nina alert, but they always have more strict criteria. The Coral Sea is still maintaining its warmth which is an imposing sign for NE feeding moisture in spring and summer. Trade winds remain above average, and the overall cog is working like a well-oiled machine. Plenty more rain and flooding to come for the Eastern states in the next 4-6months.

Figure 1: Current SST anomalies – source Tropical Tidbits

The Indian is steadily marching towards a moderate to strong strength negative IOD. We have warmed off Java and Sumatra compared to the last update yet we still have the same heat if not even more heat in the Coral Sea so there is going to be plenty of supply to drift west across the Top End. The Western Indian remains cooler than average although it’s not hugely significant. The BoM’s latest reading was –1.15C. I’m expecting final minimum values around –1.2 to –1.3C. For reference 2016 reached –1.5C which was the strongest since accurate records began in the 80’s. The expectation is for a wetter September to December with increased cut off lows and unstable troughs resulting in broadly above average rainfall for most districts. Flooding will be likely in parts at times. Traditionally, September is the ‘big’ month for most solid negative IOD’s and at this stage I agree with that for this year however it could well be staggered into October also. So will keep you posted there. The MJO is weak and in the Western Indian Ocean allowing a fair surge of convection to develop across more Central Parts. This is a great sign and is ready to shift slowly east in the coming weeks. Most of the WA moisture from recent large cloudbands and fronts has come from the Central Indian large convective mass. The SAM has been sitting in neutral range as expected with neither here nor there movement, though perhaps a trend to a little more negative values in the coming days is likely before pushing back to weakly positive into early September.

Districts:

Eyre Peninsula

• Northern & Eastern Eyre Peninsula

Shortwave trough on 18th with thickening cloudband bringing areas of rain from the west, easing later in the day. Possible isolated thunder. Weak front then extending from SW on 19th during afternoon with isolated to scattered showers but not a lot in it. Follow up weak front on 20th with isolated to scattered showers, chiefly during the afternoon with surface heating. Mostly fine until 22nd when a cold front and associated band of showers extends from the SW during the early afternoon with slightly better inland penetration. Moist onshore flow till 26th but only isolated showers at times, chiefly in the south and about the Eastern EP coastline. Potential for cut off low type system very end of month or more likely early September. Slight frost risk 24th & 28th. Rainfall for period 12-18mm. Isolated totals to 25mm in the south with decreasing totals in the far NE down to 7-10mm.

• Western & Lower Eyre Peninsula

Shortwave trough on 18th with thickening cloudband bringing areas of rain from the west, though tending to showers moreso about the Lower Eyre Peninsula, easing later in the day. Possible isolated thunder. Weak front then extending from SW on 19th from around lunchtime with scattered showers. Follow up weak front on 20th with further scattered showers from early morning with surface heating generating some renewed afternoon development further inland. Possible light showers on 21st but minimal recordings ahead of a stronger front on 22nd when an associated band of showers and colder air extends from the SW during the early afternoon with slightly better inland penetration. Moist onshore flow till 26th but only isolated showers at times, chiefly about the Lower EP coastline. Potential for cut off low type system very end of month or more likely early September. Slight frost risk 24th & 28th. Rainfall for period 15-25mm. Isolated totals to 30mm in the south and west coasts with decreasing totals in the far NE down to 10-15mm.

Upper North

Shortwave trough on 18th with thickening cloudband bringing areas of rain from the west, easing later in the day. Possible isolated thunder. Weak front then extending from SW on 19th from the evening with scattered showers. Follow up weak front on 20th with further scattered showers from early morning with surface heating generating some renewed afternoon development and showers, chiefly about higher and elevated terrain/ranges. Stronger front on 22nd with associated band of showers and colder air extending from the SW during the early afternoon with slightly better inland penetration. Moist onshore flow till 26th but only isolated showers at times, chiefly about elevated terrain and windward slopes. Potential for cut off low type system very end of month or more likely early September. Slight frost risk 21st, 24th & 28th. Rainfall for period 10-20mm. Isolated totals to 30mm about known wet spots/higher terrain with decreasing totals in the far NE/E down to 7-10mm.

Lower North

Shortwave trough on 18th with thickening cloudband bringing areas of rain from the west, easing later in the day. Possible isolated thunder. Weak front then extending from SW on 19th from the late

afternoon or evening with scattered showers. Follow up weak front on 20th with further scattered showers from early morning with surface heating generating some renewed afternoon development and showers, chiefly about higher and elevated terrain/ranges and wet spots that have water pooling from previous heavy falls. Stronger front on 22nd with associated band of showers and colder air extending from the SW during the early afternoon with slightly better inland penetration. Moist onshore flow till 26th but only isolated showers at times, chiefly about elevated terrain and windward slopes. Potential for cut off low type system very end of month or more likely early September. Slight frost risk 21st, 24th & 28th. Rainfall for period 15-25mm. Isolated totals to 35mm about known wet spots/higher terrain with decreasing totals in the far NE/E down to 10-15mm.

Yorke Peninsula

• Northern Yorke Peninsula

Shortwave trough on 18th with thickening cloudband bringing areas of rain from the west, easing later in the day. Possible isolated thunder. Weak front then extending from SW on 19th from the late afternoon or evening with scattered showers. Follow up weak front on 20th with further scattered showers from early morning with surface heating generating some renewed afternoon development. Stronger front on 22nd with associated band of showers and colder air extending from the SW during the early afternoon with slightly better inland penetration. Moist onshore flow till 26th but only isolated showers at times, possibly drizzly. Potential for cut off low type system very end of month or more likely early September. Slight frost risk 24th & 28th. Rainfall for period 12-20mm. Isolated totals to 25mm closer to higher terrain in far NE with decreasing totals in the far west or coastal areas down 10-15mm.

• Lower Yorke Peninsula

Shortwave trough on 18th with thickening cloudband bringing areas of rain from the west, though tending to showers moreso over the foot, easing later in the day. Possible isolated thunder. Weak front then extending from SW on 19th from around lunchtime with scattered showers. Follow up weak front on 20th with further scattered showers from early morning with surface heating generating some renewed afternoon development further inland and up the centre of the Peninsula.

Possible light showers on 21st but minimal recordings ahead of a stronger front on 22nd when an associated band of showers and colder air extends from the SW during the early afternoon with slightly better inland penetration. Possible small hail in colder air behind front. Moist onshore flow till 26th but only isolated showers at times, chiefly about the foot. Potential for cut off low type system very end of month or more likely early September. Slight frost risk 24th & 28th. Rainfall for period 15-25mm. Isolated totals to 30mm about the foot with decreasing totals in the far N down to 12-15mm.

Kangaroo Island

Shortwave trough on 18th with thickening cloudband bringing areas of rain and a band of showers, easing later in the day. Possible isolated thunder and cold air showers to follow. Weak front then extending from SW on 19th from around late morning to midday with scattered showers. Follow up weak front on 20th with further scattered showers from early morning. Possible light showers on 21st but minimal recordings ahead of a stronger front on 22nd when an associated band of showers and colder air extends from the SW during the early afternoon with slightly better inland penetration. Small hail likely in colder air behind front. Moist onshore flow till 26th but only isolated showers at times. Potential for cut off low type system very end of month or more likely early September. Slight frost risk 24th & 28th about island centre. Rainfall for period 20-30mm. Isolated totals to 35-40mm about the western end with decreasing totals in the far E down to 15-20mm.

Adelaide Plains

Shortwave trough on 18th with thickening cloudband bringing areas of rain from the west, easing later in the day to showers. Possible isolated thunder. Weak front then extending from SW on 19th from the mid afternoon early evening with scattered showers. Follow up weak front on 20th with further scattered showers from early morning with surface heating generating some renewed afternoon development and showers, chiefly about outer northern plains. Stronger front on 22nd with associated band of showers and colder air extending from the SW during the early afternoon with slightly better inland penetration. Possible small hail in colder air behind front. Moist onshore flow till 26th but only isolated showers at times, chiefly about elevated terrain and windward slopes of the foothills. Potential for cut off low type system very end of month or more likely early September. Slight frost risk 21st, 24th & 28th. Rainfall for period 15-25mm. Isolated totals to 35mm about foothills with decreasing totals in outer northern plains down to 15mm.

Mt.Lofty Ranges

Shortwave trough on 18th with thickening cloudband bringing areas of rain from the west, easing later in the day to showers. Possible isolated thunder. Weak front then extending from SW on 19th from the mid afternoon or early evening with scattered showers, freqent about higher peaks. Follow up weak front on 20th with further scattered showers from early morning with surface heating generating some renewed afternoon development and showers. Stronger front on 22nd with associated band of showers and colder air extending from the SW during the early afternoon with slightly better inland penetration. Small hail in colder air behind front. Moist onshore flow till 26th but only isolated showers at times, chiefly about elevated terrain and windward slopes with orographic lift. Potential for cut off low type system very end of month or more likely early September. Slight frost risk 21st, 24th & 28th. Rainfall for period 30-40mm. Isolated totals to 45mm about known wet spots with decreasing totals about northern ranges down to 15-20mm.

Riverland/Mallee

• Murray Valley

Shortwave trough on 18th with thickening cloudband bringing areas of rain from the west, easing later in the day. Possible isolated thunder. Weak front then extending from SW on 19th from the evening with isolated showers though rain shadow being fairly stout. Follow up weak front on 20th with further isolated to scattered showers from early morning with surface heating generating some renewed afternoon development. Stronger front on 22nd with associated band of showers and colder air extending from the SW during the early afternoon with slightly better inland penetration. Moist onshore flow till 26th but only isolated showers at times, chiefly about eastern slopes of the ranges at best. Potential for cut off low type system very end of month or more likely early September. Slight frost risk 21st, 24th & 28th. Rainfall for period 7-15mm. Isolated totals to 18mm with decreasing totals in the far NE/E down to 5-7mm with fairly stout rain shadowing.

• Riverland/Northern Mallee

Shortwave trough on 18th with thickening cloudband bringing areas of rain from the west, easing later in the evening. Possible isolated thunder. Weak front then extending from SW on 19th from the evening with isolated showers though rain shadow being fairly stout. Follow up weak front on 20th with further isolated to scattered showers from late morning with surface heating generating some renewed afternoon development. Stronger front on 22nd with associated band of showers and colder air extending from the SW during the early afternoon with slightly better inland penetration. Moist onshore flow till 26th but only isolated showers at times. Potential for cut off low type system very end of month or more likely early September. Slight frost risk 21st, 24th & 28th. Rainfall for period 7-12mm. Isolated totals to 15mm with decreasing totals about far N Riverland to less than 7mm.

• Southern Mallee

Shortwave trough on 18th with thickening cloudband bringing areas of rain from the west, easing later in the day. Possible isolated thunder. Weak front then extending from SW on 19th from the evening with isolated to scattered showers though rain shadow being fairly stout. Follow up weak front on 20th with further isolated to scattered showers from early morning with surface heating generating some renewed afternoon development. Stronger front on 22nd with associated band of showers and colder air extending from the SW during the early afternoon with slightly better inland penetration. Moist onshore flow till 26th but only isolated showers at times, chiefly about eastern slopes of the ranges. Potential for cut off low type system very end of month or more likely early September. Slight frost risk 21st, 24th & 28th. Rainfall for period 7-15mm. Isolated totals to 18mm with decreasing totals in the far N down to 5-8mm.

Upper SE

Shortwave trough on 18th with thickening cloudband bringing areas of rain or a band of showers from the west, easing later in the day. Possible isolated thunder. Weak front then extending from SW on 19th from the late afternoon or evening with scattered showers, tending to rain in the far south. Follow up weak front on 20th with further scattered showers, chiefly morning. Stronger front on 22nd with associated band of showers and colder air extending from the SW during the early afternoon with slightly better inland penetration. Showers tending to rain in the south. Possible small hail in colder air behind front. Moist onshore flow till 26th but only isolated showers at times. Potential for cut off low type system very end of month or more likely early September. Slight frost risk 21st, 24th & 28th. Rainfall for period 15-25mm. Isolated totals to 30mm in the far south with decreasing towards the Mallee border in the order of 10-12mm.

Lower SE/ SW VIC

Shortwave trough on 18th with thickening cloudband bringing areas of rain or a band of showers from the west, easing later in the day. Possible isolated thunder. Weak front then extending from SW on 19th from the late afternoon or evening with scattered showers, tending to rain areas at times. Follow up weak front on 20th with further scattered showers, chiefly morning. Lingering coastal showers into the 21st possible. Stronger front on 22nd with associated band of showers and colder air extending from the SW during the early afternoon with slightly better inland penetration. Showers tending to rain at times along the frontal band. Small hail in colder air behind front. Moist onshore flow till 26th but only isolated showers at times. Potential for cut off low type system very end of month or more likely early September. Slight frost risk 21st, 24th & 28th. Rainfall for period 30-30mm. Isolated totals to 50mm in the far south/coastal areas with decreasing towards the Wimmera and Upper SE borders in the order ot 15-20mm.

Wimmera

Shortwave trough later on 18th with thickening cloudband bringing areas of rain or a band of showers from the west. Possible isolated thunder. Activity clearing early on 19th. Weak front then extending from SW on 19th from the late afternoon or evening withpatchy rain tending to scattered showers. Follow up weak front on 20th with further scattered showers, chiefly morning. Stronger front on 22nd with associated band of showers and colder air extending from the SW during the early afternoon with slightly better inland penetration. Showers tending to rain in the south. Possible small hail in colder air behind front and snow flurries about higher peaks of Grampians. Moist onshore flow till 26th but only isolated showers at times. Potential for cut off low type system very end of month or more likely early September. Slight frost risk 21st, 24th & 28th. Rainfall for period 15-25mm. Isolated totals to 30mm in the far south with decreasing towards the Mallee border in the order of 10-15mm.

VIC Mallee

Shortwave trough later on 18th with thickening cloudband bringing areas of rain from the west, easing later and clearing on 19th. Possible isolated thunder. Weak front then extending from SW on 19th from the evening with isolated showers. Follow up weak front on 20th with further isolated to scattered showers from late morning with surface heating generating some renewed afternoon development but not a lot in it Stronger front on 22nd with associated band of showers and colder air extending from the SW during the early afternoon with slightly better inland penetration. Moist onshore flow till 26th but only isolated showers at times in far west and south. Potential for cut off low type system very end of month or more likely early September. Slight frost risk 21st, 24th & 28th. Rainfall for period 5-10mm. Isolated totals to 12mm with decreasing totals towards far N and NSW border to 5mm or less.

Finals Notes:

• Negative IOD only just beginning to hit its straps with good rainfall from September to November likely
• La Nina also having a third crack and likely to get to weak status
• Rain shadow effect still causing issues in Mallee with much patchier rainfall with westerly winds drying out some areas. September likely to assist a little with cut off lows to help get rainfall into those areas
• Watch for potentially a multi day thunderstorm event through November and or December with a combo of humid NE to NW winds into slow moving deep troughs
• Good chance of very large and slow moving cut off low in September or early October

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

Happy Farming and Cheers from The Weatherman.

Photos Left to Right; Calibre Wheat Wynarka - Roped Out (Not Wynarka). 

*Note – For interest only, all thoughts are of The Weatherman and may not be reproduced without my consent.