April 2023 update SA
April 2023 update SA
*Forecast prepared between April 18th-20th 2023
Review:
G’Day everyone, hope you are keeping well, and I trust that you all enjoyed a Happy and safe Easter with your family. But as much as it is a time to relax for some, many of you no doubt will have been busy out in the field getting ready for the growing season. So far, this month has been kind to us with the later than expected draining of the tropics really assisting in getting moisture into the ground without a lot of heat to remove it with a number of systems with different dynamics delivering a range of precipitation both from steady and soaking to intense and even some hail.
The main systems so far began with a complex low with troughs and fronts subtending off of it on the 5th and 6th of April. The 6th especially saw some strong instability during the afternoon with a little extra heating than expected really taking advantage of the already impressive upper-level divergence, lift and steep lapse rates present and as a result, numerous afternoon and evening thunderstorms were generated, especially over the eastern portions of the Upper and Lower North and the Riverland, Mallee and Upper SE districts. Notably, these thunderstorms remained fairly discrete in nature which allowed their updrafts to remain strong rather than spread out to messier areas of rain and un-organised activity which would have limited their severity. The resultant strong updrafts caused strong to damaging winds and hail in the strongest cores, with one of those directly impacting Waikerie in the evening of the 6th causing quite a bit of damage as well as flash flooding with around 43mm in 20minutes. Other areas to receive strong storms were Terowie on the Barrier Hwy with a left moving supercell causing similar hail and flash flooding. All in all, some very useful pockets of rainfall were recorded across the Mallee and Riverland which will be an area to suffer the most across winter and spring as the expected dry pattern becomes more established.
The second notable event was the infeed of tropical moisture from Severe Tropical Cyclone Ilsa. As mentioned in the special update issued on the 14th, this cyclone recorded the strongest sustained 1-minute winds in Australian history with 218km/hr. A gust of 289km/hr was also recorded and no doubt, higher winds occurred on the eastern section of the eyewall. The damage in and around Pardoo station as well as to the Pardoo Roadhouse was significant and devastating with anything not built to category 5 code, completely destroyed.
Downstream however the resultant rainfall with the cold front that linked up with the cyclone moisture was useful, but significant falls were mostly confined to the Upper Eyre Peninsula, Southern NW Pastoral & Mt.Lofty Ranges with Tarcoola receiving an impressive 42.4mm. Further across the Pastoral areas big falls included Lake Everard 55mm, Todmorden 53mm, Mobella 50mm, Mt Dare 47mm & Coober Pedy 47mm. Over the agricultural areas notable totals included Ashton 62mm, Uraidla 60mm, Woodhouse and Crafers both 55mm, Hope Valley 41mm, Golden Grove 40mm, Clare 36mm, Wirrabara Forest 33mm, Penong 32mm, Mintaro 31mm, & Yacka 30mm. A large bulk of totals across the remainder of the agricultural areas except east of the ranges fell in the 15-30mm bracket.
Meteorological Discussion:
Now as we move into the latter parts of the month, we will finally start to see the leading edge of a broader drier period that I have been expecting for some time. The general characteristics of this period will be more high pressure with lighter winds for the most part and even pseudo–Indian Summer like conditions. In between highs though we should still see cold fronts or weak cut off lows attempt to form, primarily being supplied with moisture from the Southern Ocean for the fronts and the eastern states for the cut offs but this will be limited with the overall downwelling portion of the MJO and Hadley cell forcing a weakening of any systems that generate uplift. That being said, it will take time to establish so the next system due after ANZAC Day will still have a little punch despite the dwindling broadscale pattern. Useful totals may still occur in each system but a lack of reliability and trust in the spread of rainfall will begin to slowly increase over the coming months so essentially, the wheel hasn’t turned bad just yet.
Currently we are now on the back edge of that burst of moisture largely thanks to the MJO and sunspot activity which caused a meandering of the jetstreams resulting in high amplitude upper troughs to tap into that tropical moisture and bring it SE towards our state. Now with that 80-85% behind us we begin to enter a new type of pattern as mentioned so for the next few days we will see the first of many large high-pressure systems dominate the state resulting in mild to warm days and cool to even cold nights in some areas. These will be some of the most pleasant days of the autumn so far to be outside and will last right up until about ANZAC Day. At this stage for those of your participating in the dawn service and other morning services for that special day, the weather should be fine but with some freshening northerly winds ahead of a trough and cold front extending from the west later in the day bringing with it a band of rain and potentially isolated thunderstorms, though the latter are more likely in the Far West of the state.
Given there is still reasonable moisture pushing west across Cape York and into the Eastern portion of the Top End thanks to the back end of the MJO, we will still have a little bit of moisture to feed down ahead of this system. With the strong high pressure ahead of it, it will likely cut off well to the west of the state before slowly pivoting and dragging in some moisture with mid-level convergence. The rainband along the troughline should develop across the West Coasts through the 26th reaching the Eyre Peninsula by roughly late afternoon and into Adelaide/Central areas by approx. midnight on 27th with the band of rain becoming general. If, however, the cut off low develops a little too far west the rainband may struggle but early indications are that we should at least be good for a general 10-15mm. With isolated much higher totals. I will likely issue a special update the day before to really hone in on totals but this would be a welcome boost to cap off a decent month in most districts and certainly exceed the initial expectations rainfall wise.
The rainband should contract eastwards during the mid to late afternoon but be followed by colder air with a mid-level upper low aloft to move in behind generating scattered showers and potentially an isolated afternoon/early evening thunderstorm as shear should be reasonably strong.
Beyond this a moderate strength high should bring settled conditions ahead of another weak front either around the 30th or just after into early May. May should then see a decline in overall lift with more high pressure/longer fine spells.
Climate Drivers
Looking at the Pacific and we can see the changes occurring with each and every update now with significant warming off Peru showing a complete retraction of the Humboldt Current – a classic sign of a developing El Nino in its early stages. We still have time however before we really see the ocean to atmosphere response in maturity plus trade winds in the Western Pacific have been quite strong – some of this in response to winds pushing into the strong MJO. The Westerly Wind Burst following the MJO will further kick start the El Nino into gear. It should be noted that the Coral Sea has warmed up which is assisting in bringing some late season moisture into QLD on those trade winds for the time being.
The Indian has shown a fair change off the NW coast of WA with cooling occurring and could well be the first signs of a Ningaloo Nina. This can majorly impact rainfall through late autumn and winter through Southern Australia and can be found during positive IOD years through May most commonly (the opposite Ningaloo Nino is the warming in the same area which can often assist rainfall). The key now is how long does this cooler area persist as getting the positive IOD setting up later in winter and early spring will effectively act as the triple whammy with the Pacific El Nino and operate like a well-oiled machine towards drier conditions, I will state though that it is unlikely we will see such dryness as experience in 1994 and 2006 to name a couple of very bad years but potentially closer to 2002. A very close watch this space now for the Indian as I do expect a moderate strength positive IOD to peak in September or October.
The MJO as mentioned earlier has been quite strong across Northern Australia and was responsible for generating Category 5 Ilsa as well as a late season burst of heavy rainfall, chiefly over the Eastern Top End with Gove Airport receiving 304mm in a day to 9am on the 18th. The latter was also aided by a westward moving strong Equatorial Rossby Wave, but I won’t bore you with the dynamics of that in this update!
Essentially the MJO is a Kelvin Wave wave which moves eastwards around the equator where as a Rossby Wave moves westwards. Where these intersect or meet can result in enhanced tropical convection and rainfall. With the main part of the MJO over the Pacific now in phase 6 to 7, we should expect tropical activity to begin to die down and potentially see more dry season like conditions at times in surges across the Top End as large high-pressure systems to the south drive those SE surges north and effectively clean out the tropics of moisture.
The SAM has been neutral to negative so far this month with a couple of cold fronts allowing that. I expect an overall upwards trend to positive into May with only a brief spike to negative near the end of this month. Large high-pressure systems till be the main influence on this index becoming positive which in the cooler months causes drier conditions with less reliable rainfall and more frost inland.
Districts:
Eyre Peninsula
Dominant high pressure with mild sunny days at first but then becoming warm to hot around ANZAC Day with temperatures near 30C inland. Trough and associated complex cut off low to develop from late 26th through till about 28th with a band of rain developing, tending to showers from 27th with colder air aloft. Isolated thunderstorms in the far west of the district. Strong SW winds behind low if it deepens over the East of the state. Resumption of settled conditions from 28th as new ridge moves in ahead of weak front with scattered showers, chiefly coastal on 30th or into early May. Due to complex nature of system rainfall figures at this point will be unreliable however a general 12-20mm with isolated 25mm is possible.
Upper North
Dominant high pressure with mild sunny days at first but then becoming warm to hot around ANZAC Day with temperatures near 30C in the north near Port Augusta. Trough and associated complex cut off low to develop from late 26th or early 27th through till about 28th with a band of rain developing, tending to showers from 27th with colder air aloft. Showers more isolated in the north. Isolated thunderstorms possible in the afternoon of 27th with heating. Strong SW winds behind low if it deepens over the East of the state. Resumption of settled conditions from later 28th as new ridge moves in ahead of weak front with isolated light showers in the south on 30th or into early May. Due to complex nature of system rainfall figures at this point will be unreliable however a general 8-15mm with isolated 20mm is possible.
Lower North
Dominant high pressure with mild sunny days at first but then becoming quite warm around ANZAC Day with temperatures in the high 20’s over flatter terrain. Trough and associated complex cut off low to develop from late 26th or early 27th through till about 28th with a band of rain developing, tending to showers from 27th with colder air aloft. Showers more isolated in the north. Isolated thunderstorms possible in the afternoon of 27th with heating. Strong SW winds behind low if it deepens over the East of the state. Resumption of settled conditions from later 28th as new ridge moves in ahead of weak front with isolated light showers in the south on 30th or into early May. Due to complex nature of system rainfall figures at this point will be unreliable however a general 10-18mm with isolated 20mm is possible.
Yorke Peninsula
Dominant high pressure with mild sunny days at first but then becoming warm to hot around ANZAC Day with temperatures in the high 20's in the north. Trough and associated complex cut off low to develop from late 26th through till about 28th with a band of rain developing, tending to showers from 27th with colder air aloft. Isolated thunderstorms possible in north on afternoon of 27th. Strong SW winds behind low if it deepens over the East of the state. Resumption of settled conditions from later 28th as new ridge moves in ahead of weak front with scattered showers, chiefly about the foot on 30th or into early May. Due to complex nature of system rainfall figures at this point will be unreliable however a general 12-20mm with isolated 25mm is possible.
Kangaroo Island
Dominant high pressure with mild sunny days at first but then becoming warm around ANZAC Day with temperatures in the mid 20's. Trough and associated complex cut off low to develop from late 26th through till about 28th with a band of rain developing, tending to showers from 27th with colder air aloft. Strong SW winds behind low if it deepens over the East of the state, especially about exposed coasts. Resumption of settled conditions from later 28th as new ridge moves in ahead of weak front with scattered showers and onshore winds on 30th or into early May. Due to complex nature of system rainfall figures at this point will be unreliable however a general 15-20mm with isolated 25mm is possible.
Adelaide Plains
Dominant high pressure with mild sunny days at first but then becoming warm to hot around ANZAC Day with temperatures in the high 20's in the north. Trough and associated complex cut off low to develop from late 26th through till about 28th with a band of rain developing, tending to showers from 27th with colder air aloft. Isolated thunderstorms possible about outer plains and northern suburbs on afternoon of 27th. Strong SW winds behind low if it deepens over the East of the state, especially about the coast. Resumption of settled conditions from later 28th as new ridge moves in ahead of weak front with isolated to scattered showers on 30th or into early May. Due to complex nature of system rainfall figures at this point will be unreliable however a general 12-20mm with isolated 25-30mm is possible, the latter especially about foothills and elevated suburbs.
Mt.Lofty Ranges
Dominant high pressure with mild sunny days at first but then becoming warm around ANZAC Day with temperatures in the mid to high 20's towards the northern ranges and off the back of the lower portions of the Southern Ranges. Trough and associated complex cut off low to develop from late 26th through till about 28th with a band of rain developing, tending to showers from 27th with colder air aloft. Isolated thunderstorms possible about the Northern Ranges on afternoon of 27th. Strong SW winds behind low if it deepens over the East of the state, especially about exposed peaks and Southern coasts and Ranges. Resumption of settled conditions from later 28th as new ridge moves in ahead of weak front with scattered showers on 30th or into early May. Due to complex nature of system rainfall figures at this point will be unreliable however a general 15-30mm with isolated 40-50mm is possible, the latter especially about higher peaks and known wet locations.
Riverland/Mallee
Dominant high pressure with mild sunny days at first but then becoming warm to hot around ANZAC Day with temperatures near 30C in the Riverland. Trough and associated complex cut off low to develop from early 27th through till about 28th with a band of rain developing, tending to showers from 27th with colder air aloft. Showers more isolated in the north of Mallee and the Riverland. Isolated thunderstorms possible in the afternoon of 27th if enough clearance behind rainband occurs. Strong SW winds behind low if it deepens over the East of the state and moves over VIC. Resumption of settled conditions from later 28th as new ridge moves in ahead of weak front with isolated light showers in the south of Mallee on 30th or into early May. Due to complex nature of system rainfall figures at this point will be unreliable however a general 5-15mm with isolated 20mm is possible towards the southern Mallee.
Upper SE
Dominant high pressure with mild sunny days at first but then becoming warm around ANZAC Day with temperatures in the mid to high 20’s. Trough and associated complex cut off low to develop from early 27th through till about 28th with a band of rain developing, tending to showers from 27th with colder air aloft. Showers will likely increase in evening or early 28th with approach of embedded trough in westerly/SW flow, chiefly about coasts. Strong SW winds behind low if it deepens over the East of the state and moves over VIC. Resumption of settled conditions from later 28th with lingering shower activity becoming isolated as new ridge moves in ahead of weak front with isolated to scattered showers on 30th or into early May. Due to complex nature of system rainfall figures at this point will be unreliable however a general 10-18mm with isolated 20-25mm is possible, chiefly towards the Lower SE.
Lower SE
Dominant high pressure with mild sunny days at first but then becoming warm around ANZAC Day with temperatures in the mid to high 20’s. Trough and associated complex cut off low to develop from early 27th through till about 28th with a band of rain developing, tending to showers from later 27th with colder air aloft. Showers will likely increase in evening or early 28th with approach of embedded trough in westerly/SW flow, chiefly about coasts and potentially may be heavy at times. Strong SW winds behind low if it deepens over the East of the state and moves over VIC. Resumption of settled conditions from later 28th with lingering shower activity becoming isolated as new ridge moves in ahead of weak front with isolated to scattered showers on 30th or into early May. Due to complex nature of system rainfall figures at this point will be unreliable however a general 15-25mm with isolated 30-35mm is possible, chiefly towards the SW/S coastline.
Western VIC
Dominant high pressure with mild sunny days at first but then becoming warm around ANZAC Day with temperatures in the mid to high 20’s. Trough and associated complex cut off low to develop from early 27th through till about 28th with a band of rain developing, tending to showers from later 27th with colder air aloft. Showers will likely increase in evening or early 28th with approach of embedded trough in westerly/SW flow, chiefly about the SW District with less northern penetration for the Wimmera and Mallee. Showers may be heavy at times. Strong SW winds behind low if it deepens enough over the state. Resumption of settled conditions from later 28th with lingering shower activity becoming isolated over the SW District as new ridge moves in ahead of weak front with isolated to scattered showers on 30th or into early May. Due to complex nature of system rainfall figures at this point will be unreliable however a general 12-20mm with isolated 30mm is possible, chiefly towards the Southern coastline.
Final Notes:
- Slowly entering a broadscale drying pattern about 3-4 weeks later than expected but enough moisture will remain from back of last MJO pulse to be utilised for the next system after ANZAC Day
- Cut off low with a band of rain the most likely, however strong high pressure to the east may delay system and shift it north a little though rain moths have been noted which is usually a good sign of moisture
- Cooling seas off WA combined with warming Pacific a cause for concern through winter and spring but early rainfall now will temper the dryness at least
- Frost still likely to be above average inland through winter with dominant high pressure
- Spring will feature a moderately strong positive IOD combined with El Nino, possibly a modoki El Nino with fast troughs and fronts struggling for moisture akin to 2002 but not as severe as 1994 or 2006.
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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