Weekly update - 3 September 2020

During the week ending 2 September 2020 westerly airstreams and cold fronts moved over far southern Australia and moist onshore flow developed on the north-eastern coast, bringing moderate rainfall to isolated areas. Rainfall totals of between 10 and 50 millimetres were recorded across parts of north-eastern Queensland, eastern Victoria, south-western Western Australia, and northern and western Tasmania. Rainfall in excess of 50 millimetres was recorded across parts western Tasmania.

In Australia’s cropping regions, rainfall totals of between 5 and 10 millimetres were recorded across parts of the western and central Western Australian wheat belt. Little to no rainfall was recorded across remaining cropping regions during the week ending 2 September 2020.

These drier conditions are likely to be a concern for yield prospect and pasture growth in regions that recorded below average rainfall during winter and have low soil moisture reserves. In contrast these drier conditions are likely to benefit production and alleviate water logging across parts of New South Wales and Victoria which recorded substantial winter rainfall.

Rainfall for the week ending 2 September 2020

©Commonwealth of Australia 2020, Australian Bureau of Meteorology - Issued: 01/09/2020

Note: The rainfall analyses and associated maps utilise data contained in the Bureau of Meteorology climate database, the Australian Data Archive for Meteorology (ADAM). The analyses are initially produced automatically from real-time data with limited quality control. They are intended to provide a general overview of rainfall across Australia as quickly as possible after the observations are received. For further information go to http://www.bom.gov.au/climate/rainfall/

August 2020 was warmer than average nationally, with a national mean temperature of 1.38°C above average.

Maximum temperatures for August were above average across the northern half of Australia and much of Western Australia, with very much above average to highest on record temperatures across north-western Australia.

Minimum temperatures were above average across large areas of south-western, central and northern Australia and scattered parts of eastern Australia. While warmer than normal temperatures during early winter accelerated crop and pasture development boosting yield potential, closer to average August temperatures have slowed grow rates and decreased the risk of crops now flowering within the frost window in many southern cropping regions.

Maximum temperature deciles for August 2020

©Commonwealth of Australia 2020, Australian Bureau of Meteorology - Issued: 01/09/2020

Minimum temperature deciles for August 2020

©Commonwealth of Australia 2020, Australian Bureau of Meteorology - Issued: 01/09/2020

Note: Maximum and minimum temperatures for August 2020 compared with temperature recorded for that period during the historical record (1900 to present). For further information go to: http://www.bom.gov.au/jsp/awap/temp/index.jsp.

August 2020 rainfall was above average to extremely high across parts of central, north-eastern, western and south-eastern Australia, with isolated areas of below average rainfall across parts of south-western Australia and generally average rainfall across the remainder of Australia.

August 2020 rainfall totals were generally average across most cropping regions. Although rainfall totals were low during the first week of August, this was followed by heavy rainfall that was beneficial for most cropping regions. This rainfall increased crop yield potential and pasture growth in areas that had been doing well and provided a timely boost to areas that had started to dry out following low rainfall in July.

During the remainder of the month moderate rainfall was recorded across cropping regions in parts of New South Wales, south-eastern Queensland, southern Victoria, South Australia and Western Australia. In parts of Victoria, South Australia and Western Australia that generally recorded average or lower August rainfall, these falls provided a boost to soil moisture and stabilised crop yield potential and pasture growth rates.

Rainfall percentiles for August 2020

Source: Bureau of Meteorology

Note: Rainfall for August  2020 is compared with rainfall recorded for that period during the historical record (1900 to present). For further information, go to http://www.bom.gov.au/jsp/awap/

Winter 2020 rainfall was below average nationally, with mixed conditions throughout the season. The season began with below average to average rainfall across Australia’s cropping regions. Despite low rainfall at the start of winter, plant growth across New South Wales, Victoria and South Australia was supported by soil moisture that had built up during autumn. Substantial falls later in June supported ongoing crop and pasture development in these regions and initiated further planting and growth in Queensland and Western Australia.

Similarly, although July started with low rainfall, most cropping regions had average to above average levels of root-zone soil moisture from June rainfall. Substantial rainfall across much of eastern New South Wales and south-western Western Australia for the remainder of July supported favourable crop yield potential and pasture growth in these areas. In contrast, cropping regions in Queensland, Victoria and South Australia generally recorded low July rainfall.

August rainfall was generally average across most cropping regions. Substantial rainfall during August increased crop yield potential and pasture growth rates in areas that has been doing well and provided a boost of soil moisture and stabilised crop yield potential and pasture growth rates in areas that recorded generally below average rainfall through winter. 

Winter 2020 rainfall was well below average to average across most cropping regions in New South Wales, Queensland, Victoria, South Australia and Western Australia. Severely deficient to extremely low rainfall was recorded in cropping regions across parts of the west and central Western Australian wheat belt and isolated parts of Victoria and South Australia. In contrast, above average to well above average rainfall was recorded in cropping regions across parts of central New South Wales.

Rainfall percentiles for winter 2020 (1 June 2020 to 31 August 2020)

Source: Bureau of Meteorology

Note: Rainfall for June 2020 to August 2020 is compared with rainfall recorded for that period during the historical record (1900 to present). For further information, go to http://www.bom.gov.au/jsp/awap/

Australia’s major climate drivers were neutral during winter, allowing minor drivers, such as the Southern Annual Mode (SAM) to have a stronger influence on Australia’s climate. The SAM refers to the north-south shift of the band of rain-bearing westerly winds and weather systems in the Southern Ocean compared to the usual position. SAM fluctuated during winter, displayed below with the Antarctic Oscillation (AAO) Index. There was a prolonged positive SAM event in June, followed by several short negative SAM events in July, a short positive SAM event at the beginning of August and a negative SAM event for the remainder of August.

When SAM is positive during winter, the band of westerly winds is further south than normal, allowing for more east coast lows to develop and more rainfall in the east, however it also reduces rainfall across parts of the far south and reduces snow in alpine areas. In contrast, when SAM is negative during winter, the band of westerly winds is further north than normal, allowing rain bearing systems to move over southern Australia, more rainfall in the south-west and south-east and more snow in alpine areas. However, a negative SAM event in winter also typically increases the flow of dry air from central Australia to the east, reducing rainfall in parts of eastern Australia.

Antarctic Oscillation (AAO) Index for 120 days ending 31 August 2020

Source: NOAA National Weather Service

Note: The Antarctic Oscillation Index (AAO) is an index related to the strength and phase of the SAM. The AAO index has been standardised by the standard deviation of the monthly AAO index from 1979-2000. For further information, go to https://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/ao_index.html

Upper layer soil moisture in August 2020 was above average to extremely high across large parts of central, western, north-eastern and south-eastern Australia for this time of year, largely reflecting rainfall patterns during the month. Soil moisture was average across parts of northern, eastern and south-western Australia. Upper layer soil moisture is important at the beginning of the summer cropping season as plant germination and establishment will utilise this moisture.

Relative upper layer soil moisture was above average to well above average for this time of year across cropping regions in parts of central New South Wales, north-western Victoria, eastern and western South Australia and the northern and southern Western Australian wheat belt. Upper layer soil moisture was generally average across remaining cropping regions, with well below average to below average soil moisture in parts of the central Western Australia wheat belt.

Modelled upper layer soil moisture for August 2020

Source: Bureau of Meteorology (Australian Water Resources Assessment Landscape model)

Note: This map shows the levels of modelled upper layer soil moisture (0 to 10 centimetres) during August  2020. This map shows how modelled soil conditions during August  2020 compare with August  conditions modelled over the reference period (1911 to 2016). Dark blue areas on the maps were much wetter in August  2020 than during the reference period. The dark red areas were much drier than during the reference period. The bulk of plant roots occur in the top 20 centimetres of the soil profile. Soil moisture in the upper layer of the soil profile is therefore useful indicator of the availability of water, particularly for germinating seed.

Lower layer soil moisture for August  2020 was above average to extremely high across large areas of central and south-eastern Australia and scattered parts of central-western and north-eastern Australia for this time of year. In contrast, it was extremely low to below average across parts of far northern and southern Australia, and inland north-eastern Australia.

In cropping regions, lower layer soil moisture was average to above average for much of New South Wales, central and southern Queensland, Victoria, and parts of eastern South Australia and the northern and southern Western Australia wheat belt. Lower layer soil moisture was well above average in cropping regions across parts of central New South Wales, south-eastern Queensland and the northern Western Australia wheat belt.

In contrast, relative lower layer soil moisture was extremely low to well below average for cropping regions across much of South Australia and parts of northern Queensland, south-western Victoria and the central Western Australia wheat belt. Production outcomes in cropping regions with below average or worse lower layer soil moisture, particularly south-western Victoria, western and central South Australia and the central Western Australia wheat belt, will be heavily reliant on rainfall during early spring.

Modelled lower layer soil moisture for August 2020

Source: Bureau of Meteorology (Australian Water Resources Assessment Landscape model)

Note: This map shows the levels of modelled lower layer soil moisture (10 to 100 centimetres) during August  2020. This map shows how modelled soil conditions during August  2020 compare with November conditions modelled over the reference period (1911 to 2016). Dark blue areas on the maps were much wetter in August  2020 than during the reference period. The dark red areas were much drier than during the reference period. The bulk of plant roots occur in the top 20 centimetres of the soil profile. The lower layer soil moisture is a larger, deeper store that is slower to respond to rainfall and tends to reflect accumulated rainfall events over longer time periods.

Below average winter rainfall and low soil moisture limited pasture production across large areas of South Australia and Western Australia. In contrast, close to average rainfall and ample stored soil moisture benefited pasture production across much of eastern Australia.

For the 3 months to August 2020, modelled pasture growth was extremely low to well below average across large areas of Western Australia and South Australia, and scattered areas of southern Queensland, western New South Wales and southern Victoria. In contrast, modelled pasture growth was well above average to extremely high across central Victoria, eastern New South Wales, eastern Queensland, and parts of south-eastern South Australia.

Modelled pasture growth and soil moisture levels were well below average over winter across large areas of South Australia and Western Australia. As a result, livestock producers in these regions will be heavily reliant on spring rainfall and supplementary feed to maintain current stock numbers. In contrast, average to above average pasture production across much of New South Wales, Victoria, and southern and eastern Queensland is likely to enable farmers to rebuild stock numbers and provide opportunities to replenish fodder supplies.

Relative pasture growth for winter 2020 (1 June 2020 to 31 August 2020)

Notes: AussieGRASS pasture growth estimates are relative to the long-term record and shown in percentiles. Percentiles rank data on a scale of zero to 100. This analysis ranks pasture growth for the selected period against average pasture growth for the long-term record (1957 to 2016). Pasture growth is modelled at 5km2 grid cells.
Source: Queensland Department of Science, Information Technology and Innovation

The northern rainfall onset outlook provides an indication of whether the first significant rains after the dry season are likely to be earlier or later than normal. The onset occurs when the total rainfall after 1 September reaches 50 millimetres, this is considered approximately the amount of rainfall required to stimulate plant growth. 

The latest northern rainfall onset outlook released by the Bureau of Meteorology on 27 August 2020 suggests that an early northern rainfall onset for the 2020-21 season is more likely across most of northern Australia. The chance of an early rainfall onset is greater than 70% for the south of the Northern Territory, central and western Queensland, and isolated parts of the central-eastern Queensland coast. This would contrast the past two northern wet seasons that had later than normal onsets, reducing the length of the summer growing season and the recharge of water storages. An early onset of the 2020­-21 northern wet season is likely to boost soil moisture and water storages, and benefit summer crop production and northern pasture growth.

The likelihood of an earlier than normal northern rainfall onset is higher across most of northern Australia due to an increased chances of a La Niña and a negative Indian Ocean Dipole (IOD) developing in spring.

Chance of early northern rainfall onset

Cold fronts and troughs are expected to bring rainfall to limited parts of far eastern and southern Australia, with high-pressure systems expected to prevent rain bearing systems from moving further over Australia during the week. Rainfall totals of between 10 and 50 millimetres are forecast for parts of north-eastern New South Wales, south-western Western Australia, Tasmania and isolated parts of north-eastern Queensland, southern Victoria and south-eastern South Australia. Rainfall in excess of 50 millimetres is expected across western Tasmania.

In cropping regions, rainfall of between 5 and 25 millimetres is expected across parts of the western and southern Western Australian wheat belt. Rainfall totals of between 5 and 10 millimetres is expected in cropping regions across parts of eastern New South Wales and south-western Victoria. Little to no rainfall is expected across cropping regions in Queensland and South Australia and remaining cropping regions in New South Wales, Victoria and Western Australia during the next eight days.

Total forecast rainfall (mm) for the period 3 September 2020 to 10 September 2020

©Commonwealth of Australia 2020, Australian Bureau of Meteorology - Issued: 3/09/2020

Note: This rainfall forecast is produced from computer models. As the model outputs are not altered by weather forecasters, it is important to check local forecasts and warnings issued by the Bureau of Meteorology.

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