Weekly update - 13 August 2020

During the week ending 12 August 2020 complex low-pressure systems moved west to east over Australia, bringing substantial rainfall to much of central and southern Australia, with heavy falls, strong winds and flooding in some areas. Rainfall totals of between 10 and 50 millimetres were recorded across much of New South Wales, southern Queensland, Victoria, South Australia, south-western Western Australia and parts of the far south of the Northern Territory. Rainfall in excess of 50 millimetres was recorded across parts of south-eastern New South Wales, and isolated parts of south-western Queensland and the south-western coastline of Western Australia.

In Australia’s cropping regions, rainfall totals of between 10 and 50 millimetres were recorded across much of New South Wales, southern Queensland, Victoria, South Australia and Western Australia, excluding parts of the southern wheat belt. Lower rainfall totals between 1 and 10 millimetres were recorded across cropping regions in the remainder of northern Queensland and southern Western Australia during the week ending 12 July 2020.

These significant falls will be beneficial for most southern cropping regions, increasing crop yield potential and pasture growth rates in areas that had been doing well, particularly parts of New South Wales, and providing a timely boost of soil moisture and stabilising crop yield potential and pasture growth rates to areas that had started to dry out following low July rainfall.

Rainfall for the week ending 12 August 2020

©Commonwealth of Australia 2020, Australian Bureau of Meteorology - Issued: 12/08/2020

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/

Above average rainfall and warmer-than-average daytime temperatures during the 2020 winter have seen crops develop two to three weeks ahead of schedule across large areas of southern Australia, which is increasing the risk of frost damage this season. There is the potential that severe frosts (minimum temperatures below -2°C) in September could cause some damage to barley and canola crops in particular.

For the week ending 11 August 2020, lowest minimum temperatures were -6°C to 0°C across most cropping regions in New South Wales, Victoria and eastern South Australia. Severe frost events on the 6th and 7th of August across parts of New South Wales may have impacted early planted canola and cereal crops that are currently heading and flowering. However, due to the timing of this event, the potential impact on final yield outcomes is likely to be lessened as most crops have not begun grain filling and recent substantial rainfall will more than likely make up for potential losses in yield.

Lowest minimum temperature (°C) for the week ending 11 August 2020

©Commonwealth of Australia 2020, Australian Bureau of Meteorology - Issued: 12/08/2020

Long term rainfall deficiencies are monitored to help determine whether rainfall conditions have been favourable or unfavourable for catchment hydrology and what this means for irrigated agricultural production. If below-average rainfall continues over months and years, water storages are depleted. Dam and reservoir levels drop, river and groundwater levels fall (some may dry up completely). Longer-term rainfall deficiencies that encompass the majorly of the main growing season within an area also give an early indication if that area is entering a period of drought, but where the period analyzed extends into previous production years it is not always the best indicator that a drought has finished and more normal production conditions have commenced.

Within season rainfall deficiencies give an early indication if we are entering a period of drought, and what is likely to happen to soil moisture and crop and pasture production. Short term deficiencies can be offset by stored soil moisture or alleviated by one or two substantial rainfall events, and so are not always a reliable indicator of a protracted downturn in crop and pasture production.

Below average rainfall across much of southern Australia during July 2020 increased short term deficiencies across south-western Australia. Reflecting this lower than average rainfall, root-zone soil moisture remains below average across large areas of southern Australia.

Across southern Western Australia the most severe short-term deficiencies have emerged in areas largely outside of the major agricultural regions and as yet have had little impact on current crop development. While, rainfall totals for the growing season-to-date (1 April to 12 August) have been sufficient to maintain average or slightly lower crop yield potential, they have not been sufficient to build any meaningfully soil water store. As temperatures begin to increase into spring this lack of stored soil moisture is likely to impact final yield potential if timely and sufficient rainfall does not eventuate during the remainder of August and September.

Rainfall deficiencies for the 4-month period 1 April 2020 to 31 July 2020

©Commonwealth of Australia 2020, Australia Bureau of Meteorology Issued: 06/08/2020

Rainfall total for the southern wet season to date 1 April 2020 to 12 August 2020

©Commonwealth of Australia 2020, Australia Bureau of Meteorology Issued: 12/08/2020

Following a generally favourable winter cropping season in Australia, there is interest in how the remainder of winter and early spring may pan out. To gain some insight it is important to look at the major climate drivers—the Indian Ocean Dipole (IOD) and the El Nino Southern Oscillation (ENSO)—that influence winter and spring rainfall across southern Australia. If La Niña conditions were to eventuate as forecast by a range of international climate models, this key climate driver would become the major influencing factor for spring rainfall outlook across eastern Australia.

The Bureau of Meteorology’s ENSO Outlook remains at La Niña WATCH. This means that while the El Niño-Southern Oscillation is currently neutral, the chance of a La Niña forming during the southern hemisphere spring has increased to around 50% - twice the normal likelihood.

A La Niña WATCH is not a guarantee that a La Niña will occur; it is an indication that some of the typical precursors of an event are in place. The potential impact of a La Niña event on agricultural production across southern Australia decreases if the event forms during spring or summer, as it has a shorter time to strengthen and will persist for a shorter time before it decays in late summer. If a La Niña event was to eventuate during spring, it is unlikely to create more favourable seasonal conditions for winter crop production than those embodied in the ABARES June 2020 edition of Agricultural commodities.

Cooling in the tropical Pacific Ocean has continued and has extended further into the central Pacific. Five of the surveyed international climate models suggest the cooling will exceed La Niña thresholds in spring.

Difference from average sea surface temperature observations 27 July to 2 August 2020

While sea surface temperatures suggest a La Niña-like pattern could emerge in spring, the Southern Oscillation Index (SOI) and the trade winds are consistent with neutral ENSO conditions, indicating that the atmosphere has not linked up with the cooling Pacific Ocean patterns. Sustained SOI values above 7 typically indicate La Niña and the latest 30-day SOI value was 3.2 and the latest 90-day value was -0.4.

30-day Southern Oscillation Index (SOI) values ending 2 August 2020

The El Niño–Southern Oscillation (ENSO) is currently neutral and likely to remain neutral for the remainder of winter and early spring. It is expected that further cooling in the central and eastern tropical Pacific Ocean will occur and a La Niña-like pattern could emerge, potentially contributing to the above average late winter and spring rainfall outlook for parts of Australia.

Monthly sea surface temperature anomalies for NINO3.4 region

The Indian Ocean Dipole (IOD) is currently neutral and the Bureau’s model suggests it is likely to remain neutral. Three of the six international models surveyed suggest a negative IOD could develop and persist through spring. A negative IOD typically brings above average rainfall to southern Australia during spring.

Monthly sea surface temperature anomalies for IOD region

The Southern Annular Mode (SAM) is currently positive and expected to remain positive for August. 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. When SAM is positive during winter, this band of westerly winds is further south than normal. This allows 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.

Southern Annular Mode (SAM) daily index

These climate outlooks are generated by ACCESS–S (Australian Community Climate Earth-System Simulator–Seasonal). ACCESS–S is the Bureau of Meteorology's dynamical (physics-based) weather and climate model used for monthly, seasonal and longer-lead climate outlooks.

For further information, go to http://www.bom.gov.au/climate/ahead/about/

The latest rainfall outlook released by the Bureau of Meteorology suggests above average rainfall is likely for much of central and eastern Australia and average rainfall is likely for much of Western Australia during September. The rainfall outlook for September to November 2020 suggests that wetter than average conditions are likely for much of the eastern half of Australia and drier than average conditions are likely for parts of north-western Australia. There are roughly equal chances of a wetter or drier than average three months across the remainder of Australia (Bureau of Meteorology ‘National Climate Outlook’, 6 August 2019). Bureau of Meteorology rainfall outlooks for September and November have greater than 65% past accuracy across most of the eastern half of Australia and lower accuracy, less than 55%, across much of north-western Australia.

Chance of exceeding the median rainfall September to November 2020

©Commonwealth of Australia 2020, Australia Bureau of Meteorology Issued: 06/08/2020

The outlook for September 2020 suggests that there is a 75% chance of rainfall totals between 10 and 50 millimetres across south-eastern and far southern Australia. Rainfall totals in excess of 50 millimetres are likely across isolated parts of south-eastern Australia, far south-western Australia and much of Tasmania.

There is a high chance of recording close to average September rainfall totals across most agricultural regions, with the exception of south-western Australia. These totals are likely to support above average crop yield potentials across New South Wales and Victoria, and parts of South Australia. In cropping regions there is a 75% chance of rainfall totals between 10 and 50 millimetres across much of New South Wales, Victoria and South Australia. In Queensland and Western Australia there is a 75% chance of rainfall totals between 10 and 25 millimetres across most cropping regions, with rainfall totals up to 50 millimetres across parts of south-east Queensland and the far south and west of Western Australian wheat belt for September 2020.

Rainfall totals that have a 75% chance of occurring September 2020

©Commonwealth of Australia 2020, Australia Bureau of Meteorology Issued: 06/08/2020

The outlook for September to November 2020 suggests that there is a 75% chance of rainfall totals between 50 and 200 millimetres across much of eastern Australia and parts of south-western and northern Australia. Lower rainfall totals between 25 and 50 millimetres are likely across much of central Australia and the remainder of northern Australia. Rainfall totals in excess of 200 millimetres are likely across isolated parts of north-eastern New South Wales, alpine regions in south-eastern Australia and western Tasmania.

These totals indicate that the northern rainfall onset may occur in parts of central and eastern Queensland and the north of the Northern Territory during October and November. 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. 

In many areas where soil moisture is close to average to above average for this time of year, there is a high chance of recording September to November rainfall totals sufficient to sustain above average crop and pasture production through spring. In cropping regions, there is a 75% chance of receiving between 100 and 200 millimetres across most of New South Wales, Queensland and southern Victoria. Rainfall totals between 25 and 100 millimetres are likely across cropping regions in south-western New South Wales, far northern Queensland, northern Victoria, and much of South Australia and Western Australia between September and November 2020.

With the exception of parts of Western Australia these high chance expected rainfall totals are equivalent to the seasonal median between (1990 and 2012) and represent an excellent finish to the 2020 winter growing season across southern Australia and start of the wet season across northern Australia. 

Rainfall totals that have a 75% chance of occurring September to November 2020

©Commonwealth of Australia 2020, Australia Bureau of Meteorology Issued: 06/08/2020

The temperature outlook for September to November 2020 indicates that temperatures are likely to be between 1°C to 2°C above the 1990-2012 average across parts of northern Australia during the daytime and much of northern and north-eastern Australia during the night-time. Average (- 1°C to 1°C) daytime and night-time temperatures are likely for the remainder of the country (Bureau of Meteorology ‘National Climate Outlook’, 6 August 2020).

Predicted maximum temperature anomaly for September to November 2020

Predicted minimum temperature anomaly for September to November 2020

Low-pressure systems and associated troughs and fronts are expected to bring rainfall to parts of south-eastern and south-western Australia. High-pressure systems are expected to prevent rain bearing systems from moving over the remainder of Australia. Rainfall totals of between 10 and 50 millimetres are forecast for much of eastern New South Wales and Victoria, and parts of south-eastern Queensland, south-eastern South Australia, south-western Western Australia and central Tasmania. Rainfall in excess of 50 millimetres is expected for alpine regions in New South Wales and Victoria, and much of Tasmania.

In cropping regions, rainfall of between 10 and 50 millimetres is expected across much of New South Wales, Victoria and South Australia, and parts of south-eastern Queensland and the far south and west of the Western Australia wheat belt. Rainfall totals of between 5 and 10 millimetres are expected across the remainder of cropping regions in Western Australia and parts of southern Queensland during the next eight days.

Total forecast rainfall (mm) for the period 13 August 2020 to 20 August 2020

©Commonwealth of Australia 2020, Australian Bureau of Meteorology - Issued: 13/08/2020

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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