Summary

The Department of Primary Industries and Regional Development’s (DPIRD) Statistical Seasonal Forecast (SSF) outlook for July to September is neutral (40-60%) chance of exceeding median rainfall, with higher (70-80%) odds in the south and parts of the north and eastern grainbelt. The July to October 2020 outlook is also close to neutral odds.

• For July to September, the SSF is indicating neutral (40-60%) chance of exceeding median rainfall, with higher (70-80%) odds in the south and parts of the north and eastern grainbelt. The most probable rainfall decile map indicates decile 8-9 for most of the SWLD, with decile 4-7 elsewhere. Predictive skill based on June conditions is poor to moderate (50-70% consistent).
• For July to October, the SSF is close to neutral (40-60%), with higher chances of exceeding median rainfall for parts of the north, east and southern grainbelt. The most probable rainfall decile map indicates decile 8-9 for the majority of the SWLD, with decile 4-7 elsewhere. Predictive skill based on June conditions is poor to moderate (50-70% consistent).
• The Bureau of Meteorology’s current seasonal outlook is indicating close to neutral chances 45-60% of exceeding median rainfall for July to September 2020. Predictive skill is poor to good (45-75% consistent). The longer-term outlook for August to October is mostly neutral, with slightly higher chances (60-65%) of a wetter 3-months in the far north. Predictive skill is 50-75% consistent. 
• Temperature outlooks for July to September 2020, from the Bureau indicate a 65-80% chance of above average day-time maxima (skill 55-75%) and 70-80% chance of above average night-time minima (skill mostly low to very low below 55%) for the SWLD.
• June rainfall was generally average in the SWLD, and below average in the South Coastal and South East Coastal Districts.June maximum temperatures were above average to very much above average, and highest on record for the whole of Western Australia. June minimum temperatures were above average.
• Cooling Sea Surface Temperatures in the Indian Ocean north of Australia and higher than normal atmospheric pressure over Australia are currently the most dominant climate drivers. This reduces rainfall.

Three Month Outlook for the south-west of Western Australia

Statistical Seasonal Forecasting (SSF)

DPIRD’s Statistical Seasonal Forecast (SSF) system uses historical relationships between global sea surface temperature and sea level pressure with rainfall in south-west Australia to produce forecasts of rainfall for the coming months. Users can click on any station indicated on the map for location-specific forecast information from the Seasonal Climate Information [1] web page. 

For July to September, the SSF is indicating neutral 40-60% chance of exceeding median rainfall, with higher odds in the south and parts of the north and eastern grainbelt. The most probable rainfall decile map indicates decile 8-9 for the majority of the SWLD, with decile 4-7 elsewhere. Predictive skill based on June conditions is poor to moderate (50-70% consistent).

[2]

SSF forecast of the probability of exceeding median rainfall for July to September 2020 using data up to and including June.

[3]

Percent Consistent skill of the SSF at forecasting July to September rainfall using data up to and including June.

Bureau of Meteorology seasonal climate outlook

The Bureau of Meteorology's climate forecast system for monthly and seasonal climate outlooks is the Australian Community Climate Earth-System Simulator – Seasonal (ACCESS–S [4]). It is a dynamical (physics-based) forecast modelling system and is a collaboration between the Bureau of Meteorology and the UK Meteorological Office.

The Bureau of Meteorology’s current seasonal outlook is indicating close to neutral chances 45-60% of exceeding median rainfall for July to September 2020. Predictive skill is poor to good (45-75% consistent). The longer-term outlook for August to October is mostly neutral, with slightly higher chances (60-65%) of a wetter 3-months in the far north. Predictive skill is 50-75% consistent.

Temperature outlooks for July to September 2020, from the Bureau indicate a 65-80% chance of above average day-time maxima (skill 55-75%) and 70-80% chance of above average night-time minima (skill mostly low to very low below 55%) for the SWLD.

[5]

Rainfall outlook for July to September 2020 for Western Australia from the Bureau of Meteorology.

[6]

Percent Consistent skill of the Bureau of Meteorology’s outlook for July to September.

Looking at other models, the majority are indicating above chances of exceeding median rainfall for July to September.

[7]

Model distribution summary of 13 models which forecast July to September, rainfall in the South West Land Division.

SSF Forecast for July to October

For July to October, the SSF is close to neutral (40-60%), with higher chances of exceeding median rainfall for parts of the north, east and southern grainbelt. The most probable rainfall decile map indicates decile 8-9 for the majority of the SWLD, with decile 4-7 elsewhere. Predictive skill based on June conditions is poor to moderate (50-70% consistent).

[8]

SSF forecast of the probability of exceeding median rainfall for July to October 2020 using data up to and including June.

[9]

Percent Consistent skill of the SSF at forecasting July to October rainfall using data up to and including June.

Recent climate

June rainfall was generally average in the SWLD, and below average in the South Coastal and South East Coastal Districts.June maximum temperatures were above average to very much above average, and highest on record for the whole of Western Australia. June minimum temperatures were above average. Rainfall from 1 April to date has generally been below average in the SWLD.

[10]

Rainfall decile map for 1 April to 5 July 2020 in the South West Land Division.

In June, the atmospheric pressure [11] [11]was higher than normal over the SWLD.

Recent cooling of the Indian Ocean sea surface temperatures [12] (SST) north-west of Western has reduced the moisture available to weather systems passing the SWLD. The July to September 2020, SST forecast by the Bureau of Meteorology [13] indicates SSTs are likely to become warmer, north of WA.

The Southern Annular Mode (SAM), also known as the Antarctic Oscillation (AAO), describes the north–south movement of the westerly wind belt that circles Antarctica, dominating the middle to higher latitudes of the southern hemisphere. The Southern Annular Mode (SAM) is currently near neutral and is forecast to remain near neutral until the end of July. The Bureau now forecasts SAM in their Climate Driver Update [14].

The Indian Ocean Dipole (IOD) is currently neutral. Half of the international climate models surveyed by the Bureau suggest the development of a negative IOD by the September. Recent cooling in the Indian Ocean has eased outlooks for a negative IOD. A negative IOD typically means increased rainfall for far eastern grainbelt, and average elsewhere.

The El Niño–Southern Oscillation (ENSO) is currently neutral, however, the Bureau has raised their ENSO outlook to a La Nina WATCH. This is due to cooling in the tropical Pacific Ocean in recent months, and model outlooks suggest the likelihood of further cooling. A La Nina is often associated with above average rainfall over eastern, central and northern Australia. For further information, see the Bureau of Meteorology’s new Climate Driver Update [14] .

The table below gives a summary of past month and three-month South West Land Division (SWLD) climate conditions, and can indicate what is likely to occur in the near future if climate conditions follow the current pattern.

Climate Indicator