Summary

The Department of Primary Industries and Regional Development’s (DPIRD) Statistical Seasonal Forecast (SSF) outlook for August to October 2019 is indicating 20-60% chance of exceeding median rainfall for the majority of the South West land Division (SWLD).

• For August to October 2019, the SSF is indicating 20-60% chance of exceeding median rainfall for the South West Land Division. With higher chances (above 60%) for parts of Esperance. Predictive skill based on July conditions is poor to good (50 -75% consistent).
• The Bureau of Meteorology’s current seasonal outlook is indicating 30-45% chance of exceeding median rainfall for August to October 2019, for the South West Land Division. Predictive skill is moderate to good (55-75% consistent).
• Temperature outlooks for August to October 2019, from the Bureau indicate a 60-80% chance of above average day-time maxima for the SWLD. Skill is poor to moderate at 50-65% consistent. Minimum temperature outlooks indicate a 50-65% chance of above average night-time minima for the SWLD, with skill poor 45-55% consistent. 
• July rainfall was below average to average for eastern parts. July maximum temperatures were above average and minimum temperatures were average to above average.
• The majority of other models indicate dry condiitons excepted for August to October with positive Indian Ocean Dipole (IOD) conditions are forecast until November. With cloud-free days and nights expected, there is an increase of frost in susceptible areas. 

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 August to October 2019, the SSF is indicating 20-60% chance of exceeding median rainfall for the South West Land Division. With higher chances (above 60%) for parts of Esperance. The most probable decile range varies across the SWLD. Predictive skill based on July conditions is poor to good (50 -75% consistent).

[2]

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

[3]

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

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 30-45% chance of exceeding median rainfall for August to October 2019, for the South West Land Division. Predictive skill is moderate to good (55-75% consistent). 

Temperature outlooks for August to October 2019, from the Bureau indicate a 60-80% chance of above average day-time maxima for the SWLD. Skill is poor to moderate at 50-65% consistent. Minimum temperature outlooks indicate a 50-65% chance of above average night-time minima for the SWLD, with skill poor 45-55% consistent. With more cloud-free days and nights expected, there is an increase of frost in susceptible areas.

[5]

Rainfall outlook for August to October 2019 for Western Australia from the Bureau of Meteorology.

[6]

Percent Consistent skill of the Bureau of Meteorology’s outlook for August to October.

Recent climate

July rainfall was below average to average in eastern parts.July maximum temperatures were above average and minimum temperatures were average to above average. Rainfall sice 1 April has been generally below deicle 5 for the SWLD with wetter areas in parts of the Central West and Central Wheatbelt (Bureau of Meteorology forecast districts [7]).

[8]

Rainfall decile map for 1 April to 5 August 2019 for the South West Land Division.

In July, the atmospheric pressure [9] [9]was above normal over the SWLD suppressing cloud formation and to keep cold fronts further south than normal.

Indian Ocean sea surface temperatures [10]  off the WA coastline have been cooler than average. The August to October SST forecast by the Bureau of Meteorology [11] indicates SSTs are likely to be near normal 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. For July, SAM was negative and is forecast to switch to positive, meaning the belt of westerly winds contracts towards Antarctica. This results in weaker than normal westerly winds and higher pressures over southern Australia, restricting the passage of cold fronts inland. Generally, this means that there are fewer rain events in winter for southern Australia. 

Indian Ocean Dipole (IOD) index values have been near the threshold for a positive IOD in recent weeks. The broad pattern across the Indian Ocean has remained positive IOD-like, with warmer than average sea surface temperatures near Africa and cooler than average waters to Australia's northwest. Most climate models surveyed forecast positive IOD conditions for the remainder of winter and spring. Typically, a positive IOD brings below average winter–spring rainfall, above average temperatures, and an earlier start to the fire season for southern and central Australia.

The El Niño–Southern Oscillation (ENSO) is expected to remain neutral for the remainder of 2019. For further information, see the Bureau of Meteorology’s ENSO Wrap Up [12].

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.