Summary

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

• For October to December 2019, the SSF is indicating less than 40% chance of exceeding median rainfall for parts of the Central West, Lower West and South West and Great Southern Districts. Neutral (40-60%) chances, elsewhere. The most probable decile range is decile 2-3 parts of the Central West, Lower West and South West and Great Southern Districts and decile 4-7 for elsewhere. See Bureau of Meteorology forecast districts [1]. Predictive skill based on September conditions is poor to good (50 -70% consistent).
• The Bureau of Meteorology’s current seasonal outlook is indicating 20-30% chance of exceeding median rainfall for October to December 2019, for the South West Land Division. Predictive skill is moderate (55-65% consistent).
• Temperature outlooks for October to December 2019, from the Bureau indicate over 80% chances of above average day-time maxima for the SWLD. Skill is very good at over 75% consistent. Minimum temperature outlooks indicate a 60-80% chance of above average night-time minima for the SWLD, with very good skill of over 75% consistent.
• September rainfall, was below average for the SWLD. September maximum temperatures were very much above average, with highest on record in some locations and minimum temperatures were above average.
• The positive Indian Ocean Dipole (IOD) is forecast to continue until the end of spring.

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 [2] web page. 

For October to December 2019, the SSF is indicating less than 40% chance of exceeding median rainfall for parts of the Central West, Lower West and South West and Great Southern Districts. Neutral (40-60%) chances, elsewhere. The most probable decile range is decile 2-3 parts of the Central West, Lower West and South West and Great Southern Districts and decile 4-7 for elsewhere. Predictive skill based on September conditions is poor to good (50 -70% consistent).

[3]

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

[4]

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

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 [5]). 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 20-30% chance of exceeding median rainfall for October to December 2019, for the South West Land Division. Predictive skill is moderate (55-65% consistent).

Temperature outlooks for October to December 2019, from the Bureau indicate over 80% chances of above average day-time maxima for the SWLD. Skill is very good at over 75% consistent. Minimum temperature outlooks indicate a 60-80% chance of above average night-time minima for the SWLD, with very good skill of over 75% consistent.

[6]

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

[7]

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

Recent climate

September rainfall,was below average for the SWLD. September maximum temperatures were very much above average, with highest on record in some locations and minimum temperatures were above average.

Rainfall since 1 April has been generally below average (decile 2-3) for the SWLD, with some northern and eastern areas tracking at average (decile 4-7). 

[8]

Rainfall decile map for 1 April to 3 October 2019 for the South West Land Division.

New soil water maps, for 10 soil types and the mean, are now available on the DPIRD website [9]. These maps are different to the pervious soil water maps created in the early part of the season as they are created using evapotranspiration of a generic wheat crop which is estimated using the FAO crop factor method. This is the same method used to calculate the crop soil water in the soil water tool [10].

In September, the atmospheric pressure [11] [11]was above normal over the SWLD.

Indian Ocean sea surface temperatures [12]  off the WA coastline have been cooler than average. The October to December SST forecast by the Bureau of Meteorology [13] indicates SSTs are likely to be warming up 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.  SAM has been negative since 1 July and is forecast to remain negative during October and November. This is largely due to a sudden warming of the stratosphere over Antarctica. A negative SAM in the spring months often brings drier than average conditions to parts of eastern mainland Australia by reducing onshore flow, but also brings wetter than average conditions to western Tasmania, but has little impact on SWLD spring rainfall.

The positive Indian Ocean Dipole (IOD) is forecast to continue until the end of spring. Typically, a positive IOD brings below average winter–spring rainfall to southern and central Australia, above average daytime temperatures for the southern two-thirds of Australia, and increased fire risk in the southeast. IOD events typically decay towards the end of the year, having little influence on Australian climate from December to April. 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 [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.