Summary

The Department of Primary Industries and Regional Development’s (DPIRD) Statistical Seasonal Forecast (SSF) outlook for spring, September to November, 2022 is indicating mostly neutral (40-60%) probability of above median rainfall for the majority of the South West Land Division.

• For spring, September to November, the SSF forecast is indicating mostly neutral (40-60%) probability of above median rainfall for the majority of the South West Land Division. Greater than 60% probability for part of the Central West, Lower West and western part of Great Southern forecast districts. With less than 40% probability for parts of the Central Wheatbelt and Great Southern forecast districts. The most likely decile range map is indicating decile 8-9 for parts of the Central West, Central Wheatbelt, Lower West and Great Southern forecast districts. Decile 2-3 for parts of the Central Wheatbelt, Great Southern forecast districts, and Ravensthorpe shire, and decile 4-7 elsewhere. Skill is poor to good at 50 to 75 % consistent.
• The Bureau of Meteorology’s seasonal outlook for spring, September to November 2022 is indicating 25-55% chance of exceeding median rainfall for the SWLD.  Predictive skill is poor to good (45-75% consistent). The current longer-term outlook (at time of writing) for October to December 2022 is 30-50% chance of exceeding median rainfall with mostly poor skill (45-65%).
• Temperature outlooks for spring, September to November 2022, from the Bureau indicate a 30-80% chance of above average day-time maxima, with the lower chances inland. Skill is 75-100%. For night-time minima for the SWLD, the Bureau is indicating 50- 80% chance of exceeding above average temperatures, with the higher chances along the coast. Skill is moderate to high at 55-100%.
• August rainfall was above average for the majority of the SWLD. The main rain event 16-18 August was due to a rainband linking up with a cold front. August maximum temperatures were average, with minimum temperatures mostly close to average. Widespread frost has recorded for the last three days of August. 
• The main climate driver influencing South West Land Division climate is the positive Southern Annular mode, which will have a drying influence for parts of south-west. SAM values are expected to be generally positive throughout spring and into early summer. The Indian Ocean Dipole (IOD) is currently negative and likely to remain through to November. Past negative IOD events generally increase rainfall in the eastern grainbelt.

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 DPIRD’s Seasonal Climate Information [1] pages.

For spring, September to November, the SSF forecast is indicating mostly neutral (40-60%) probability of above median rainfall for the majority of the South West Land Division. Greater than 60% probability for part of the Central West, Lower West and western part of Great Southern forecast districts. With less than 40% probability for parts of the Central Wheatbelt and Great Southern forecast districts. The most likely decile range map is indicating decile 8-9 for parts of the Central West, Central Wheatbelt, Lower West and Great Southern forecast districts. Decile 2-3 for parts of the Central Wheatbelt, Great Southern forecast districts, and Ravensthorpe shire, and decile 4-7 elsewhere. Skill is poor to good at 50 to 75 % consistent.

[2]

SSF forecast of the probability of exceeding median rainfall for September to November 2022 using data up to and including August.

[3]

Percent consistent skill of the SSF at forecasting September to November rainfall using data up to and including August.

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 seasonal outlook for spring, September to November 2022 is indicating 25-55% chance of exceeding median rainfall for the SWLD.  Predictive skill is poor to good (45-75% consistent). The current longer-term outlook (at time of writing) for October to December 2022 is 30-50% chance of exceeding median rainfall with mostly poor skill (45-65%).

Temperature outlooks for spring, September to November 2022, from the Bureau indicate a 30-80% chance of above average day-time maxima, with the lower chances inland. Skill is 75-100%. For night-time minima for the SWLD, the Bureau is indicating 50- 80% chance of exceeding above average temperatures, with the higher chances along the coast. Skill is moderate to high at 55-100%.

[5]

Rainfall outlook for spring, September to November 2022 for Western Australia from the Bureau of Meteorology

[6]

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

Looking at other rainfall forecasting models, the majority of models are indicating neutral chance of exceeding median rainfall for the SWLD for spring, September to November 2022.

[7]

Model distribution summary of 12 models outlook for September to November 2022 rainfall in the South West Land Division.

Recent climate

August rainfall was above average for the majority of the SWLD. The main rain event 16-18 August, was due to a rainband linking up with a cold front. August maximum temperatures were average, with minimum temperatures mostly close to average. Widespread frost has recorded for the last three days of August. Rainfall decile map for 1 April to 4 September 2022 shows decile 8-10 rainfall for the majority of the South West Land Division.

In August the atmospheric pressure [8] [8]was near normal over the SWLD.

Sea surface temperatures [9] have remained warmer than average around much of the Australian coastline, particularly to the north and west. The sea surface temperature outlook for September to October by the Bureau of Meteorology [10] indicates SSTs are likely to remain warmer than normal around Western Australia and the north and south-east of Australia.

A negative Indian Ocean Dipole (IOD) event continues, and model outlooks indicate it is likely to persist into late spring (November). A negative IOD increases the chances of above average winter–spring rainfall for much of Australia. Past negative IOD events generally increase rainfall in the eastern grainbelt.

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 is currently strongly positive. Values are expected to be generally positive throughout spring and into early summer. A positive SAM has a drying influence for parts of south-west and south-east Australia at this time of year.  For more information see the Bureau of Meteorology’s Climate Driver Update [11].

The El Niño–Southern Oscillation is currently neutral. The Bureau's ENSO Outlook continues at La Niña ALERT, indicating at least a 70% chance of La Niña reforming later this year. This is around triple the normal likelihood. La Niña events increase the chances of above-average rainfall for northern and eastern Australia during spring and summer. In past La Niñas, summer rainfall has been generally average in the South West Land Division.

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.