Seasonal Climate Outlook

Summary

The Department of Primary Industries and Regional Development’s (DPIRD) Statistical Seasonal Forecast (SSF) outlook for May to July 2021, is indicating 30-60% chance of exceeding median rainfall for the majority of the South West Land Division (SWLD). The SSF longer outlook for May to October is mostly neutral at 40-60% chance.

• For May to July 2021, the SSF is indicating 30-60% chance of exceeding median rainfall for the majority of the South West Land Division (SWLD). The most probable rainfall decile map indicates decile 2-3 for the SWLD. Predictive skill based on April conditions is mostly moderate to good (60-100% consistent).
• The longer lead SSF forecast of May to October 2021 rainfall is indicating mostly neutral (40-60%) chance of exceeding median rainfall for the majority of the SWLD, with wetter chances in south eastern parts. The most likely decile range map is indicating decile 4-7 for the SWLD. Skill is moderate to good at 60 to 100 % consistent.
• The Bureau of Meteorology’s seasonal outlook for May to July indicated 35-55% chance of exceeding median rainfall for the SWLD. Predictive skill is poor to good (55-75% consistent). The longer term outlook for winter, June to August is mostly neutral (40-65%) chance for the majority of the SWLD with poor to moderate predictive skill (45-65%).
• Temperature outlooks from the Bureau for May to July 2021 indicate a 65-80% chance of above average daytime maxima (skill 75-100%), and 60-80% chance of exceeding above average night time minima for most of the SWLD (skill 45-65%).
• April rainfall was very much above average for SWLD due to tropical cyclone Seroja.  April maximum and minimum temperatures were above average. 
• The El Niño–Southern Oscillation (ENSO) is neutral. For SWLD, the current climate influence is warmer Sea Surface Temperatures in the Indian Ocean.

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 web page.

For May to July 2021, the SSF is indicating 30-60% chance of exceeding median rainfall for the majority of the South West Land Division (SWLD). The most probable rainfall decile map indicates decile 2-3 for the SWLD. Predictive skill based on April conditions is mostly moderate to good (60-100% consistent).

SSF forecast of the probability of exceeding median rainfall for May to July 2021 using data up to and including April.

Percent consistent skill of the SSF at forecasting May to July rainfall using data up to and including April.

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). It is a dynamic (physics-based) forecast modelling system and is a collaboration between the Bureau of Meteorology and the Meteorological Office of the United Kingdom.

The Bureau of Meteorology’s seasonal outlook for May to July indicated 35-55% chance of exceeding median rainfall for the SWLD. Predictive skill is poor to good (55-75% consistent). The longer term outlook for winter, June to August, is mostly neutral (40-65%) chance of exceeding median rainfall for the majority of the SWLD, with poor to moderate predictive skill (45-65%).

Temperature outlooks for May to July 2021 from the Bureau indicate a 65-80% chance of above average daytime maxima (skill 75-100%), and 60-80% chance of exceeding above average night time minima for most of the SWLD (skill 45-65%).

Rainfall outlook for May to July 2021 for Western Australia from the Bureau of Meteorology

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

Looking at other rainfall forecasting models, the majority of models are suggesting neutral to better chances of receiving above median rainfall for the SWLD for May to July 2021.

Model distribution summary of 12 models (not including the SSF) which forecast May to July 2021 rainfall in the South West Land Division.

SSF forecast for May to October

The SSF forecast for SWLD May to October 2021 rainfall is indicating mostly neutral (40-60%) chance of exceeding median rainfall for the majority of the SWLD, with wetter chances in south eastern parts. The most likely decile range map is indicating decile 4-7 for the SWLD. Skill is moderate to good at 60-100% consistent.

SSF forecast of the probability of exceeding median rainfall for May to October 2021 using data up to and including April.

Percent consistent skill of the SSF at forecasting May to October rainfall using data up to and including April.

Recent climate

April rainfall was very much above average for SWLD due to tropical cyclone Seroja. April maximum and minimum temperatures were above average. Decile rainfall to date map from 1 April to 4 May indicates decile 8-10 rainfall for most of the SWLD.

Rainfall decile map for the South West Land Division for 1 April to 4 May 2021.

In April, the atmospheric pressure was near normal over the SWLD.

Sea surface temperatures are near normal to the north west of Australia due to the activity of tropical cyclones Seroja and Odette taking the warm moisture out of the ocean in April. The May to July 2021 SST forecast by the Bureau of Meteorology indicates SSTs are likely to warm 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 is currently neutral and expected to remain neutral until mid May. A neutral SAM has little influence on Australian rainfall. During autumn, SAM has less influence on rainfall than during other times of the year. For more information. see the Bureau of Meteorology’s Climate Driver Update.

The Indian Ocean Dipole (IOD) is currently neutral. Outlooks indicate the IOD is most likely to remain neutral for the remainder of autumn and early winter. Two models indicate a positive IOD (positive means less rainfall for eastern grainbelt) in May, and one model indicates a negative IOD in September (meaning more rainfall for eastern grainbelt). However, it should be noted that model accuracy is low at this time of the year, so the current outlooks should be viewed with caution.

The El Niño–Southern Oscillation (ENSO) is neutral. Model outlooks predict neutral ENSO conditions through to the start of spring. The table below summarises South West Land Division (SWLD) climate conditions of the past month and three months, and can indicate what is likely to occur in the near future if climate conditions follow the current pattern.