Summary

The Department of Primary Industries and Regional Development’s (DPIRD) Statistical Seasonal Forecast (SSF) outlook for autumn March to May 2021 is indicating less than 40% chance of exceeding median rainfall for Esperance and the western part of the South West Land Division (SWLD). Neutral chances elsewhere.

• For autumn, March to May 2021, the SSF is indicating less than 40% chance of exceeding median rainfall for Esperance and the western part of the South West Land Division (SWLD). Neutral chances elsewhere. The most probable rainfall decile map indicates decile 2-3 for Esperance and western and decile 4-7 elsewhere. Predictive skill based on February conditions is mostly poor to moderate (50-70% consistent).
• The Bureau of Meteorology’s seasonal outlook for autumn March to May indicated generally neutral (40-55%) chance of exceeding median rainfall for the SWLD. Predictive skill is poor to moderate (50-65% consistent). The longer term outlook for April to June has  chances below 40% for the majority of the SWLD with poor to moderate predictive skill (50-65%).
• Temperature outlooks for autumn March to May 2021, from the Bureau indicate a 60-80% chance of above average day-time maxima along the coast, with lower chances (40-60%) for interior (skill 45-55%) and 60-80% chance of above average night-time minima along the coast, with neutral chances (45-55%) for interior (skill 45-65%).
• February rainfall was above average for the South West Land Division due to tropical low early on in the month, with Jurien receiving 100 mm of rain. February maximum temperatures were below average and minimum temperatures were average. 
• Tropical lows from the north are currently the main climate driver bringing rain to the South West Land Division.

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 autumn, March to May 2021, the SSF is indicating less than 40% chance of exceeding median rainfall for Esperance and the western part of the South West Land Division (SWLD). Neutral chances elsewhere. The most probable rainfall decile map indicates decile 2-3 for Esperance and western and decile 4-7 elsewhere. Predictive skill based on February conditions is mostly poor to moderate (50-70% consistent).

[2]

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

[3]

Percent consistent skill of the SSF at forecasting autumn March to May rainfall using data up to and including February.

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 autumn March to May indicated generally neutral (40-55%) chance of exceeding median rainfall for the SWLD. Predictive skill is poor to moderate (50-65% consistent). The longer term outlook for April to June has chances below 40% for the majority of the SWLD with poor to moderate predictive skill (50-65%).

Temperature outlooks for autumn March to May 2021, from the Bureau indicate a 60-80% chance of above average day-time maxima along the coast, with lower chances (40-60%) for interior (skill 45-55%) and 60-80% chance of above average night-time minima along the coast, with neutral chances (45-55%) for interior (skill 45-65%).

[5]

Rainfall outlook for autumn March to May 2021 for Australia from the Bureau of Meteorology.

[6]

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

Looking at other rainfall forecasting models, the majority of models are suggesting above average rainfall for March to May 2021 for the South West Land Division.

[7]

Model distribution summary of 13 models (not including the SSF) which forecast March to May 2021 rainfall in the South West Land Division.

Recent climate

February rainfall was above average for the South West Land Division due to tropical low early on in the month, with Jurien receiving 100 mm of rain.  Rainfall since 1 January has been above average for the majority of the South West Land Division. February maximum temperatures were below average and minimum temperatures were average.

[8]

Rainfall decile map for the South West Land Division for 1 January to 3 March 2021

In February the atmospheric pressure [9] [9]was lower than normal over the SWLD.

Sea surface temperatures  [10] are warmer than average around much of the north, west and southeast of Australia. These warm SSTs are likely to be contributing to above-average temperature outlooks for adjacent land areas.

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 excepted to remain close to neutral throughout March. 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 [11].

The Indian Ocean Dipole (IOD) is neutral and is expected to remain neutral through autumn. IOD events are typically unable to form between April and December. This is because the monsoon trough shifts south over the tropical Indian Ocean and alters wind patterns, preventing the IOD pattern from being able to form

The current La Niña is forecast to end during autumn. La Niña typically increases the likelihood of above average rainfall across eastern Australia during early autumn.

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.