Summary

The Department of Primary Industries and Regional Development’s (DPIRD) Statistical Seasonal Forecast (SSF) outlook for February to April 2021 is indicating greater than 40% chance of exceeding median rainfall for the eastern parts of the South West Land Division.

• For February to April 2021, the SSF is indicating greater than 60% chance of exceeding median rainfall for the eastern part of the Central West, parts of the Great Southern and South East Coastal forecast Districts of the South West Land Division (SWLD). Less than 60% chance elsewhere.  The most probable rainfall decile map indicates decile 8-10 for eastern parts and South East Coastal forecast District in the SWLD and decile 2-7 elsewhere. Predictive skill based on January conditions is mostly poor to moderate (50-70% consistent).
• The Bureau of Meteorology’s seasonal outlook for February to April 2021 indicated 45-70% of exceeding median rainfall for the SWLD, with the higher chances for the South Coastal and South East Coastal forecast Districts. Predictive skill is poor to good (45-75% consistent). The autumn outlook, March to May has similar chances with predictive skill poor to moderate (45-65%).
• Temperature outlooks for February to April 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 45-65%) and 70-80% chance of above average night-time minima (skill 55-75%) for the SWLD.
• January rainfall was generally average for the South West Land Division. In January the western coast of the SWLD saw above average maximum and minimum temperatures, but the Southeast Coastal forecast district had below average maximum temperatures. 
• Warmer than normal Sea Surface Temperatures off the Western Australian coast, together with a positive SAM are currently the main climate drivers influencing SWLD climate. Warmer SSTs are likely to be contributing to above-average rainfall. A positive SAM means westerly winds are further south than normal and this may increase temperatures in the SWLD.

Three Month Outlook for the South West Land Division

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 February to April 2021, the SSF is indicating greater than 60% chance of exceeding median rainfall for the eastern part of the Central West, parts of the Great Southern and South East Coastal forecast Districts of the South West Land Division (SWLD). Less than 60% chance elsewhere.  The most probable rainfall decile map indicates decile 8-10 for eastern parts and South East Coastal forecast District in the SWLD and decile 2-7 elsewhere. Predictive skill based on January conditions is mostly poor to moderate (50-70% consistent).

[2]

SSF forecast of the probability of exceeding median rainfall for February to April 2021 using data up to and including January. Indicating above 40% chance of exceeding median rainfall for the eastern part of the South West Land Division.

[3]

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

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 February to April 2021 indicated 45-70% of exceeding median rainfall for the SWLD, with the higher chances for the South Coastal and South East Coastal forecast Districts. Predictive skill is poor to good (45-75% consistent). The autumn outlook, March to May has similar chances with predictive skill is poor to moderate (45-65%).

Temperature outlooks for February to April 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 45-65%) and 70-80% chance of above average night-time minima (skill 55-75%) for the SWLD. These outlooks can be found at http://www.bom.gov.au/climate/ahead/outlooks/archive/20210128-outlook.shtml [5].

[6]

Rainfall outlook for February to April 2021 for Australia from the Bureau of Meteorology.

[7]

Percent Consistent skill of the Bureau of Meteorology’s outlook for February to April indicating 45 to 75 percent consistent skill over the SWLD.

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

[8]

Model distribution summary of 13 models (not including the SSF) which forecast February to April 2021 rainfall in the South West Land Division. Higher chances of neutral to above average rainfall.

Recent climate

January rainfall was generally average for the South West Land Division. In January the western coast of the SWLD saw above average maximum and minimum temperatures, but the Southeast Coastal forecast district had below average maximum temperatures. 

[9]

Rainfall decile map for the South West Land Division for 1-31 January 2021, showing generally average rainfall for the SWLD.

In January, the atmospheric pressure [10] [10]was higher than normal over the SWLD.

Sea surface temperatures [11] are warmer than average around much of Australia, particularly off the western coast. These warm SSTs are also likely to be contributing to above average rainfall for parts of the country.

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 positive and is expected to remain so until the end of February. For the SWLD, a positive SAM in summer has little effect on rainfall over WA, but increases temperatures as westerly winds are further south than normal, reducing the sea breeze along the west coast, but increases southeasterlies on the south coast. For more information see the Bureau of Meteorology’s  Climate Driver Update [12].

The Indian Ocean Dipole (IOD) is neutral. With the Australian monsoon commencing, the IOD is likely to remain neutral through summer.

La Niña continues in the tropical Pacific. Australian and international climate models suggest returning to neutral conditions during autumn.  A La Niña is often associated with increased chance of widespread rains and flooding over eastern, central and northern Australia, more tropical cyclones than average and prolonged very warm periods in the south. In a La Niña, there is an increased chance of above average number of tropical systems (cyclones and lows) across northern Australia. For further information, see the Bureau of Meteorology’s Australian Tropical Cyclone Outlook [13]  and the Northern Rainfall Onset [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.