Seasonal Climate Outlook

Summary

The Department of Primary Industries and Regional Development’s (DPIRD) Statistical Seasonal Forecast (SSF) outlook for January to March 2021 is indicating neutral (40-60%) chance of exceeding median rainfall for the majority of the South West Land Division.

• For January to March 2021, the SSF is indicating neutral (40-60%) chance of exceeding median rainfall for the majority of the South West Land Division (SWLD). With higher chances (60-80%) for the Central West forecast District, eastern parts of the Central Wheatbelt and along the south coast. The most probable rainfall decile map indicates decile 8-10 for the Central West District and along the south coast, and decile 4-7 elsewhere in the SWLD. Predictive skill based on December conditions is mostly poor to moderate (50-75% consistent).
• The Bureau of Meteorology’s seasonal outlook for January to March 2021 indicated 65-75% of exceeding median rainfall for the majority of the SWLD. Predictive skill is moderate to good (55-75% consistent). The current outlook for February to April and longer-term outlook for March to May is for neutral to wetter conditions. Predictive skill is moderate (50-65%).
• Temperature outlooks for February to April 2021, from the Bureau indicate a 70-80% chance of above average day-time maxima along the coast, with lower chances (50%) for interior (skill 50-75%) and 70-80% chance of above average night-time minima (skill 55-75%) for the SWLD.
• December rainfall was generally below average for the South West Land Division. December maximum temperatures were above average and minimum temperatures were generally average.  
• The La Niña in the tropical Pacific Ocean, together with a positive SAM are currently the main climate drivers influencing SWLD climate. In a La Niña, there is an increased chance of above average number of tropical systems (cyclones and lows) across northern Australia. This may lead to more summer rainfall in the SWLD. 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 web page. 

For January to March 2021, the SSF is indicating neutral (40-60%) chance of exceeding median rainfall for the majority of the South West Land Division (SWLD). With higher chances (60-80%) for the Central West forecast District, eastern parts of the Central Wheatbelt and along the south coast. The most probable rainfall decile map indicates decile 8-10 for the Central West District and along the south coast, and decile 4-7 elsewhere in the SWLD. Predictive skill based on December conditions is mostly poor to moderate (50-75% consistent).

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

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

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 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 January to March 2021 indicated 65-75% of exceeding median rainfall for the majority of the SWLD. Predictive skill is moderate to good (55-75% consistent). The current outlook for February to April and longer-term outlook for March to May is for neutral to wetter conditions. Predictive skill is moderate (50-65%).

Temperature outlooks for February to April 2021, from the Bureau indicate a 70-80% chance of above average day-time maxima along the coast, with lower chances (50%) for interior (skill 50-75%) 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/20201029-outlook.shtml .

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

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

Looking at other rainfall forecasting models, the odds are for neutral to above average rainfall for January to March 2021 for the South West Land Division.

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

Recent climate

December rainfall was generally below average for the South West Land Division. December maximum temperatures were above average and minimum temperatures were generally average.  

In December, the atmospheric pressure was higher than normal over the SWLD.

Sea surface temperatures of the Indian Ocean north of Western Australia are warmer than average, this may be favourable for enhancing rainfall over the continent.

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 and is expected to remain so until the end of January. La Niña tends to favour positive SAM during the spring to summer months, which typically enhances the wet signal of La Niña in parts of eastern Australia. 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 WA coast. For more information see the Bureau of Meteorology’s  Climate Driver Update.

The Indian Ocean Dipole (IOD) is neutral after tending towards positive values in recent months. 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 it is likely to continue at least 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. The first Australian landfall typically occurs in early December, which is about 3 weeks earlier than usual. For further information, see the Bureau of Meteorology’s Australian Tropical Cyclone Outlook  and the Northern Rainfall Onset.

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.