Summary

The Department of Primary Industries and Regional Development’s (DPIRD) Statistical Seasonal Forecast (SSF) outlook for January to February 2023, is indicating mixed probabilities of above median rainfall for South West Land Division (SWLD), although skill is very low this time of the year.

• For January to February 2023, the SSF forecast is indicating greater than 60% probability of above median rainfall for part of the Central West and Central Wheatbelt forecast districts, below 40% probability for part of the Central West, South West, Great Southern, South Coastal and South East Coastal Forecast districts, and 40-60% chance elsewhere. Skill is mostly poor at 50-70% consistent.  
• The Bureau of Meteorology’s seasonal outlook for January to March 2023 is indicating 25-50% chance of exceeding median rainfall for the SWLD, with moderate to good skill (55-75%).  The current longer-term outlook (at time of writing) for February to April 2023 and March to May 2023 is less than 40% chance of exceeding median rainfall with mostly moderate skill (50-65%).
• Temperature outlooks for February to April 2023, from the Bureau indicate a 60-80% chance of above average day-time maxima, with the higher chances for the Central West and Central Wheatbelt . Skill is moderate 55-75%. For night-time minima for the SWLD, the Bureau is indicating 60-80% chance of exceeding above average temperatures, with higher chances along the coast. Skill is moderate at 55-65%.
• December rainfall was below average to average. December maximum temperatures were below average to average. Minimum temperatures were average.
• The warmer sea surface temperatures north of Australia increases the likelihood of tropical cyclones developing. January to March rainfall in the SWLD is usually caused by isolated thunderstorms and tropical cyclones and are therefore harder to (long-lead) forecast, than winter rainfall which are mostly from westerly frontal systems.

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 January to February 2023, the SSF forecast is indicating greater than 60% probability of above median rainfall for part of the Central West and Central Wheatbelt forecast districts, below 40% probability for part of the Central West, South West, Great Southern, South Coastal and South East Coastal Forecast districts, and 40-60% chance elsewhere. Skill is mostly poor at 50-70% consistent.  

[2]

SSF forecast of the probability of exceeding median rainfall for January to February using data up to and including November

[3]

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

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

The Bureau of Meteorology’s seasonal outlook for January to March 2023 is indicating 25-50% chance of exceeding median rainfall for the SWLD, with moderate to good skill (55-75%).  The current longer-term outlook (at time of writing) for February to April 2023 and March to May 2023 is less than 40% chance of exceeding median rainfall with mostly moderate skill (50-65%).

Temperature outlooks for February to April 2023, from the Bureau indicate a 60-80% chance of above average day-time maxima, with the higher chances for the Central West and Central Wheatbelt . Skill is moderate 55-75%. For night-time minima for the SWLD, the Bureau is indicating 60-80% chance of exceeding above average temperatures, with higher chances along the coast. Skill is moderate at 55-65%.

[5]

Rainfall outlook for January to March 2023 for Western Australia from the Bureau of Meteorology

[6]

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

Looking at other rainfall forecasting models, the majority of models are indicating neutral to below chance of exceeding median rainfall for the SWLD for January to March 2023.

[7]

Model distribution summary of 11 models outlook for January to March 2023 rainfall in the South West Land Division.

Recent climate

December rainfall was below average to average. December maximum temperatures were below average to average. Minimum temperatures were average.

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

Sea surface temperatures [9] (SST) remain warmer than average in the western Pacific, much of the Maritime Continent, and around northern Australia. Warmer Australian waters, especially in the tropics, can result in greater evaporation, humidity, cloudiness, and rainfall. The sea surface temperature outlook for January to March by the Bureau of Meteorology [10] indicates SSTs will be normal to warm around Western Australia. Warmer temperatures marginally increase the likelihood of tropical cyclones developing.

The Indian Ocean Dipole (IOD) is neutral and has little influence on Australian climate while the monsoon trough is in the southern hemisphere (typically December to April).

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. The Southern Annular Mode (SAM) is strongly positive and is likely to be positive until at least mid-January.  SAM has no influence on rainfall in the SWLD in summer.  For more information see the Bureau of Meteorology’s Climate Driver Update [11].

La Niña slowly easing in the tropical Pacific. ENSO events typically peak in late (southern hemisphere) summer and decay during the autumn; current outlooks indicate this La Niña may decay slightly earlier than usual (February).

The table below gives a summary of past month and three-month SWLD climate conditions, and can indicate what is likely to occur in the near future if climate conditions follow the current pattern.