Seasonal Climate Outlook

Summary

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

  • For October to December, the SSF is indicating neutral chances (40-60%) exceeding median rainfall for the majority of the South West Land Division (SWLD). Lower chances (less than 40%) for the northern part of the Central West forecast district and higher chances (60-70%) for parts of the Central wheatbelt and South West forecast districts. The most probable rainfall decile map indicates decile 4-7 for the majority of SWLD. Predictive skill based on September conditions is mostly poor to moderate (50-70% consistent).
  • The Bureau of Meteorology’s seasonal outlook for October to December 2021 is indicating 45-75% chance of above median rainfall for the SWLD, with the higher chances for areas of the Great Southern, South Coastal and South East Coastal forecast districts.  Predictive skill is poor to moderate (45-65% consistent). The longer term outlook for November 2021 to January 2021 is similar. Skill is poor to good (45-75%).
  • Temperature outlooks for October to December 2021, from the Bureau indicate a 25-65% chance of above average day-time maxima, with lower chances for the southern coast and interior (skill 75-100%). The Bureau indicates a 50-80% chance of exceeding above average night-time minima for the SWLD, with higher chances for the Central West forecast district (skill 75-100%).
  • September rainfall was below average to average for the SWLD. September maximum temperatures were average to above average and minimum temperatures were below average to average, with some frost damage reported.
  • The main climate driver influencing the SWLD is warmer than normal Sea Surface Temperatures in the Indian Ocean following the weakened Indian Ocean Dipole, favouring above average rainfall for parts of Australia. 

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 rainfall forecasts for the coming months. Users can click on any weather station indicated on the map for location-specific forecast information from DPIRD’s Seasonal Climate Information pages.

For October to December, the SSF is indicating neutral chances (40-60%) exceeding median rainfall for the majority of the South West Land Division (SWLD). Lower chances (less than 40%) for the northern part of the Central West forecast district and higher chances (60-70%) for parts of the Central wheatbelt and South West forecast districts. The most probable rainfall decile map indicates decile 4-7 for the majority of SWLD. Predictive skill based on September conditions is mostly poor to moderate (50-70% consistent).

SSF forecast of the probability of exceeding median rainfall for October to December 2021 using data up to and including September. Indicating neutral chances (40-60%) of exceeding median rainfall for the majority of the South West Land Division.
SSF forecast of the probability of exceeding median rainfall for October to December 2021 using data up to and including September.
Percent consistent skill of the SSF at forecasting October to December rainfall using data up to and including September. Skill is 50 to 70 percent consistent.
Percent consistent skill of the SSF at forecasting October to December rainfall using data up to and including September.

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 October to December 2021 is indicating 45-75% chance of above median rainfall for the SWLD, with the higher chances for areas of the Great Southern, South Coastal and South East Coastal forecast districts. Predictive skill is poor to moderate (45-65% consistent). The longer term outlook for November 2021 to January 2021 is similar. Skill is poor to good (45-75%).

Temperature outlooks for October to December 2021, from the Bureau indicate a 25-65% chance of above average day-time maxima, with lower chances for the southern coast and interior (skill is good at 75-100%). The Bureau indicates a 50-80% chance of exceeding above average night-time minima for the SWLD, with higher chances for the Central West forecast district (skill is good 75-100%).

Rainfall outlook for October to December 2021 for Western Australia from the Bureau of Meteorology, indicating 45-75% chance of above median rainfall for the SWLD.
Rainfall outlook for October to December 2021 for Western Australia from the Bureau of Meteorology.
Per cent Consistent skill of the Bureau of Meteorology’s outlook for October to December indicating 45 to 65 percent consistent skill over the SWLD.
Per cent Consistent skill of the Bureau of Meteorology’s outlook for October to December.

 

The majority of other rainfall forecasting models are indicating neutral chances of exceeding above median rainfall for the SWLD for October to December 2021.

Model distribution summary of 12 models (not including the SSF) which forecast October to December 2021 rainfall in the South West Land Division. The majority are indicating neutral chance of exceeding median rainfall for the next three months.
Model distribution summary of 12 models (not including the SSF) which forecast October to December 2021 rainfall in the South West Land Division.

Recent climate

September rainfall was below average to average for the SWLD. September maximum temperatures were average to above average and minimum temperatures were below average to average, with some frost damage reported,

In September, the atmospheric pressure was higher than normal over the SWLD, reducing rainfall to the SWLD.

Indian Ocean sea surface temperatures north-west of Western Australia are warmer than average, which can favour above average rainfall for parts of Australia.The October to December 2021 SST forecast by the Bureau of Meteorology indicates SSTs are likely to remain warm north of WA as the negative IOD remains.

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 SAM index is currently positive and is expected to remain so until the end of the month. In a positive SAM phase, the belt of westerly winds contracts towards Antarctica. This results in weaker than normal westerly winds and higher pressures over southern Australia, restricting the passage of cold fronts inland. For more information, see the Bureau of Meteorology’s Climate Driver Update.

The negative Indian Ocean Dipole (IOD) has weakened, with latest IOD values falling shy of negative thresholds. While the negative IOD is near its end, the pattern of sea surface temperatures in the Indian Ocean may still be providing conditions conducive for rainfall across some parts of Australia.

The El Niño–Southern Oscillation (ENSO) is neutral. Climate models indicate further cooling of the tropical Pacific is likely over the coming months, with three of the seven models surveyed by the Bureau' cooling enough to meet La Niña criteria, while two additional models briefly touch La Niña thresholds. As a result, the Bureau's ENSO Outlook is at La Niña WATCH, which means around a 50% chance of La Niña forming. This is approximately double the normal likelihood.

The table below summarises the 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.

Climate indicator Past month Past 3 months

SWLD Rainfall

below average to average

average to above average

SWLD Mean Temperature

average to above average

above average 

SWLD atmospheric pressure

Higher

Higher

Indian Ocean Sea surface temperature

Warmer

Warmer

El Niño/Southern Oscillation (ENSO)

Neutral

Neutral

Indian Ocean Dipole (IOD)

Neutral

Negative

Southern Annular Mode (SAM)

Positive

Positive