Seasonal Climate Outlook

SSF forecast for July to October

The SSF forecast for the SWLD for June to October 2021 rainfall is indicating less than 40% chance of exceeding median rainfall for the majority of the SWLD. Most likely decile range maps indicate decile 2-3 for the SWLD. Skill is poor to good at 50 to 70 % consistent.

SSF forecast of the probability of exceeding median rainfall for July to October 2021 using data up to and including June. Indicating less than 40% chance of exceeding median rainfall for the majority of the South West Land Division.
SSF forecast of the probability of exceeding median rainfall for July to October 2021 using data up to and including June.
Percent consistent skill of the SSF at forecasting July to October rainfall using data up to and including June. Skill is 50 to 70 percent consistent.
Percent consistent skill of the SSF at forecasting July to October rainfall using data up to and including June.

Recent climate

June rainfall was average in the north, below average in the central grainbel,t and above average along the southern coast and Esperance.  June maximum and minimum temperatures were generally average to below average. The Rainfall decile map for 1 April to 4 July shows decile 8-10 rainfall for the majority of the SWLD.

Rainfall decile map for the South West Land Division for 1 April to 4 July 2021, showing decile 8-10 rainfall for the majority of the SWLD.
Rainfall decile map for the South West Land Division for 1 April to 4 July 2021.

In June. the atmospheric pressure was higher than normal over the SWLD, restricting rainfall in the central grainbelt.

Large parts of the eastern Indian Ocean have sea surface temperatures that are warmer than average, which can favour above average rainfall for parts of Australia. The July to September 2020 SST forecast by the Bureau of Meteorology indicates SSTs are likely to remain warm to the north of WA as the negative IOD develops.

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 positive until the end of July. A positive SAM could see a decrease in rainfall over South West WA. For more information, see the Bureau of Meteorology’s Climate Driver Update.

The Indian Ocean Dipole (IOD) has been negative for the past five consecutive weeks. A negative IOD event is declared when there have been at least eight weeks below the IOD index threshold of −0.4 °C.  Four out of five models indicate a negative IOD will develop by August and persist to October, with forecast accuracy improving. In a negative IOD, rainfall is increased in eastern and southern Australia, with cooler days in the south. The last negative IOD was in 2016.

The El Niño–Southern Oscillation (ENSO) is neutral. Climate model outlooks show the neutral ENSO state is likely to continue through winter and into spring. Neutral ENSO has little influence on Australian weather and climate, and means the effects of the other climate drivers may dominate this season.

The table below summarises climate conditions for the South West Land Division (SWLD) over the past month and three-months, 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

Mixed

Average to Above average

SWLD Mean Temperature

Below 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) Negative Neutral
Southern Annular Mode (SAM) Positive Positive