Seasonal Climate Outlook

Summary

The Department of Primary Industries and Regional Development’s (DPIRD) Statistical Seasonal Forecast (SSF) system is indicating less than 30% chance of exceeding median rainfall for spring, September to November 2018.

• The SSF is indicating less than a 30% chance of exceeding median rainfall for the grainbelt for September to November 2018. The most probable decile range is decile 2-3 for most of the grainbelt. Predictive skill based on August conditions is mostly poor to good (50- 75% consistent).
• The Bureau of Meteorology’s current seasonal outlook is indicating a 20-40% chance of exceeding median rainfall for spring, September to November for the South West Land Division (SWLD). Predictive skill is mostly poor to moderate (45-65% consistent).
• Temperature outlooks for September to November 2018, from the Bureau indicate mostly an 80% chance of above normal day-time maxima for the SWLD. Skill is poor to good at 45-75% consistent. Minimum temperature outlooks indicate a 60-80% chance of above normal night-time minima for the SWLD, with skill poor to moderate at 45-65% consistent. For the individual month of September, the chance of exceeding median minima is 30-50%, so with clear skies likely, the risk of frost and cold nights continue, especially into September.
• August rainfall was near average or above average for much of the grainbelt. August maximum and minimum temperatures were generally near average.

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 the Seasonal Climate Information page.

For spring, September to November, the SSF is indicating less than a 30% chance of exceeding median rainfall for the grainbelt. The most probable decile range is decile 2-3 rainfall for most of the grainbelt. Predictive skill based on August conditions is poor to good (50-75% consistent).

SSF forecast of the probability of exceeding median rainfall for September to November 2018 using data up to and including August

Percent Consistent skill of the SSF at forecasting September to November rainfall using data up to and including August

Bureau of Meteorology seasonal climate outlook

The Bureau of Meteorology's climate forecast system for monthly and seasonal climate outlooks is called 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 current seasonal outlook is indicating a 20-40% chance of exceeding median rainfall for spring, September to November for the SWLD. Predictive skill is mostly poor to moderate (45-65% consistent).

Temperature outlooks for spring, September to November 2018, from the Bureau mostly indicate an 80% chance of above normal day-time maxima for the SWLD. Skill is poor to good at 45-75% consistent. Minimum temperature outlooks indicate a 60-80% chance of above normal night-time minima for the SWLD, with skill poor to moderate at 45-65% consistent. For the individual month of September, the chance of exceeding median minima is 30-50%, so with clear skies likely, the risk of frost and cold nights continue, especially into September.

Rainfall outlook for September to November 2018 for Western Australia from the Bureau of Meteorology

Percent Consistent skill of the Bureau of Meteorology’s outlook for September to November rainfall

Recent climate

August rainfall was generally above average for the grainbelt and average in the Great Southern. Winter rainfall (June to August) was generally average, with small parts of the Great Southern receiving below rainfall. The SSF was indicating a below average winter and the Bureau’s POAMA model indicated neutral conditions in the June Seasonal Climate Outlook. August maximum and minimum temperatures were generally average. The rainfall deciles map for 1 April-2 September shows large parts of the grainbelt have received average rainfall (decile 4-7), some parts of the northern agricultural region has received decile 8-9 rainfall, and the southern grainbelt and northern Esperance region is tracking at decile 2-3.

Rainfall decile map for 1 April-2 September 2018

In August the atmospheric pressure was lower than normal over southern Australia, bringing in cold fronts to SWLD. The Indian Ocean sea surface temperatures continue to be cooler than average to Australia's northwest, which is likely acting to suppress rainfall over southern and central Australia.

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. In a positive SAM event, the belt of strong westerly winds contracts towards Antarctica, resulting in weaker than normal westerly winds and higher pressures over southern Australia, restricting the penetration of cold fronts inland. The Bureau’s POAMA model suggests that SAM is likely to remain positive until mid-September.

The Indian Ocean Dipole (IOD) is neutral. However, models suggest a brief positive IOD event may form during spring. A positive IOD event typically reduces winter–spring rainfall in central and southern Australia, and can exacerbate any El Niño driven rainfall deficiencies. See the Bureau of Meteorology’s IOD and Pacific Ocean interaction for details.

The El Niño-Southern Oscillation is currently neutral, but there is a 50% chance (double the normal risk) of El Niño forming in the coming months. El Niño typically means below average rainfall during spring for northern and eastern Australia and warmer days for the southern two-thirds of the country.

The table below gives a summary of past month and three month south-west Western Australia (SWWA) climate conditions, and can be used as an indication of what is likely to occur in the near future if climate conditions follow the current pattern.