Seasonal Climate Outlook

Summary

The Department of Primary Industries and Regional Development’s (DPIRD) Statistical Seasonal Forecast (SSF) system is indicating less than 40% chance of exceeding median rainfall for November 2018 to January 2019 for the northern grainbelt and south-west corner and neutral (40-60%) chances elsewhere.

• The SSF is indicating less than less than 40% chance of exceeding median rainfall for November 2018 to January 2019 for the northern grainbelt and south-west corner and neutral (40-60%) chances elsewhere. The most probable decile range is decile 2-3 for most of the grainbelt. Predictive skill based on October conditions is mostly poor to good (50-70% consistent).
• The Bureau of Meteorology’s current seasonal outlook is indicating a 30-45% chance of exceeding median rainfall for November 2018 to January 2019 for the South West Land Division (SWLD). Predictive skill is mostly low to moderate (45-65% consistent).
• Temperature outlooks for November 2018 to January 2019, from the Bureau indicate 75-80% chance of above average day-time maxima for the SWLD. Skill is moderate to high at 55-75% consistent. Minimum temperature outlooks indicate 65-75% chance of above average night-time minima for the SWLD, with skill mostly poor to high at 50-75% consistent.
• October rainfall was average for the grainbelt. October maximum temperatures were average to above average and minimum temperatures were very much above average.
• The combination of possible El Nino and positive IOD increases the risk of heatwaves and bushfire weather in the south, while there are typically fewer tropical cyclones in the Australian region.

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 the next three months, November 2018 to January 2019, the SSF is indicating less than 40% chance of exceeding median rainfall for November 2018 to January 2019 for the northern grainbelt and south-west corner and neutral (40-60%) chances elsewhere. The most probable decile range is decile 2-3 for most of the grainbelt. Predictive skill based on October conditions is mostly poor to good (50-70% consistent).

SSF forecast of the probability of exceeding median rainfall for November 2018 to January 2019 using data up to and including October

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

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 30-45% chance of exceeding median rainfall for November 2018 to January 2019 for the SWLD. Predictive skill is mostly low to moderate (45-65% consistent).

Temperature outlooks for November 2018 to January 2019, from the Bureau indicate 75-80% chance of above average day-time maxima for the SWLD. Skill is moderate to high at 55-75% consistent. Minimum temperature outlooks indicate 65-75% chance of above average night-time minima for the SWLD, with skill mostly poor to high at 50-75% consistent.

Rainfall outlook for November 2018 to January 2019 for Western Australia from the Bureau of Meteorology

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

Recent climate

October rainfall was generally average for the grainbelt. The last three months rainfall (August to October) was generally average to above average. In the August Seasonal Climate Outlook, the SSF and the Bureau model both indicated below median rainfall likely. October maximum temperatures were average to above average and minimum temperatures were very much above average. October maximum temperatures were average to above average and minimum temperatures were very much above average.

April to October rainfall in 2018 was generally average for the northern and central grainbelt and below average for the southern grainbelt. The SSF April to October decile rainfall outlook was correct for the southern grainbelt. This growing season, some stations received above average rainfall from localised systems from cloud band activity. Cloud bands develop quickly over a number of days (not months) and are therefore usually not identified by long lead statistical models such as the SSF.

Rainfall decile map for 1 April to 31 October 2018 for the WA grainbelt

Most probable decile range for April to October 2018 rainfall indicated by SSF

In September, the atmospheric pressure was near normal over southern Australia, contributing to the average rainfall. The Indian Ocean sea surface temperatures continue to be cooler than average to Australia's north-west. This can lead to reduced rainfall, as less atmospheric moisture is available for rain-bearing weather systems.

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 near neutral. The National Oceanic and Atmospheric Administration (NOAA) suggests that SAM is likely to go positive in mid-November. 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.

It is likely that a positive Indian Ocean Dipole (IOD) event is underway, as five of the past six weeks have exceeded positive IOD thresholds. The Bureau's model suggests positive IOD values will continue through October and return to neutral during November. A positive IOD typically decays in late spring or early summer. A positive IOD during spring typically reduces rainfall across much of the eastern two-thirds of Australia and can exacerbate any El Niño-driven rainfall deficiencies. During December to April, the IOD typically has little effect on Australian climate.See the Bureau of Meteorology’s IOD and Pacific Ocean interaction for details.

The tropical Pacific Ocean has been warming in recent weeks and currently the Bureau has issued an El Niño alert. There is approximately a 70% chance of El Niño occurring in 2018, around triple the normal likelihood, with El Niño likely to develop before the end of the year. El Niño typically results in below-average spring rainfall for northern and eastern Australia, and warmer days for the southern two-thirds of the country. By summer, the rainfall influence from El Niño contracts to the tropical north, while warmer days remain likely for large parts of the country.

The combination of El Niño and positive IOD increases the risk of heatwaves and bushfire weather in the south, while there are typically fewer tropical cyclones in the Australian region.

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