Summary

The Department of Primary Industries and Regional Development’s (DPIRD) Statistical Seasonal Forecast (SSF) outlook for winter, June to August 2019 and June to October 2019 is indicating less than 40% chance of exceeding median rainfall for the majority of the grainbelt. Most (80%) of international models expect winter rainfall to be below average over southern WA.

• For winter, June to August 2019 and June to October, the SSF is indicating less than 40% chance of exceeding median rainfall for the central wheatbelt, great southern, south coastal and south west (as classified by the Bureau of Meteorology forecast districts [1]). For the central west and south east coastal districts, SSF is indicating neutral (40-60%) chance of exceeding median rainfall. Most probable decile range is decile 2-3 for the central wheatbelt, great southern, south coastal and south west and decile 4-7 for central west and south east coastal districts. Predictive skill based on May conditions is mostly poor to good (50 -75% consistent).

• The Bureau of Meteorology’s current seasonal outlook is indicating neutral (45-55%) chance of exceeding median rainfall for winter, June to August 2019, for the majority of the grainbelt. Lower chances (30-45%) are for coastal areas. Predictive skill is poor to good (45-75% consistent).

• Temperature outlooks for winter, June to August 2019, from the Bureau indicate 70-80% chance of above average day-time maxima for the SWLD. Skill is poor to moderate at 45-65% consistent. Minimum temperature outlooks indicate 65-80% chance of above average night-time minima for the SWLD, with skill poor to moderate 45-65% consistent. With more cloud-free days and nights expected, there is an increase of frost in susceptible areas.

• May rainfall was very much below average in the SWLD. May maximum temperatures were average to above average and minimum temperatures were very much below average.

• The Indian Ocean is expected to become the dominant influence on Australian climate, with models predicting a positive phase of the Indian Ocean Dipole (IOD) likely to develop in the coming months. The dry years of 2006, 2012 and 2015 (2006 and 2015 were also El Niño years) were all positive IOD years.

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 [2] web page.

For winter, June to August 2019, the SSF is indicating less than 40% chance of exceeding median rainfall for the central wheatbelt, great southern, south coastal and south west (as classified by the Bureau of Meteorology forecast districts [1]). For the central west and south east coastal districts, SSF is indicating neutral (40-60%) chance of exceeding median rainfall. Most probable decile range is decile 2-3 for the central wheatbelt, great southern, south coastal and south west and decile 4-7 for central west and south east coastal districts. Predictive skill based on May conditions is mostly poor to good (50 -75% consistent).

[3]

SSF forecast of the probability of exceeding median rainfall for winter, June to August using data up to and including May.

[4]

Percent Consistent skill of the SSF at forecasting June to August rainfall using data up to and including May.

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 [5]). 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 neutral (45-55%) chance of exceeding median rainfall for winter, June to August 2019, for the majority of the grainbelt. Lower chances (30-45%) are for coastal areas Predictive skill is poor to good (45-75% consistent).

Temperature outlooks for winter, June to August 2019, from the Bureau indicate 70-80% chance of above average day-time maxima for the SWLD. Skill is poor to moderate at 45-65% consistent. Minimum temperature outlooks indicate 65-80% chance of above average night-time minima for the SWLD, with skill poor to moderate 45-65% consistent. With more cloud-free days and nights expected, there is an increase of frost in susceptible areas.

[6]

Rainfall outlook for June to August 2019 for Western Australia from the Bureau of Meteorology.

[7]

Percent Consistent skill of the Bureau of Meteorology’s outlook for June to August.

SSF June to October outlook

The SSF is indicating less than 40% chance of exceeding median rainfall for June to October for the central wheatbelt, great southern, south coastal and south west (as classified by the Bureau of Meteorology forecast districts [1]). For the central west and southeast coastal districts, SSF is indicating neutral (40-60%) chance of exceeding median rainfall. Most probable decile range is decile 2-3 for the central wheatbelt, great southern, south coastal and south west. Predictive skill based on May conditions is mostly poor to good (50 -75% consistent).

[8]

SSF forecast of the probability of exceeding median rainfall for June to October using data up to and including May.

[9]

Percent Consistent skill of the SSF at forecasting June to October rainfall using data up to and including May.

Recent climate

May rainfall was very much below average in the SWLD.May maximum temperatures were average to above average and minimum temperatures were very much below average. Rainfall since 1 January has been below decile 3 for the majority of the grainbelt.

[10]

Rainfall decile map for 1 January  to 30 May 2019 for the South West Land Division.

In May, the atmospheric pressure [11] [11]was very much above normal over the SWLD, reducing rainfall across parts of southern Australia. The current state of the drivers means that higher than average pressure is likely over southern Australia for winter. This can act to keep cold fronts further south than normal, thereby reducing winter rainfall to the southern states.

The Indian Ocean sea surface temperatures [12]  off the western WA coastline have been cooler than average. The June to August SST forecast, by the Bureau of Meteorology [13] indicates that SST in the Indian Ocean will be near normal north of WA.

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. For May, SAM was near neutral and is expected to remain so for the next couple of months.

The current positive IOD value is being driven by warmer than average sea surface temperatures across most of the Indian Ocean basin, and closer to average temperatures in parts of the eastern node of the IOD, near the Indonesian island of Sumatra. For an event to be considered to have occurred, the IOD value needs to be above the threshold value for at least eight weeks. Climate outlooks indicate the IOD is likely to be positive from June through to November. A positive IOD typically means drier than average conditions for southern and central Australia during winter-spring. The dry years of 2006, 2012 and 2015 (2006 and 2015 were also El Niño years) were all positive IOD years.

The Bureau's ENSO Outlook is currently on El Niño WATCH. Models indicate the tropical Pacific Ocean will ease away from El Niño levels, becoming neutral during winter. For further information, see the Bureau of Meteorology’s ENSO Wrap Up [14].

The table below gives a summary of 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.