Summary

The Department of Primary Industries and Regional Development’s (DPIRD) Statistical Seasonal Forecast (SSF) for April to June 2019 and April to October 2019 is indicating less than 40% chance of exceeding median rainfall for the South West Land Division (SWLD).

• For April to June 2019, the SSF is indicating less than 40% chance of exceeding median rainfall for SWLD. Most probable decile range is decile 2-3. Predictive skill based on March conditions is mostly moderate to good (60 -100% consistent). 

• The SSF long lead outlook for April to October also indicates less than 40% chance of exceeding median rainfall. Most probable decile range is decile 2-3. Predictive skill based on March conditions is mostly poor to good (50 -100% consistent). Past outlooks have been particularly good for Esperance, being correct 5 out of 6 seasons. Three times in the northern (Northampton to Chittering) and southern (Wagin to Ravensthorpe) agricultural regions and twice in the central (Toodyay to Lake Grace) agricultural region.

• The Bureau of Meteorology’s current seasonal outlook is indicating roughly equal chance (45-55%) of exceeding or below median rainfall for April to June 2019 for the majority of the SWLD, and chances of exceeding median rainfall are lower  (30-40%) along  the coastal areas between Jurien Bay and Albany. Parts of Esperance shire have higher chances at 60-65%. Predictive skill is poor to good (50-75% consistent).

• Temperature outlooks for April to June 2019, from the Bureau indicate 60-80% chance of above average day-time maxima for the SWLD. Skill is mostly good at 55-65% consistent. Minimum temperature outlooks indicate 55-80% chance of above average night-time minima for the SWLD, with skill mostly poor at 45-55% consistent.

• March rainfall was above average in the Great Southern, but below average to average elsewhere in the SWLD.March maximum temperatures were generally average to above average and minimum temperatures were average to above average. The past two to three decades have seen a decline in autumn rainfall for southern Australia.

Three month outlook for South West Land Division

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 [1] web page.

For April to June 2019, the SSF is indicating less than 40% chance of exceeding median rainfall for SWLD. Most probable decile range is decile 2-3. Predictive skill based on March conditions is mostly moderate to good (60 -100% consistent).

[2]

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

[3]

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

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 [4]). 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 roughly equal chance (45-55%) of exceeding or below median rainfall for April to June 2019 for the majority of the SWLD, and chances of exceeding median rainfall are lower  (30-40%) along  the coastal areas between Jurien Bay and Albany. Parts of Esperance shire have higher chances at 60-65%. Predictive skill is poor to good (50-75% consistent).
Temperature outlooks for April to July 2019, from the Bureau indicate 60- 80% chance of above average day-time maxima for the SWLD. Skill is mostly moderate at 55-65% consistent. Minimum temperature outlooks indicate 55-80% chance of above average night-time minima for the SWLD, with skill mostly poor at 45-55% consistent.

[5]

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

[6]

Percent Consistent skill of the Bureau of Meteorology’s outlook for autumn, March to May.

SSF April to October outlook

The SSF is indicating less than 40% chance of exceeding median rainfall for April to October. The most probable decile map is indicating decile 2-3 rainfall most likely. Skill is 50 to 100 percent consistent.

Past success of the SSF for April to October indicates: that the outlook was correct in Esperance for the years 2013 and then 2015 until 2018. In the northern agricultural area (Northampton to Chittering) April to October of 2013, 2016 and 2017 was corectly indicated by the SSF. In the southern (Wagin to Ravensthorpe) agricultural area, 2016 until 2018 were correctly indicated by the SSF. In the central (Toodyay to Lake Grace) agricultural area, the SSF had success in 2016 and 2017.

[7]

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

[8]

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

Recent climate

March rainfall was above average in the Great Southern, but below average to average elsewhere in the SWLD.March maximum temperatures were generally average to above average and minimum temperatures were average to above average. The past two to three decades have seen a decline in autumn rainfall for southern Australia.

The rainfall to date decile map for 1 January to 3 April 2019 shows some of the grainbelt has received less rainfall (decile 4 and below) than usual based on historical rainfall for the years 1975-2018.

[9]

Rainfall decile map for 1 January to 3 April 2019 for the South West Land Division.

In March, the atmospheric pressure [10] [10]was slightly above normal over the SWLD, reducing rainfall across parts of southern Australia.

The Indian Ocean sea surface temperatures [11]  off the western WA coastline have been slightly cooler than average. The April to June SST forecast by the Bureau of Meteorology [12] indicate that SST will be slightly warmer, but not warm enough to influence moisture flowing into Western 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. For the last two weeks SAM has been positive. The Bureau model ACCESS is indicating SAM to remain positive until June. In a positive SAM event, the belt of strong westerly winds contracts towards Antarctica. This results in weaker than normal westerly winds and higher pressures over southern Australia, restricting the penetration of cold fronts inland and therefore less rainfall.

The Indian Ocean Dipole (IOD) [13] is currently neutral and forecast to remain neutral through autumn. The Bureau's ENSO Outlook is currently at El Niño ALERT. This means the chance of El Niño developing in 2019 has increased to approximately 70%, around triple the normal likelihood. However, the Bureau's model suggests that any El Niño is likely to be weak and short-lived, and is unlikely to have a significant effect on the rainfall patterns for April to June. 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 southwest Western Australia (SWWA) climate conditions, and can indicate what is likely to occur in the near future if climate conditions follow the current pattern.