Seasonal Climate Outlook

Summary

The Department of Primary Industries and Regional Development’s (DPIRD) Statistical Seasonal Forecast (SSF) system is indicating below median rainfall is likely for the majority of the South-West Land Division (including the wheatbelt) for July to September 2017.

• The SSF is indicating less than a 40% chance of exceeding median rainfall for July to September for the majority of the wheatbelt and South West. The most probable seasonal rainfall is decile 2-3. For eastern parts of the wheatbelt, chances of exceeding median rainfall are higher at 40-60%. Predictive skill based on June conditions is poor to moderate.
• The Bureau of Meteorology’s seasonal outlook for July to September indicates the South-West Land Division (SWLD) has a 25-40% chance of exceeding median rainfall for this period. Predictive skill is moderate.
• Temperature outlook for July to September from the Bureau indicate more than an 80% chance of above normal day-time maxima for the SWLD. Skill is moderate for most of the SWLD. Minimum temperature outlook shows a 60-80% chance of above normal night-time minima, but skill is low. There remains a risk of frost for the inland parts of the SWLD.
• DPIRD’s SSF is also indicating no more than a 40% chance of exceeding median rainfall for July to October over northern, western and southern parts of the SWLD. The most probable four month rainfall is decile 2-3. Predictive skill based on June conditions is poor to moderate.
• June rainfall in the South-West Land Division was very much below average, to record low totals. Maximum temperatures were above average while minimum temperatures were near average.

Three month outlook for the south-west of Western Australia

Statistical Seasonal Forecasting

DPIRD’s 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.

The SSF forecast for July to September 2017 is indicating no more than a 40% chance of exceeding median rainfall for the majority of the SWLD (Figure 1). Over eastern parts of the wheatbelt, this has increased to 40-60% chance of exceeding median. The most probable rainfall is decile 2-3 for the majority of the SWLD, and decile 4-7 for eastern parts of the wheatbelt. Predictive skill for this period, based on June conditions, varies from poor over central and eastern parts, to moderate skill for the north and south (Figure 2). While the improvement in rainfall chances for eastern parts of the wheatbelt is encouraging, the SSF has relatively low skill for this region and time of year.

Figure 1 SSF forecast of probability of exceeding median rainfall for July to September 2017

Figure 2 Percent consistent skill of the SSF at forecasting July to September rainfall using data up to and including June

Bureau of Meteorology seasonal climate outlook

The Bureau of Meteorology’s climate outlooks are generated by a dynamical (physics based) coupled atmosphere-ocean climate model.

The Bureau of Meteorology’s seasonal rainfall outlook indicates the SWLD has a 25-40% chance of exceeding median rainfall for July to September (Figure 3). Predictive skill is moderate (55-75% consistent, see Figure 4). The Bureau’s drier outlook is influenced by expected higher than average atmospheric pressure over southern Australia during the outlook period.

Temperature outlooks for July to September indicate an 80% chance of above normal day-time and night-time temperatures for the SWLD. Historical predictive skill is moderate to good over most of the region. The Bureau cautions that while nights are likely to be warmer on average, very cold nights also remain likely during periods of high pressure and clear skies.

Figure 3 Rainfall outlook for July to September 2017 from the Bureau of Meteorology

Figure 4 Percent Consistent skill of the Bureau of Meteorology’s outlook for July to September rainfall

July to October outlook for the south-west of Western Australia

DPIRD’s SSF is indicating a no more than a 40% chance of exceeding median rainfall for the majority of the SWLD for July to October 2017. This pattern is very similar to the July to September outlook. Chances of above median rainfall have increased to 40-60% over eastern parts of the wheatbelt. The most probable four month rainfall is decile of 2-3 for the majority of the SWLD. Predictive skill based on June conditions is poor to moderate (50-70% consistent).

Figure 5 SSF forecast of probability of exceeding median rainfall for July to October 2017

Figure 6 Percent consistent skill of the SSF at forecasting July to October rainfall using data up to and including June

Recent climate

June rainfall was well below average, to record low totals in some parts of the SWLD. The early part of the growing season (April to June) was dominated by higher than average sea level pressure and clear skies, which resulted in seasonal rainfall which was well below average to record low across the SWLD.

The seasonal rainfall outlook for April to June from the Bureau indicated below average rainfall was highly likely. DPIRD’s SSF also indicated below median rainfall, with decile 2-3 rainfall most likely (see April edition of the Seasonal Climate outlook).

The potential wheat yield map (Figure 7) indicates low yields for the north and north-east regions in particular. The model uses observed rainfall to date at each location, with decile 3 rainfall for the rest of the growing season. This was chosen to reflect likely rainfall in coming months (see Figures 1 and 5). Importantly, the model also assumes crop has germinated in April or May, and is likely to over-estimate yields where crops have germinated late in the season.

Check the rainfall to date tool and potential yield tool for rainfall totals and modelled potential yield estimates for individual locations.

Figure 7 Potential wheat yield map using rainfall November 2016 to 6 July 2017. The model uses the French-Schultz relationship, with a water use efficiency of 12kg/mm, and assumes a decile 3 finish to growing season rainfall

In June, the SWLD maximum temperatures were above average while minimum temperatures were mostly near-average to cooler than normal in some inland areas (likely the result of clear night skies and light winds).

Mean sea level atmospheric pressure continued to be higher than normal over the southwest. Higher than average pressure is forecast for southern parts of Australia for the next three months, which means fewer cold fronts and low pressure systems are likely to cross southern Australia.

Cooler than normal Indian Ocean sea surface temperatures, north of Western Australia have probably contributed to dry conditions by reducing moisture availability to weather systems.

The El Niño-Southern Oscillation (ENSO) status remains neutral, with tropical Pacific Ocean waters only slightly warmer than average. The majority of climate models indicate ENSO neutral conditions are likely for the second half of 2017. Accordingly, the Bureau’s El Niño outlook is classed as inactive, meaning no signs of an El Nino or La Nina developing in coming months.

The Indian Ocean Dipole (IOD) remains neutral, with half the climate models indicating a positive IOD may develop in spring. A positive IOD typically means above normal maximum and minimum temperatures for the SWLD and reduced rainfall in southern parts.

The Southern Annular Mode (SAM), also known as the Antarctic Oscillation (AAO), describes the north–south movement of the westerly wind belt dominating the middle to higher latitudes of the southern hemisphere. SAM is currently positive and is expected to remain so through July. A positive SAM can mean drier conditions for the south-west corner and northern and central wheatbelt in winter.

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.

Additional information

• Soil water tool
• Rainfall to date tool