Seasonal Climate Outlook

Summary

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

  • The SSF is indicating less than a 40% chance of exceeding median rainfall for spring, September to November, for the majority of the wheatbelt. For western areas of the central agricultural area, chances are higher at 40-60%. For eastern areas of the metropolitan region, chances are higher still at 60-70%. The most probable decile range map indicates decile 2-3 rainfall is most likely for the majority of the wheatbelt. Predictive skill based on August conditions is poor to good (50-75% consistent).
  • The Bureau of Meteorology’s current seasonal outlook indicates the South-West Land Division (SWLD) has a 30-50% chance of exceeding median rainfall for spring, September to November. Predictive skill is poor to moderate (45-65% consistent).
  • Temperature outlooks for September to November from the Bureau indicate a 30-60% chance of above normal day-time maxima for the SWLD. Skill is poor to good at 45-75% consistent. Minimum temperature outlooks also indicate 30-60% chance of above normal night-time minima for the SWLD, with poor to moderate skill at 45-65% consistent. Frost risk remains in areas with clear skies and dry soils.
  • August rainfall in the SWLD was average to above average, with parts of the Great Southern and Esperance shire below average. Maximum temperatures were near average and minimum temperatures were above average.

Three month outlook for the south-west of Western Australia

Statistical Seasonal Forecasting (SSF)

DPIRD’s Statistical Seasonal Forecast 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 web page.

The SSF forecast for spring, September to November 2017 is indicating less than a 40% chance of exceeding median rainfall for the majority of the wheatbelt. For western areas of the central agricultural area, chances are higher at 40-60%. For eastern areas of the metropolitan region chances are higher still at 60-70%. Predictive skill based on August conditions is poor to good (50-75% consistent). The most probable decile range map indicates decile 2-3 rainfall is most likely for the majority of the wheatbelt.

SSF forecast of probability of exceeding median rainfall for spring, September to November 2017 indicating 0-40% chance of exceeding median rainfall for the majority of the wheatbelt.
SSF forecast of probability of exceeding median rainfall for spring, September to November 2017

Percent Consistent skill of the SSF at forecasting September to November rainfall using data up to and including August. Skill is between 50 to 75 percent consistent at this time of the year.
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 outlooks are generated by a dynamical (physics based) coupled atmosphere-ocean climate model.

The Bureau of Meteorology’s current seasonal outlook indicates the SWLD has a 30-50% chance of exceeding median rainfall for spring, September to November. Predictive skill is poor to moderate (45-65% consistent). The Bureau’s drier outlook is influenced by localised factors such as cooler than normal Indian Ocean sea surface temperatures and higher sea level pressures. Less cold fronts are likely for spring.

Temperature outlooks for spring, September to November, from the Bureau indicate a 30-60% chance of above normal day-time maxima for the SWLD. Skill is poor to good at 45-75% consistent. Minimum temperature outlooks also indicate a 30-60% chance of above normal night-time minima for the SWLD, with poor to moderate skill at 45-65% consistent. Frost risk remains in areas with clear skies and dry soils.

Rainfall outlook for spring, September to November 2017 from the Bureau of Meteorology. Indicating 30 to 45 percent chance of exceeding spring rainfall.
Rainfall outlook for spring, September to November 2017 from the Bureau of Meteorology

Percent Consistent skill of the Bureau of Meteorology’s outlook for spring, September to November rainfall. Skill is 45 to 65 percent consistent.
Percent Consistent skill of the Bureau of Meteorology’s outlook for spring, September to November rainfall

Recent climate

August rainfall in the SWLD was average to above average, with parts of the Great Southern and Esperance shire below average. Winter rainfall, June to August, was generally below average for the SWLD. Outlooks for winter from both the SSF and from the Bureau were for below average rainfall (see the June edition of the Seasonal Climate outlook).

The Bureau of Meteorology’s drought statement indicates areas in the northern and central wheatbelt have had serious to severe rainfall deficiency since March 2017.

Rainfall deficiency map from 1 March to 31 August 2017. This map indicates that northern and central agricultural areas of the Western Australian wheatbelt has serious to severe rainfall deficiency.
 Rainfall deficiency map from 1 March to 31 August 2017

In August, the SWLD maximum temperatures were near average and minimum temperatures were above average. The winter maximum temperatures for the SWLD was the fourth highest since records began in 1910.

In August the atmospheric pressure was lower than normal over the southwest, partly as a result of a low pressure system which brought wide-spread rain on 29 August, as well as frequent cold fronts during the month. Higher pressures are favoured south of Australia in September, meaning greater easterly flow across southern Australia. This would favour increased rainfall on the east coast, and drier conditions in the west (due to drier inland air moving over the SWLD and less onshore flow).

Cooler than normal Indian Ocean sea surface temperatures northwest and west of Western Australia have reduced the potential for cloud-band development.

The El Niño-Southern Oscillation (ENSO) remains neutral. All climate models surveyed by the Bureau indicate the tropical Pacific Ocean is likely to stay ENSO-neutral for the rest of 2017.

The Indian Ocean Dipole (IOD) is neutral. Most climate models suggest a neutral IOD is likely to continue. However, two of six climate models surveyed suggest a positive IOD may develop during spring. Positive IOD events are typically associated with below average winter–spring rainfall, and increased spring–summer fire potential over central and southern Australia. See the Bureau of Meteorology’s IOD and Pacific Ocean interaction for details.

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 and is expected to remain positive for the next two weeks. During a positive SAM event, the belt of strong westerly winds contracts towards the south pole. This results in weaker than normal westerly winds and higher pressure over southern Australia. A positive SAM has a weak influence on spring rainfall in SWLD.

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.

Climate Indicator Past month Past three months
SWWA rainfall Average to above average Below average
SWWA mean temperature Above average Above average
SWWA atmospheric pressure Low Low
Indian Ocean sea surface temperature Cooler Cooler
El Niño/Southern Oscillation (ENSO) Neutral Neutral
Indian Ocean Dipole (IOD) Neutral Neutral
Southern Annular Mode (SAM) Positive Positive

Additional information