Seasonal Climate Outlook

Summary

The Department of Agriculture and Food, Western Australia’s Statistical Seasonal Forecast (SSF) system is indicating below median rainfall for the majority of the South-West Land Division (which includes the wheatbelt) for June to July and June to October 2017.The month of May is not included in this forecast as SSF was indicating a wet May which is inconsistent when compared to the rest of the outlook and to other models and to Indian Ocean and mean sea level pressure climate conditions.

  • The SSF is indicating 0-40% chance of exceeding median rainfall for June to July for the majority of the wheatbelt. For the south coast this has increased to 40-70% chance of exceeding median. The most probable decile range map indicates decile 2-3 rainfall is most likely for the majority of the wheat-belt for June to July. Predictive skill based on April conditions is poor to moderate (50-75% consistent).
  • The Bureau of Meteorology’s current seasonal outlook indicates the South-West Land Division (SWLD) has a 20-35% chance of exceeding median rainfall for May to July. Predictive skill is moderate to good (55-75% consistent).
  • Temperature outlooks for May to July from the Bureau indicate a 65-80% chance of above normal day-time maxima for the SWLD. Skill is moderate to good at 55-75% consistent. Minimum temperature outlooks indicate a 55-75% chance of above normal night-time minima for the SWLD, with skill poor to moderate at 50-65% consistent.
  • DAFWA’s SSF is indicating a 20-60% chance of exceeding median rainfall for April to October. The most probable decile range map indicates rainfall decile of 2-3 for the majority of the SWLD. Predictive skill based on March conditions is poor to good (50-75% consistent).
  • April rainfall in the SWLD was the eighth driest on record. Maximum temperatures were above average to average whilst minimum temperatures were below average to average.

Three month outlook for the south-west of Western Australia

Statistical Seasonal Forecasting (SSF)

DAFWA’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 web page.

For the month of May, SSF was indicating very high chances of receiving median rainfall which is not in-line with the other months in the outlook or with other models. For the month of April the Pacific Ocean was warmer than normal and this is what was driving the wet outlook for May. For this reason May is not included in the outlooks.

The SSF forecast for June to July 2017 is indicating a 0-40% chance of exceeding median rainfall for the majority of the wheatbelt. For the south coast this has increased to 40-70% chance of exceeding median. The most probable decile range map indicates decile 2-3 rainfall is most likely for June to July for the majority of the SWLD and decile 4-7 along the south coast and decile 8-9 for the south-west corner and eastern parts of the Esperance shire. Predictive skill based on April conditions is poor to moderate (50-75% consistent).

SSF forecast of probability of exceeding median rainfall for June to July 2017, indicating 0-40 per cent chance of exceeding median rainfall
SSF forecast of probability of exceeding median rainfall for June to July 2017
Percent Consistent skill of the SSF at forecasting June to July rainfall using data up to and including April. Skill is 50–75% consistent.
Percent consistent skill of the SSF at forecasting June to July rainfall using data up to and including April

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 20-35% chance of exceeding median rainfall for May to July. Predictive skill is moderate to good (55-75% consistent). The Bureau’s drier outlook is influenced by warming in tropical Pacific sea surface temperatures and a cooler eastern Indian Ocean.

Temperature outlooks for May to July from the Bureau indicate a 65-80% chance of above normal day-time maxima for the SWLD. Skill is moderate to good at 55-75% consistent. Minimum temperature outlooks indicate a 55-75% chance of above normal night-time minima for the SWLD, with skill poor to moderate at 50-65% consistent.

Rainfall outlook for May to July 2017 from the Bureau of Meteorology. Indicating below median rainfall expected.
Rainfall outlook for May to July 2017 from the Bureau of Meteorology
Percent Consistent skill of the Bureau of Meteorology’s outlook for May to July rainfall. Skill is 55 to 75 per cent consistent.
Percent consistent skill of the Bureau of Meteorology’s outlook for May to July rainfall

June to October Outlook for the south-west of Western Australia

DAFWA’s SSF is indicating a 0-40% chance of exceeding median rainfall for the majority of the South West Land Division for June to October 2017. The most probable decile range map indicates rainfall decile of 2-3 for the majority of the SWLD. Predictive skill based on April conditions is poor to moderate (50-75% consistent).

SSF forecast of probability of exceeding median rainfall for June to October 2017 indicating 0-40% chance of exceeding median rainfall.
SSF forecast of probability of exceeding median rainfall for June to October 2017
Percent consistent skill of the SSF at forecasting June to October rainfall using data up to and including April. Skill is 50–75% consistent.
Percent consistent skill of the SSF at forecasting June to October rainfall using data up to and including April

Recent climate

April rainfall in the SWLD was the eighth driest on record. The last three months, February to April was very much above average, the Bureau outlook saw probabilities close to 50% for this period and DAFWA outlook was indicating above median for the Great Southern but below average to average for elsewhere, so offered little guidance (See February edition of the Seasonal Climate outlook). The latest soil water map (2 May) indicates high soil moisture for southern region, but low levels in the north and southwest corner after receiving little to no rainfall in April. Check DAFWA rainfall to date tool and DAFWA soil water tool for rainfall totals and modelled stored soil moisture estimates.

Plant available soil water map using data from 1 November 2016 to 4 May 2017.The map shows good soil moisture levels in the central wheatbelt, Great Southern and Esperance region and lower levels in northern wheat-belt and south-west corner.
Plant available soil water map using data from 1 November 2016 to 4 May 2017

In April, the SWLD maximum temperatures were above average to average whilst minimum temperatures were below average to average.

The El Niño-Southern Oscillation (ENSO) remains neutral but tropical Pacific Ocean waters have been steadily warming since January. Five out of eight international climate models indicate an El Niño is likely to develop by September. The Bureau’s El Niño watch continues and the odds of an El Niño developing in 2017 are double the normal chance. An El Niño typically biases Australia's climate towards a drier than average winter-spring (for eastern Australia), and warmer daytime temperatures in the south.

The Indian Ocean Dipole (IOD) remains neutral. Four out of six international climate models indicate the development of a positive IOD is possible by July, which means a greater likelihood of reduced rainfall for large parts of the SWLD in winter and spring. Positive IOD also typically means above normal maximum and minimum temperatures for the SWLD. In the past, a positive IOD combined with an El Niño has generally meant reduced rainfall for winter and spring for SWWA. See the Bureau of Meteorology’s IOD and Pacific Ocean interaction for details.

Indian Ocean sea surface temperatures (SSTs) north and west of Australia remain cooler than normal. This means that heavy rainfall events from cold fronts are less likely as there is a weaker north/south temperature gradient, which suppresses frontal development, and less likelihood of tropical moisture from the north interacting with cold fronts. For a summary of Pacific and Indian Ocean outlooks, see the Climate Model Summary produced by the Bureau.

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 so through May. A positive SAM in autumn and winter can mean drier conditions for the south-west corner though has a limited impact on the wheatbelt.

In April the atmospheric pressure was higher than normal over the southwest. The mean sea level pressure is forecast to remain higher in the coming months, meaning fewer cold fronts are likely to cross the coast.

The table below gives a summary of past month and three month southwest 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 Below average Above average
SWWA mean temperature Average Average to below average
SWWA atmospheric pressure High High
Indian Ocean sea surface temperature Cooler Cooler
El Nino/Southern Oscillation (ENSO) Neutral Neutral
Indian Ocean Dipole (IOD) Neutral Neutral
Southern Annular Mode (SAM) Positive Positive

Additional information