Seasonal Climate Outlook

Summary

The Department of Agriculture and Food, Western Australia’s Statistical Seasonal Forecast (SSF) system is indicating an above median rainfall outlook for the majority of the wheatbelt for March to May, autumn 2017.

  • The SSF is indicating 60-100% chance of exceeding median rainfall for the majority of the wheatbelt, 40-60-% chance of exceeding median rainfall in some southern shires and Esperance and 30-40% for Manjimup and Denmark shires. Predictive skill based on February conditions is poor to moderate (50-70% consistent).
  • The Bureau of Meteorology’s current seasonal outlook indicates that SWWA and the southern wheatbelt have 25-40% chance of exceeding the median rainfall for autumn. However, due to the decile 10 rainfall in February, soil moisture reserves are excellent for most parts of the wheatbelt. Predictive skill is poor to moderate (45-65% consistent).
  • Temperature outlooks from the Bureau indicate a 55-75% chance of above normal day-time maxima and 45-65% chance of above normal night-time minima across the region for the March to May period (poor to moderate skill, 45-65% consistent).
  • Rainfall in February was second highest for the South-West Land Division, behind 1955. Maximum temperatures were 1.6°C below average and the lowest since 2001, whilst minimum temperatures were near 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.

The SSF forecast for autumn, (March to May) 2017 is indicating 60-100% chance of exceeding median rainfall for the majority of the wheatbelt, 40-60% chance of exceeding median rainfall some Southern shires and Esperance and 30-40% for Manjimup and Denmark shires. The most probable decile range map indicates decile 8-10 rainfall for March to May for the northern and central wheatbelt, decile 2-3 for the south-west corner and decile 4-7 elsewhere. Predictive skill based on February conditions is poor to moderate (50-70% consistent).

SSF forecast of probability of exceeding median rainfall for March to May 2017. Indicating 60-100% chance of exceeding median rainfall for the majority of the wheatbelt
SSF forecast of probability of exceeding median rainfall for March to May 2017
Percent Consistent skill of the SSF at forecasting March to May rainfall using data up to and including February. Skill is 50–70% consistent.
Percent Consistent skill of the SSF at forecasting March to May rainfall using data up to and including February

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 that SWWA and the southern wheat-belt have 25-40% chance of exceeding the median rainfall for autumn. Predictive skill is poor to moderate (45-65% consistent). This differs from the wetter SSF outlook, and outlooks must be used with caution at this time when skill is low due to a predictability barrier in the Pacific Ocean. The Bureau’s drier outlook is likely a result of forecast higher than normal pressure over western and southern Australia, meaning fewer cold fronts are likely to cross the coast. International dynamical climate models, similar to POAMA, are mostly indicating a below median autumn for SWWA based on expected higher mean sea level pressures.

Temperature outlooks from the Bureau indicate a 55-75% chance of above normal day-time maxima and 45-65% chance of above normal night-time minima across the region for the March to May period (poor to moderate skill, 45-65% consistent).

Rainfall outlook for March to May 2017 from the Bureau of Meteorology. Indicating below median rainfall expected.
Rainfall outlook for March to May 2017 from the Bureau of Meteorology
Percent Consistent skill of the Bureau of Meteorology’s outlook for March to May rainfall. Skill is 45-65 per cent consistent at this time of the year.
Percent Consistent skill of the Bureau of Meteorology’s outlook for March to May rainfall

Recent climate

Rainfall in February was second wettest for the South-West Land Division and also the second wettest summer behind 1955. Thunderstorms on 2 March, lead to wide spread rainfall throughout the wheatbelt with 39mm recorded in Tardun Rock and 13mm in Wongan Hills. Further thunderstorms on 12-13 March led to weekly rainfall totals of 58mm for Ardath in the central wheatbelt. Check DAFWA soil water tool for modelled stored soil moisture estimates.

February South-West Land Division maximum temperatures were 1.6°C below average, and lowest since 2001, whilst minimum temperatures were near average.

The El Niño–Southern Oscillation (ENSO) remains neutral, but the Pacific Ocean is warming. Six out of eight models are suggesting an El Niño to develop by July. The Bureau is currently on an El Niño watch.

Indian Ocean sea surface temperatures (SSTs) north of Australia have warmed in the past week and now are slightly above average. The Madden Julian Oscillation is in an active phase but is beginning to weaken as it approaches northern Australia. Surprisingly, there has only been one tropical cyclone cross the Australian coast this season, Blanche on 6 March, despite expectations of an average to above average tropical cyclone season. However, numerous tropical lows and thunderstorms have had an impact on the climate of WA over the past months, bringing record rainfall to the Kimberley, as well as the southwest. Although the continuation of this activity appears unlikely as the MJO weakens, tropical activity in March and April can lead to more rainfall for the northern and eastern wheatbelt if lows move through the region (Olwyn in March 2015 and Elaine in March 1999). For a summary of Pacific and Indian Ocean outlooks, see the Climate Model Summary produced by the Bureau.

The influence of the Indian Ocean Dipole (IOD) on Australian climate is weak during December to April. This is due to the monsoon trough shifting south over the tropical Indian Ocean and changing the overall wind circulation, which in turn prevents an IOD ocean temperature pattern from being able to form. Current outlooks suggest a neutral IOD for the end of autumn.

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 near zero and is expected to remain so up to end of March. The negative SAM in summer was partly responsible for the out of season cold fronts and cooler conditions.

In February the atmospheric pressure was above normal over the southwest. These higher than normal pressures over western and southern Australia are forecast to continue, 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 Very much above average Very much above average
SWWA mean temperature Below average Below average
SWWA atmospheric pressure Higher Normal
Indian Ocean sea surface temperature Cooler Cooler
El Nino/Southern Oscillation (ENSO) Neutral Neutral
Indian Ocean Dipole (IOD) Neutral Neutral
Southern Annular Mode (SAM) Neutral Negative

Additional information