Seasonal Climate Outlook

Summary

The Department of Primary Industries and Regional Development’s (DPIRD) Statistical Seasonal Forecast (SSF) system is indicating less than a 40% chance of exceeding median rainfall for January 2018 to February 2018 for the majority of the wheatbelt.

• This month, the SSF covers only January and February and is based on climate conditions in November 2017. Most of the eastern and south-eastern eastern agricultural regions have less than a 40% chance of exceeding median rain for the period. The most probable rainfall decile range is decile 2-3. The far northern agricultural region has higher chances (60-70%) of exceeding median rainfall, with decile 8-10 rainfall being most likely. Elsewhere the chances are close to 50%, with decile 4-7 rainfall being most likely. Predictive skill based on November conditions is poor to good (50-70% consistent).
• The Bureau of Meteorology’s current seasonal outlook indicates a 60-80% chance of exceeding median rainfall for January to March 2018 for the South West Land Division (SWLD). This wetter outlook is likely influenced by recent increases in sea surface temperatures to the north-west of Western Australia. Predictive skill is poor to good (45-75% consistent).
• Temperature outlooks for January to March 2018 from the Bureau indicate a 60-80% chance of above normal day-time maxima for the south-west region in particular. Other parts of the SWLD have near neutral chances. Skill is moderate to good at 55-75% consistent. Minimum temperature outlooks indicate a 60-80% chance of above normal night-time minima for most the SWLD, with poor to moderate skill at 45-65% consistent.
• December rainfall in the SWLD was above average along the coast, average in the central wheatbelt and below above in the eastern wheatbelt. December maximum and minimum temperatures were near average or above average.

Three month outlook for the south-west of Western Australia

Statistical Seasonal Forecasting (SSF)

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.

Due to technical difficulties, SSF outlook is for January to February 2018 and does not include March. The outlook is based on climate conditions in November 2017. The SSF is indicating 60-70% chance of exceeding median rainfall for the far northern agricultural region. In contrast, there is less than 40% chance for eastern and south-eastern agricultural regions. Neutral (40-60%) chances apply elsewhere. The most probable decile range map indicates decile 2-3 rainfall is most likely for the eastern and south-eastern agricultural regions, with decile 8-10 rainfall likely for the far northern wheatbelt and decile 4-7 elsewhere. Predictive skill based on November conditions is poor to good (50-70% consistent).

SSF forecast of probability of exceeding median rainfall for January to February 2018

Percent Consistent skill of the SSF at forecasting January to February rainfall using data up to and including November

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 generally 60-80% chance of exceeding median rainfall for January to March 2018 for the SWLD. Predictive skill is poor to good (45-75% consistent).

On average, there are seven tropical cyclones each season in the western region of Australia. The Bureau’s tropical cyclone outlook is indicating the average number is expected this coming summer. Typically between about 15-40% of tropical cyclones in the western region create coastal impacts. Wheatbelt summer rainfall is generally from localised thunderstorms, but tropical cyclones and tropical lows can generate significant rain if they travel southwards.

Temperature outlooks for January to March 2018 from the Bureau indicate a 35-80% chance of above normal day-time maxima for the SWLD, with higher probabilties concentrated in the south-west region. Skill is moderate to good at 55-75% consistent. Minimum temperature outlooks indicate a 60-80% chance of above normal night-time minima for most of the SWLD, with poor to moderate skill at 45-65% consistent.

The Heatwave Forecast, produced by the Bureau shows the location of heatwaves, severe heatwaves and extreme heatwaves for three-day periods. It may be useful to assist with livestock management. The latest bushfire outlook produced by The Bushfire and Natural Hazards Cooperative Research Centre, indicates an above normal bushfire risk for the south-west corner.

Rainfall outlook for January to March 2018 for Western Australia from the Bureau of Meteorology

Percent Consistent skill of the Bureau of Meteorology’s outlook for January to March rainfall

Recent climate

December rainfall in the SWLD was above average along the coast, near average in the central wheatbelt and below above in the eastern wheatbelt. December maximum and minimum temperatures were near average or above average.

In December the atmospheric pressure was near normal over the south-west. The Indian Ocean sea surface temperatures north-west and west of WA were cooler than normal.

Pacific Ocean climate patterns indicate a weak La Niña persists in the tropical Pacific. The event is expected to be short-lived, and end in the southern autumn of 2018. Past late developing La Niñas have had little influence on south-west Western Australia (SWWA) in summer.

The Indian Ocean Dipole (IOD) is neutral. IOD events are typically unable to form between December and April due to the influence of the monsoon trough over the tropical Indian Ocean. 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 until the end of January. In a positive SAM event, the belt of strong westerly winds contracts towards Antarctica. In spring and summer, a strong positive SAM can mean that southern Australia is influenced by the northern half of high pressure systems and hence there are more easterly winds bringing moist air from the Tasman Sea to south-east Australia. This increased moisture can turn to rain as the winds hit the coast and the Great Dividing Range. However, a positive SAM generally has little influence on spring/summer rainfall in the SWLD.

The table below gives a summary of past month and three month 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.