Summary

The Department of Primary Industries and Regional Development’s (DPIRD) Statistical Seasonal Forecast (SSF) outlook for summer, December 2020 to February 2021 is indicating neutral (40-60%) chance of exceeding median rainfall for the majority of the South West Land Division.

• For summer, December 2020 to February 2021, the SSF is indicating neutral (40-60%) chance of exceeding median rainfall for the majority of the South West Land Division (SWLD). With higher chances (60-80%) for the Central Wheatbelt forecast District. The most probable rainfall decile map indicates decile 8-10 for the Central Wheatbelt District, 2-3 for South East Coastal District and decile 4-7 elsewhere in the SWLD. Predictive skill based on November conditions is mostly poor to moderate (50-75% consistent).
• The Bureau of Meteorology’s seasonal outlook for summer, December 2020 to February 2021 is indicating 65-75% of exceeding median rainfall for the majority of the SWLD. Predictive skill is poor to good (45-75% consistent). The longer-term outlook for January to March is similar. Predictive skill is moderate to good (55-75%).
• Temperature outlooks for summer, December 2020 to February  2021, from the Bureau indicate a 60-80% chance of above average day-time maxima along the west coast, with lower chances (less than 40%) for the south coast and interior (high skill 75-100%) and 80% chance of above average night-time minima (high skill 75-100%) for the SWLD.
• November rainfall was highest on record for Perth and seventh highest for Western Australia due to a series of strong cold fronts. November maximum temperatures were below average to average and minimum temperatures were generally average.
• The La Niña in the tropical Pacific Ocean, together with a positive SAM are currently the main climate drivers influencing SWLD climate. In a La Niña, there is an increased chance of above average number of tropical systems (cyclones and lows) across northern Australia. This may lead to more summer rainfall in the SWLD. A positive SAM means westerly winds are further south than normal and this may increase temperatures in the SWLD.

Three Month Seasonal Outlook for the South West Land Division

Statistical Seasonal Forecasting (SSF)

DPIRD’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 [1] web page.

For summer, December 2020 to February 2021, the SSF is indicating neutral (40-60%) chance of exceeding median rainfall for the majority of the South West Land Division (SWLD). With higher chances (60-80%) for the Central Wheatbelt forecast District. The most probable rainfall decile map indicates decile 8-10 for the Central Wheatbelt District, 2-3 for South East Coastal District and decile 4-7 elsewhere in the SWLD. Predictive skill based on November conditions is mostly poor to moderate (50-75% consistent).

[2]

SSF forecast of the probability of exceeding median rainfall for December 2020 to February 2021 using data up to and including November.

[3]

Percent consistent skill of the SSF at forecasting December to February rainfall using data up to and including November.

Australian Bureau of Meteorology 

The Bureau of Meteorology's climate forecast system for monthly and seasonal climate outlooks is the Australian Community Climate Earth-System Simulator – Seasonal (ACCESS–S [4]). It is a dynamical (physics-based) forecast modelling system and is a collaboration between the Bureau of Meteorology and the UK Meteorological Office.

The Bureau of Meteorology’s seasonal outlook issued on 3 December is indicating 65-75% of exceeding median rainfall for summer, December 2020 to February 2021 for the majority of the SWLD. Predictive skill is poor to good (45-75% consistent). The longer-term outlook for January to March is similar. Predictive skill is moderate to good (55-75%).

Temperature outlooks for summer, December 2020 to February 2021, from the Bureau indicate a 60-80% chance of above average day-time maxima along the west coast, with lower chances (less than 40%) for the south coast and interior (high skill 75-100%) and 80% chance of above average night-time minima (high skill 75-100%) for the SWLD. These outlooks can be found at http://www.bom.gov.au/climate/ahead/outlooks/archive/20201029-outlook.shtml [5] .

[6]

Rainfall outlook for December 2020 to February 2021 for Western Australia from the Bureau of Meteorology.

[7]

Percent Consistent skill of the Bureau of Meteorology’s outlook for December to February.

Looking at other rainfall forecasting models, the odds are for neutral or wetter for December 2020 to February 2021 for the South West Land Division.

[8]

Model distribution summary of 13 models (not including the SSF) which forecast December 2020 to February 2021 rainfall in the South West Land Division.

Recent climate

November rainfall was highest on record for Perth and seventh highest for Western Australia. November maximum temperatures were below average to average and minimum temperatures were generally average.  Annual rainfall to date is generally below average in the SWLD.

[9]

Rainfall deciles for 1 January to 6 December 2020 in the South West Land Division.

In November, the atmospheric pressure [10] [10]was near normal over the SWLD.

Sea surface temperatures [11] of the Indian Ocean north of Western Australia are warmer than average, this may be favourable for enhancing rainfall over the continent.

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 over the 2020-21 summer. La Niña tends to favour positive SAM during the spring to summer months, which typically enhances the wet signal of La Niña in parts of eastern Australia. For the SWLD, a positive SAM in summer has little effect on rainfall over WA, but increases temperatures as westerly winds are further south than normal, reducing the sea breeze along the WA coast. For more information see the Bureau of Meteorology’s  Climate Driver Update [12].

The Indian Ocean Dipole (IOD) is neutral after tending towards positive values in recent months. With the Australian monsoon commencing, the IOD is likely to remain neutral through summer.

La Niña continues in the tropical Pacific. Australian and international climate models suggest it is likely to continue at least the end of February 2021.  A La Niña is often associated with increased chance of widespread rains and flooding over eastern, central and northern Australia, more tropical cyclones than average and prolonged very warm periods in the south. In a La Niña, there is an increased chance of above average number of tropical systems (cyclones and lows) across northern Australia. The first Australian landfall typically occurs in early December, which is about 3 weeks earlier than usual. For further information, see the Bureau of Meteorology’s Australian Tropical Cyclone Outlook [13]  and the Northern Rainfall Onset [14].

The table below gives a summary of past month and three-month South West Land Division (SWLD) climate conditions, and can indicate what is likely to occur in the near future if climate conditions follow the current pattern.