Summary
The Department of Primary Industries and Regional Development’s (DPIRD) Statistical Seasonal Forecast (SSF) outlook for summer, December 2019 to February 2020 is indicating greater than 60% chance of exceeding median rainfall for the majority of the South West Land Division (SWLD).
- For December 2019 to February 2020, the SSF is indicating greater than 60% chance of exceeding median rainfall for the majority of the South West Land Division. With neutral chance (40-60%) for the Lower West District. The most probable rainfall decile map indicates decile 8-9 for SWLD. Predictive skill based on November conditions is poor to good (50 -75% consistent) with slightly better skill in the north.
- The Bureau of Meteorology’s current seasonal outlook is indicating 35-60% chance of exceeding median rainfall for December 2019 to February 2020, with the higher chance in the northern part of the South West Land Division. Predictive skill is good (65-75% consistent). The longer-term outlook for January to March is near neutral with 45-65% chance but with poorer skill at 50-75% consistent.
- Temperature outlooks for December 2019 to February 2020, from the Bureau indicate an 80% chance of above average day-time maxima (skill 55-100%) and night-time minima (skill 65-100%) for the SWLD for the SWLD.
- November rainfall was below average to average for the SWLD.November maximum temperatures were above average and minimum temperatures were average to above average.
- The strong positive Indian Ocean Dipole (IOD) and negative Southern Annular Mode (SAM) continue to be the major climate drivers. Both influences are expected to progressively weaken and then dissipate by late December.
- The Bureau of Meteorology’s tropical cyclone outlook for 2019-2020 is indicating slower start to the season with fewer than average number of tropical cyclones expected in the west.
Three Month Outlook for the south-west of Western Australia
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 December 2019 to February 2020, the SSF is indicating greater than 60% chance of exceeding median rainfall for the majority of the South West Land Division. With neutral chance (40-60%) for the Lower West District. The most probable rainfall decile map indicates decile 8-9 for SWLD. Predictive skill based on November conditions is poor to good (50 -75% consistent) with slightly better skill in the north.
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[3]Bureau of Meteorology seasonal climate outlook
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 current seasonal outlook is indicating 35-60% chance of exceeding median rainfall for December 2019 to February 2020, with the higher chance in the northern part of the South West Land Division. Predictive skill is good (65-75% consistent). The longer-term outlook for January to March is near neutral with 45-65% chance but with poorer skill at 50-75% consistent.
Temperature outlooks for December 2019 to February 2020, from the Bureau indicate an 80% chance of above average day-time maxima (skill 55-100%) and night-time minima (skill 65-100%) for the SWLD for the SWLD.
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[6]Recent climate
November rainfall was below average to average for the SWLD. Year to date decile rainfall map indicates decile 1-3 rainfall for the SWLD.November maximum temperatures were above average and minimum temperatures were average to above average. The number of days above 30˚C in the SLWD in November were high.
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[8]
In November, the atmospheric pressure [9] [9]was above normal over the SWLD.
Indian Ocean sea surface temperatures [10] (SST) off the WA coastline have been cooler than average. The December 2019 to February 2020, SST forecast by the Bureau of Meteorology [11] indicates SSTs are likely to be warming up north of WA.
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 has been negative since July and is forecast to remain negative until at least late December. A negative SAM increases the chance of spring heatwaves occurring across southern Australia.
The strong positive Indian Ocean Dipole (IOD) continues to influence Australia's weather, but has weakened over the past four weeks. The IOD continues to show signs it will persisted later than usual this year, and will dissipate when the monsoon trough moves into the southern hemisphere, in mid-December. The El Niño–Southern Oscillation (ENSO) is expected to remain neutral for the remainder of 2019. For further information, see the Bureau of Meteorology’s ENSO Wrap Up [12].
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.
| Climate Indicator | Past month | Past 3 months |
|---|---|---|
| SWLD Rainfall [13] | Below average to average | Below average |
| SWLD Mean Temperature [14] | Above average | Very much above average |
| SWLD atmospheric pressure [9] | Above normal | Above normal |
| Indian Ocean Sea surface temperature [15] | Cooler | Cooler |
| El Niño/Southern Oscillation [12] (ENSO) | Neutral | Neutral |
| Indian Ocean Dipole [16] (IOD) | Positive | Positive |
| Southern Annular Mode [17] (SAM) | Negative | Negative |
