About Rainfall Outlook

Introduction

Seasonal climate outlooks should always be given as probabilities, or the chance, of exceeding a certain threshold. In the case of the Bureauís rainfall outlooks, they are given as the likelihood of being above median, i.e., the chance of rainfall being above normal. However, many users are interested in specific rainfall amounts (e.g., 200 mm for the season), or are prepared to make decisions at specific probabilities (e.g., if there is a 75% chance it will be dry).

The maps seen on these pages result from statistical techniques which transform the outputs from the Bureauís official seasonal outlook into information in two forms: the chances that the three-month rainfall will exceed particular thresholds (e.g., 200 mm), and the three-month rainfall amounts that have a specific chance of occurring (e.g., 25%).

Verification has shown that these statistical techniques do not diminish the accuracy (i.e., how well the outlooks correctly favoured above or below median rainfall) or the reliability (how the probabilities produced by the model compared to the frequency of the outcomes) of the forecasts. To get the best results out of any seasonal outlook system, it is important to consider the accuracy of the model at the time of year that is of interest.

The seasonal outlook data is prepared on a 2.5°x2.5° grid, or roughly 250 km by 250 km.

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The Outlook Model and Schedule

In May 2013, the Bureau of Meteorology introduced a dynamical (physics based) climate model (the Predictive Ocean Atmosphere Model for Australia, POAMA), replacing the statistical seasonal outlook model (Drosdowsky and Chambers; 1998, 2001). Therefore, all seasonal climate outlooks issued by the Bureau of Meteorology after this date are based upon the dynamical model. It is a state of the art long-range forecast system that takes observations of the global oceans, atmosphere, ice and land, and uses the physics of how these all evolve and interact to calculate the likely climate in the months ahead. The model was developed jointly by the Bureau of Meteorology and CSIRO Marine and Atmospheric Research.

The model maps and text are typically updated in the last week of each month. The outlook is issued for the coming three months. For example, the outlook for winter (June to August) will be issued at the end of May.

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The Outlook

Outlooks for specific areas and locations are presented in three formats, as well as the standard above and below median outlooks.

Outlook scenarios convert the seasonal outlook into the rainfall amounts which have a 75%, 50% and 25% chance of occurring. To illustrate, the map below shows the rain outlook from 1 July to 30 September 2008 for the Australian continent. The colours on the map correspond to the amount of rainfall (mm) that has a 75% chance of exceeding for this period. The location highlighted with the black circle in western Victoria, south-east Australia, has a 75% chance of at least 100 mm and possibly up to 200 mm of rain occurring during the period.

75% chance of exceeding

If the climate is not strongly pushed one way or another (i.e., if there is no El Niño or La Niña event), then the amounts with a 75% chance of exceedance will be close to the 25th percentile. By definition, that means that 75% of past years will have had more rainfall for the season and 25% will have had less rainfall.

Chance of at least rainfall outlooks present the seasonal outlook in a different way. The user can choose from twelve different rainfall amounts (mm) for the coming season. Once selected the map displays the chance of receiving that amount. To illustrate, the map below shows the chance of receiving a total rainfall amount of at least 150 mm between July to September 2008 for the Australian continent. The colours on the map correspond to the percentage chance of 150 mm of rain occurring. The location highlighted with the black circle in western Victoria, south-east Australia, has a 65Ė75% chance of 150 mm of rain occurring during the period.

Chance of at least 150 mm

Chance of at least rainfall outlooks are consistent with tbe outlook scenarios. For the same location in the examples above, the chance of at least 150 mm of rainfall was 65-75%, which is consistent with the outlook scenario which showed a 75% chance of at least 100 mm, and possibly up to 200 mm.

Table versions of the seasonal outlook provide detailed information for over 260 specific locations and towns across Australia.

The user can choose to display a table of the outlook scenarios (i.e., 75%, 50%, or 25% chance of occurrence) or the chance of exceeding given amounts for specific locations around Australia. For example, the following table shows the Australian capital cities and the percentage chance that they will receive certain amounts of rainfall. It is immediately apparent that Darwin has a very low chance of receiving substantial rain, while Perth should be the wettest capital for this particular example season (July to September).

Stations used in the tables were originally selected in accordance with three main principles.

  1. at least two stations per rainfall district,
  2. a geographical spread as even as possible, and
  3. at least thirty years of rainfall data from which to compute rainfall statistics

The development of new high resolution datasets means that we are now able to add locations not previously available. Proposals for new locations, including latitude and longitude, may be sent via the Water and the Land feedback page.

Location10 mm25 mm50 mm100 mm150 mm200 mm250 mm300 mm400 mm500 mm600 mm700 mm
Perth 100 100 100 100 100 97 96 87 35 9
Darwin 54 25 7
Adelaide 100 100 100 96 80 55 27 10
Brisbane 100 100 87 56 27 13 6
Sydney 100 99 97 79 66 46 36 24 7
Canberra 100 99 99 83 54 22
Melbourne 100 100 100 96 61 22
Hobart 100 100 100 95 57 25 9

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References

Drosdowsky, W., and Chambers, L.E., (1998), Near global sea surface temperature anomalies as predictors of Australian seasonal rainfall. BMRC Research Report No. 65.

Drosdowsky, W., Chambers, L.E. 2001. Near global sea surface temperature anomalies as predictors of Australian seasonal rainfall, J. Climate 14:1677-1687.

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