A comparison of two seasonal rainfall forecasting systems for Australia

There are at present two major governmental seasonal rainfall forecasting programs in Australia. The first of these programs is run by the Australian Government through the Bureau of Meteorology and commenced in 1989. The second is run by the Queensland Government (QG) through its Department of Environment and Resource Management (and its predecessors) and commenced in 1994. Both programs issue seasonal (three-month) rainfall outlooks, using the format of the (conditional) probability of exceeding the (climatological) seasonal median, employing empirical statistical schemes informed by climatological understanding of relevant mechanisms. In what follows, these two programs or systems will be referred to as the Bureau and QG systems, respectively. The primary known driver (apart from weather noise) of inter-annual climate variability in Australia is the El NiñoSouthern Oscillation (ENSO), which accordingly must be taken into account when devising seasonal forecasting systems for Australia. Both forecasting systems use an ENSO index as the primary predictor, currently in the form of a sea-surface temperature (SST) index for the Bureau system and the Southern Oscillation Index (SOI) for the QG system. The Bureau system additionally uses an SST index with substantial input from the tropical Indian Ocean, particularly those waters between southern India and Western Australia (Drosdowsky and Chambers 1998, 2001). In assessing forecast skill, we undertake measurements of how good a forecasting method is and/or is expected to be. There are many different metrics or skill scores, with different scores for different applications. The actual forecast format is an important ingredient in the selection of an appropriate verification score. Another important issue is the distinction between validation and verification. A comparison of two seasonal rainfall forecasting systems for Australia