Northern rainfall onset

The northern rainfall onset date occurs when the rainfall total in a particular region reaches 50 mm or more from 1 September. It is considered to be approximately the amount of rainfall required to stimulate plant growth after the dry season.

What is the normal onset date?

The northern rainfall onset occurs when enough rainfall has fallen to stimulate plant growth after the dry season. This is defined as the date when at least 50 mm has accumulated after 1 September. In general, parts of coastal Queensland and the western Top End receive the first useful rainfall by late October or early November, and this spreads further south and inland over ensuing weeks. The southern inland regions of the Northern Territory and western parts of Western Australia usually have the latest northern rainfall onset, around mid-January. The long-term median is calculated by averaging the onset date for each year from 1960–2009.

Grey shading within the Australian domain represents areas with insufficient data for the northern rainfall onset calculation. In the more isolated regions of Australia, the weather station density is not sufficient to support an analysis, and thus these data-void areas are grey.

Map showing average times of northern rainfall onset across Australia

Median date of northern rainfall onset (1960–2009)

How accurate is the outlook?

Model accuracy (also known as model confidence or model skill) is a measure of how well the model has performed in the past for that time of the year. One way the Bureau measures the accuracy of its climate models is by comparing how often the real outcomes matched the forecast (as a percentage). This measurement of accuracy is known as percent consistent, and has been tested over the period from 1981 to 2011.

Historical accuracy maps for all outlook start dates are available above. Generally, the closer to September an outlook is produced, the higher its accuracy will be. This concept is similar to a weather forecast, with forecasts for tomorrow being more accurate than one for seven days ahead. On the historical accuracy maps, the higher the percent consistent value for an area (i.e. the greener/darker the map), the higher the accuracy has been in that area in the past, and thus more confidence can be placed in the outlook for those regions. Areas of the maps that are not green/coloured do not have a good record of accuracy. In the least accurate areas, the outlooks are no better than random chance (equivalent to flipping a coin). In areas not coloured green, some caution should be taken when using the forecast, notably at times when there is not a strong driver of our climate present (e.g. when there is no El Niño or La Niña).

It must always be remembered that the outlooks provide probability-based information. They are statements of chance or likelihood. For example, a 75% (or three in four) chance of an early rainfall onset still means there is a one in four chance that the rainfall onset will be late.


Early rainfall onset likely for the tropical far north

The chance of an early rainfall onset is high for the tropical far north of Australia. This includes the Top End of NT, Cape York Peninsula, the Kimberley and the Gulf Country. This suggests that most of northern tropical Australia will receive the first rains of the wet season earlier than usual.

The chances of an early or late rainfall onset are roughly equal over the tropical inland areas of Western Australia, the Northern Territory, and Queensland. The coastal regions of eastern Queensland also have roughly equal chances of an early or late rainfall onset.

This is the final issue for 2016–17.

Influences

The Pacific Ocean and atmosphere remain in a neutral El Niño-Southern Oscillation (ENSO) state. However, a La Niña still cannot be ruled out during the northern wet season. Regardless of whether La Niña thresholds are formally reached or not, some La Niña-like impacts are likely to affect Australia, including an earlier onset of the tropical wet season. Warmer than normal ocean temperatures may persist around the northern Australia coastline, raising the likelihood of an earlier wet season onset, as more evaporated moisture is available to fall as rain.

Map showing the chance of early rainfall onset across tropical Australia

Product code: IDCKOCNRN0