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 onset likely across central parts; later onset likely for west and far northeast

The August outlook for the 2015–16 northern rainfall onset suggests an increased chance of earlier-than-normal rainfall over central and southern parts of the NT, and southwestern Queensland.

However, over parts of the Pilbara, Kimberley, and the northern Cape York Peninsula, there is a decreased chance of an early rainfall onset. In other words, in these areas the first rains after the dry season are likely to be later than normal.

Elsewhere across northern Australia, there is no strong shift in the chances towards an early or late onset.

This is the final issue for 2015–16.

Influences

El Niño persists in the tropical Pacific, and has seen reduced cloud and rainfall to the north of Australia. The Bureau's climate model indicates El Niño is likely to remain through to early 2016. Typically during El Niño, the northern rainfall onset occurs later than normal. However, other influences, such as a notably warmer than normal Indian Ocean, are raising the likelihood of an earlier onset over some parts of Australia.

Map showing the chance of early rainfall onset across tropical Australia

Product code: IDCKOCNRN0