Northern rainfall onset
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.
How accurate is the outlook?
Percent Consistent Rate (or forecast accuracy) 1981–2011
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.
The El Niño-Southern Oscillation (ENSO) and the northern rainfall onset
- The northern Australian rainfall onset is greatly affected by the phase of the El Niño-Southern Oscillation. During El Niño years, the onset date tends to be later than normal, while during La Niña years, the northern rainfall onset tends to be earlier than usual.
- The maps show the average northern rainfall onset date based on different phases of the El Niño–Southern Oscillation. The maps show years where the July–August average of the Southern Oscillation Index (SOI) has been below −8 (El Niño), above +8 (La Niña), or in between (neutral). The maps are calculated for all years between 1960 and 2009.
About the outlook
What is the northern rainfall onset?
The northern rainfall onset occurs when enough rain has fallen in northern Australia to stimulate plant growth after the dry season. This rainfall onset amount is defined as the accumulation of at least 50 mm after 1 September. The rainfall onset is significant for primary producers, including the agricultural and horticultural sectors.
Generally, parts of coastal Queensland and the western Top End regions receive the first useful rainfall from the northern rainfall onset by October or early November, and this spreads further south and inland over coming 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 rainfall onset as defined here is different from the Australian monsoon onset, which is characterised by a reversal of the prevailing winds and widespread heavy rainfall. The north Australian monsoon usually beings in late December.
Using rainfall onset outlook information
- The Bureau's northern rainfall onset outlook indicates the likelihood of the rainfall onset beginning earlier or later than the average onset date (see Normal onset). The outlooks are based on probabilities (i.e., the chance of the rains beginning earlier than normal), and are produced by the same Bureau climate model used for the climate outlooks.
- Outlooks are issued monthly from June through to the end of August each year, and are released on the same date as the Bureau's climate outlooks.
- The outlooks do not give an indication of rainfall amounts for the duration of the wet season, nor do they state categorically (i.e., 'yes' or 'no') that the northern rainfall onset will be early or late. They give a percentage chance of whether it will be early or late. Values of 60% and above indicate an increased chance of an early northern rainfall onset; values below 40% indicate an increased chance of a late northern rainfall onset. The outlooks are designed to be used as a tool in risk management and decision-making.
Rainfall onset for key tropical regions
- For key tropical regions, rainfall onset dials show the chance that the northern rains will begin earlier or later than normal, averaged over the region. The arrow represents the averaged percentage value, or average likelihood, for the region.
- The colours of the dial roughly correspond to the contours on the outlook map. However, the values are not given as precise figures and do not display the possible spread in probabilities over the specified region. The dials should be used in conjunction with the outlook map.
- The outlook maps are based upon a 2.5 degree grid, and grid boxes do not fall precisely over each region. To adequately represent the key regions, a proportion of an individual grid box is assigned to the calculation of the area-average. The map below shows the grid boxes used to calculate area-averages. The darkness of each grid square indicates the weighting of the square in the calculation, with the darkest shade having the strongest weighting.
For further information, see this research paper:
Drosdowsky, W., and M.C. Wheeler, 2014: Predicting the onset of the north Australian wet season with the POAMA dynamical prediction system. Wea. Forecasting, 29, 150-161 pdf
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.
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.
Product code: IDCKOCNRN0