Northern rainfall onset

Chances of an early or late northern rainfall onset roughly equal

Across northern Australia the chances of an early or late rainfall onset for the first significant rains after the dry season are roughly equal.

There is a slightly increased likelihood of an early rainfall onset in parts of northern QLD and the Pilbara in WA. There is also a weakly increased chance of a later than normal rainfall onset near the WA-NT border. View normal onset dates here.

This is the third and final issue of the Northern Rainfall Onset outlook for 2017–18.

Influences

Both the El Niño–Southern Oscillation (ENSO) in the Pacific and the Indian Ocean Dipole are currently neutral. They are likely to remain neutral until the end of 2017, and hence have little influence over the northern rainfall onset timing. Without a major ocean climate driver, secondary influences such as local ocean temperatures and the Madden–Julian Oscillation are likely to affect the rainfall onset date.

Map showing the chance of early rainfall onset across tropical Australia

Product code: IDCKOCNRN0


Onset outlook and rainfall observations. The rainfall observations section is active from 1 September to 31 May

Further information

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Rainfall maps are updated daily from September to the end of May

For map alternatives see: reports and summaries that describe rainfall history.

Radio buttons show maps of rainfall totals since September 1, 50mm (onset) date, and onset anomaly days.

Map showing season-to-date rainfall
Rainfall since 1 September

Observations product code: IDCKARJNTB


What are the normal onset dates?

What is the usual 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.

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 the 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.

How accurate is the outlook?


Details

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.

Rainfall onset for key tropical regions

  • Northern rainfall onset dial showing Early as greater than 60% chance of an early onset and Later as less than 40% chance of an early onsetFor 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.

Map showing grids used for each regional area-average

Further reading

  • 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


About the rainfall observation maps

Rainfall maps and schedule

  • Three maps are provided and all relate to the northern rainfall onset: rainfall totals map, rainfall onset date map and an early or late map.
  • Station data used are from the Bureau of Meteorology's rainfall network. As a result, not all areas across Australia are covered by a rainfall station.
  • A weighted analysis scheme is applied to station data, providing a broad spatial gridded coverage throughout Australia. The daily grids area available through the AWAP dataset.
  • Not all of the available data is available on the day of the rainfall being recorded – for some sites these are delivered many months later.
  • The maps are updated each Tuesday afternoon, between 1 October and 31 May.
  • From June until the start of the next northern rainfall onset season, the final end of season maps remain available for review.
  • End of season maps are available in each archive page since the 2015–2016 season.

Rainfall 'Totals' map

  • The rainfall is accumulated each day, starting from 1 September.
  • The rainfall accumlates until the end of May of the following year.

See also: Recent and past rainfall maps

Rainfall 'Onset date' map

  • The 'Onset date' map shows the dates where the 50 mm onset threshold has occurred.
  • Areas with totals below the onset threshold of 50 mm are not shown.

'Early or late' map

  • The 'Early or late?' map shows the number of days earlier or later than the mean (normal) onset date.

See also: Northern rainfall onset mean onset date

Rainfall observations archive

  • 'Totals', 'Onset date' and 'Early or late' end-of-season maps are available from each archive page.
  • Archive page dates are tied to rainfall onset outlook dates, so the latest year of archives displays the end-of-season maps from the previous season.