Northern rainfall onset

Late rainfall onset likely for much of northern Australia

The chance of an early northern rainfall onset for the 2019-20 season is low over large parts of northern Australia. Areas which have a less than 30% chance of an early rainfall onset include much of the Top End and central parts of the NT, Cape York Peninsula and eastern Queensland, and the far north Kimberley in WA. In other words, these areas have a greater than 70% chance of a late rainfall onset. Most of northern WA has roughly equal chances of an early or late rainfall onset.  

The northern rainfall onset outlook gives an indication of whether the first significant rains after 1 September are likely to be earlier or later than normal. View normal onset dates here.

This is the final of three northern rainfall onset outlook issues for the 2019–20 season.

Influences

The El Niño–Southern Oscillation (ENSO) is currently neutral, and forecasts indicate it is likely to remain neutral to at least the end of 2019. This means it is unlikely to be influencing the northern rainfall onset outlook towards either an earlier or later than average onset. 

However, waters in the western tropical Pacific Ocean remain warmer than normal, while water temperatures around northern Australia are average to cooler than average. This pattern would typically favour cloud formation away from northern Australia. 

The Indian Ocean sea surface temperature pattern remains consistent with a positive Indian Ocean Dipole (IOD). This is expected to persist for the coming months. A positive IOD typically leads to drier than average conditions across northern Australia during the second half of the year. IOD events typically break down in early December, so this driver is unlikely to have a significant influence on rains beyond early December. 

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

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 tropical dry season. This is defined as the date when at least 50 mm of rainfall has accumulated after 1 September. Parts of coastal Queensland and the western Top End typically accumulate 50 mm by late October or early November, spreading further south and inland over following weeks. The southern inland regions of the Northern Territory and Queensland, as well as 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 1960–2012.

Dark grey shading on the map represents areas where there are too few weather stations to support an analysis.

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 2012.

About the outlook

What is the northern rainfall onset?

  • The northern rainfall onset occurs when enough rain has fallen to stimulate plant growth after the northern dry season. This is defined as the date when rainfall accumulated after 1 September reaches at least 50 mm. The rainfall onset is especially significant for primary producers, including the agricultural and horticultural sectors.
  • Parts of coastal Queensland and the western Top End regions typically have a northern rainfall onset date of October or early November, and this spreads further south and inland over coming weeks. The southern inland regions of the Northern Territory and Queensland, as well as 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 arrives at Darwin in the last week of December.
  • Dark grey shading on the maps represent areas where the weather station density is not sufficient to support an analysis. Light grey shading means that the usual rainfall onset date occurs later than the currently issued forecast period.

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 from June through to the end of August each year.
  • 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 of 40% and below indicate an increased chance of a late northern rainfall onset. Values between 40% to 60% show no strong signs of the onset being earlier or later than usual. 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 1990 to 2012.

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.

Further reading


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.