Seasonal Climate Outlook Rainfall Archive

Frequently Asked Questions

Three-month Rainfall Probabilities

MEDIA RELEASE - ISSUED 14th NOVEMBER 2001
Three-month Seasonal Climate Outlook Summary: Rainfall probabilities for Summer 2001/02

Increased summer rains for SE Qld, NSW & W.A.

Rainfall odds favour above average summer rainfall in parts of eastern and western Australia, according to the latest rainfall outlook from the Bureau's National Climate Centre. For the December to February period, the chances of above average (median) falls are 60 to 70% across southeast Queensland and the eastern half of New South Wales, as well as over a large part of western and southern W.A. However, summer is usually a dry time of year in southern W.A., and high totals are uncommon.

So with climate patterns like the current, about 6 or 7 summers out of 10 are expected to be wetter than usual in these regions, whilst about 3 or 4 out of 10 are drier. On a cautionary note though, the outlook scheme has modest skill at best for summer in these regions of increased probability.

In remaining areas, the chances of above average falls are mainly between 40 and 60%, with no big swing towards wet or dry conditions.

Background Information:

  • The Outlook probabilities are based on recent Indian and Pacific Ocean temperatures. Neutral conditions prevailed in the eastern tropical Pacific in October, but the Indian Ocean remained warmer than average and this dominates the rainfall outlook.
  • Continued neutral conditions is the most likely outcome for the tropical Pacific during the remainder of this year and early 2002. Only three of twelve international computer models predict an El Niņo by mid-autumn 2002.
  • Check out the latest on El Niņo on our ENSO wrap-up web page.
  • October's value of the Southern Oscillation Index (SOI) was -2, marginally below September's +1. The approximate SOI for the 30 days ending 11th November was +2.
  • This outlook represents a summary, more detail is available from the contact people or from SILO.
  • Important: Probability outlooks should not be used as if they were categorical forecasts. More on probabilities is contained in the booklet "The Seasonal Climate Outlook - What it is and how to use it", available from the National Climate Centre.

More information on this outlook is available during normal office hours from 9:00am to 5:30pm (EST) Monday to Friday by contacting the following climate meteorologists in the National Climate Centre:

Grant Beard on (03) 9669 4527
Robert Fawcett on (03) 9669 4603
Clare Mullen on (03) 9669 4296
Bill Wright on (03) 9669 4781

THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 18th DECEMBER 2001.

Archive of previous Seasonal Climate Rainfall Outlooks

Archive of previous Seasonal Climate Temperature Outlooks

October 2001 rainfall in historical perspective

August to October 2001 rainfall in historical perspective



probability of exceeding median seasonal rainfall
Click on the map for full resolution.

Information on tropical cyclones


 

Information on tropical cyclones
The tropical cyclone season around northern Australia extends from November to May. The average number of cyclones per season is 9.4 (mean from 1949/50 season to 1993/94 season), with a standard deviation of 3.4. Cyclone activity in this region is related to the El Niņo-Southern Oscillation phenomenon, with fewer than normal cyclones during El Niņo episodes, and slightly more during La Niņa episodes. Below is a map showing the extreme bounds of the Australian Tropical Cyclone Region (Australia's area of responsibilty).


australian tropical cyclone region






Frequently Asked Questions
Q: WHAT ARE THE BUREAU OF METEOROLOGY'S SEASONAL CLIMATE OUTLOOKS?

A:General statements about the probability or risk of wetter or drier than average weather over a three-month period. The outlooks are based on the statistics of chance (the odds) taken from rainfall and sea surface temperature records. They are not, however, categorical predictions about future rainfall, and they are not about rainfall within individual months of the three-month outlook period.
Q: WHAT DO WE MEAN BY "WETTER OR DRIER THAN AVERAGE, OR "WARMER OR COOLER THAN AVERAGE""?

A:Being above or below the median rainfall, average maximum temperature, or average minimum temperature for the three-month period.
The median is a useful measure of "normal" rainfall. In the long term, rainfall is above median in one half of years, and below median in the other half.
For example, from July to September at Mackay in Queensland, one-half of 3-month rainfall totals have been below 80mm, and one-half have been above. If rainfall was above 80mm in that period it would be "wetter than average" or above median. Over the long haul there is a 50% chance of this occurring. In terms of odds this is even money.
Note that the average maximum temperature is the average of all the daily highest temperatures for the period.
Similarly, the average minimum temperature is the average of all the daily lowest temperatures for the period
Q: HOW ACCURATE ARE THE OUTLOOKS?

A: In the places and seasons where the outlooks are most skilful, the eventual outcome (above or below median) is correctly given the higher chance about 70 to 80% of the time. In the least skilful areas, the outlooks perform no better than random chance or guessing. The rainfall outlooks perform best in eastern and northern Australia between July and January, but are less useful in autumn and in the west of the continent. The skill at predicting seasonal maximum temperature peaks in early winter and drops off marginally during the second half of the year. The lowest point in skill occurs in early autumn. The skill at predicting seasonal minimum temperature peaks in late autumn and again in mid-spring. There are also two distinct periods when the skill is lowest - namely late summer and mid-winter. However, it must always be remembered that the outlooks are statements of chance or risk. For example, if you were told there was a 50:50 chance of a horse winning a race but it ran second, the original assessment of a 50:50 chance could still have been correct.
Q: WILL CATEGORICAL OUTLOOKS EVER BE ISSUED? (Eg. It WILL be drier than average.)

A: Very unlikely. There is a certain level of natural variability in the climate which is chaotic and unpredictable. This is particularly the case with rainfall. For example, rainfall in a season can be significantly above average in one region, and significantly below average less than 50km away.
Q: HOW SHOULD THE OUTLOOKS BE USED?

A: As another tool in risk management and decision making. The benefits accrue from long-term use, say over 10 years. At any given time, the probabilities may seem inaccurate, but taken over several years, the advantages of taking account of the risks will outweigh the disadvantages. For more information on the use of probabilities, farmers could contact their local departments of agriculture or primary industry.



Definitions and Explanations....

THE SOUTHERN OSCILLATION INDEX (SOI) is calculated using the barometric pressure difference between Tahiti and Darwin.

The SOI is one indicator of the stage of El Niņo or La Niņa events in the tropical Pacific Ocean. It is best considered in conjunction with sea-surface temperatures, which form the basis of the outlooks.

A strongly negative SOI (below -10) is characteristic of El Niņo, which is often associated with below average rainfall over eastern Australia, and a weaker than normal monsoon in the north.

A strongly positive SOI (above +10) is characteristic of La Niņa, which is often associated with above average rainfall over parts of tropical and eastern Australia, and an earlier than normal start to the northern monsoon season.


El Niño & La Niña

El Niņo translates from Spanish as "the boy-child", and refers to the extensive warming of the central and eastern Pacific Ocean.

La Niņa translates from Spanish as "the girl-child", and refers to the extensive cooling of the central and eastern Pacific Ocean. The term has recently become the conventional label for the opposite of El Niņo.

See http://www.bom.gov.au/climate/glossary/elnino.shtml for more on SOI and El Niņo.