Seasonal Climate Outlook Rainfall Archive

Three-month Rainfall Probabilities

MEDIA RELEASE - ISSUED 14^th September 2000

Three-month Seasonal Climate Outlook Summary: Rainfall probabilities for October to December 2000

Wetter in the north, drier in the southeast

The National Climate Centre's outlook for total October-December rainfall shows the chances of a wetter than average season exceed 60% across much of northern Australia (see map below). The highest probabilities of 70 to 80% are in central and northern Queensland where the outlook skill is high for this period. The prediction model has low to moderate skill levels elsewhere.

The chances of wetter than average conditions are generally below 50% in Victoria, Tasmania, and the south of South Australia, thereby implying that a drier than average season is the more likely outcome. The lowest chances of 35 to 40% occur in those parts of the southeast that have been deficient in rainfall for the past four years. In fact the overall pattern, of widespread above average rainfall except in the southeast, is one that has been dominant since mid-1998.

Background Information:

This outlook uses data from both the Pacific and Indian Oceans. Recently, the Indian Ocean has become increasingly warmer than average, whilst the Pacific temperatures have remained close to average. The Indian Ocean has had the most impact on the outlook probabilities.
Computer models suggest neutral conditions in the Pacific Ocean for the next 6 months - i.e. no El Ni�o or La Ni�a.
The August Southern Oscillation Index (SOI) was +5, a rise of 9 points from the July value of -4.
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.

The Bureau of Meteorology is hosting a workshop, CLI-MANAGE, in Albury from 23-25 October, and places are still open for anyone wishing to attend. Participants will learn, and discuss, how climate information can assist their decision-making in climate-sensitive industries, such as agriculture. For more information please contact Linda Sampson on (03) 9669 4072.

More information on this outlook is available during normal office hours from 8:45am 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
Scott Power on (03) 9669 4085

Archive of previous Seasonal Climate Rainfall Outlooks

Archive of previous Seasonal Climate Temperature Outlooks

August 2000 rainfall - Decile Distribution.

Winter 2000 rainfall - Decile Distribution.

probability of exceeding median seasonal rainfall

Click on the map for full resolution.

*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.

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 Ni�o or La Ni�a 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 Ni�o, 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 Ni�a, 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 Ni�o translates from Spanish as "the boy-child", and refers to the extensive warming of the central and eastern Pacific Ocean. La Ni�a 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 Ni�o. See http://www.bom.gov.au/climate/glossary/elnino.shtml for more on SOI and El Ni�o.

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 Ni�o or La Ni�a 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 Ni�o, 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 Ni�a, 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 Ni�o translates from Spanish as "the boy-child", and refers to the extensive warming of the central and eastern Pacific Ocean.

La Ni�a 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 Ni�o.

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

Seasonal Climate Outlook Rainfall Archive

Three-month Seasonal Climate Outlook Summary: Rainfall probabilities for October to December 2000

Wetter in the north, drier in the southeast

Graphical Views