Seasonal Climate Outlook Archive

Frequently Asked Questions

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MEDIA RELEASE - ISSUED 15th APRIL 1999

Three-month Seasonal Climate Outlook Summary: Rainfall probabilities for May to July 1999.

Higher rainfall more likely in some eastern parts

The National Climate Centre's rainfall outlook for May to July 1999 is that there is an increased chance (from one-half to about two-thirds) of exceeding the median rainfall for areas of southeast Queensland and northeastern New South Wales (see map). Additional analysis indicates that there is also a significantly increased chance of wetter than average rainfall in a region of northern Queensland between the towns of Bowen, Barcaldine, Georgetown and Ingham.

Areas of seasonal outlook skill for this time of year include the areas mentioned above.

A weak La Niña sea-surface temperature (SST) pattern in the tropical Pacific seen in recent months has weakened further on the surface, although other climate indicators normally associated with La Niña persist. For example, the Southern Oscillation Index (SOI) has remained moderately high since June 1998. The unofficial SOI for the 30 days ending 12th April is +11, following the +9 value for both February and March.

Computer models which attempt to predict the development of La Niña aredivided in their current results. Slightly more than half suggest that it will remain a feature for the next six months, with the others suggesting more neutral conditions. At this stage, none of the models is indicating the development of an El Niño in the remainder of 1999.

Indices of SST patterns are used to produce the outlook. The indices measure the strength of La Niña/El Niño patterns in the Pacific, and a different pattern in the Indian Ocean. The Pacific index is close to zero, but the Indian Ocean pattern is showing warmer than average temperatures, contributing to this forecast.

 

Note: "wetter than average rainfall" here means three-month rainfall totals being in the highest one-third of historical values.

 

For more information regarding this outlook please contact Robert Fawcett on (03) 9669 4603, Scott Power on (03) 9669 4085, Clare Mullen on (03) 9669 4296, or Bill Wright on (03) 9669 4781, during normal office hours from 8:45am to 5:30pm (EST) Monday to Friday.

Archive of previous Seasonal Climate Outlooks




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, MEDIAN maximum temperature, or MEDIAN minimum temperature over the three-month outlook period.
The median is the middle value in the historical record for the period in question. In the long term, rainfall or temperature are above median in one half of years, and below median in the other half.
Example 1: For the July to September period 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.
Example 2: In Sydney, one-half of summers (Dec-Feb) have a mean maximum temperature above 25.7°C, with the other half being below. Therefore 25.7°C is the median.
Note that the mean or average maximum temperature is the average of all the daily highest temperatures for the period.
Similarly, the mean or 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 category of the eventual outcome (above or below median) is consistent with the category favoured in the outlook about 75% 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.


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