WA Seasonal Rainfall Outlook: probabilities for Winter 2006,
issued 25th May 2006 |
No significant shifts in rainfall odds in Western Australia
There are no significant shifts in the odds towards
either above or below average winter (June-August) rainfall
in Western Australia,
the Bureau of Meteorology announced today. The chances of accumulating at
least average rain during winter are close to 50% throughout the state.
The pattern of winter rainfall odds is largely a result of above average
temperatures in the Pacific Ocean.
Conditions in the Indian Ocean are near normal
and have little influence on the outlook.
Outlook confidence is related to the influence of Pacific and Indian Ocean
temperatures on seasonal rainfall.
During winter, history shows this influence to be
generally weakly consistent in Western Australia,
reaching moderate only in patches
(see background information).
The Southern Oscillation Index (SOI) has been generally positive recently,
with values for February, March and April of zero, +14 and +15 respectively.
However, values have recently dropped with the approximate SOI for the 30
days ending 21st May being zero.
Most computer models indicate that neutral ENSO conditions are likely to
remain in place for most of the remainder of 2006. However, it should be
noted that model skill is at its lowest during autumn. For
routine updates and comprehensive discussion on the latest data relating to
ENSO, together with details on what the phenomenon is and how it
has affected Australia in the past,
please see the ENSO Wrap-Up.
Click on the map above for a larger version of the map. Use the reload/refresh button to ensure the latest forecast map is displayed.
More information on this outlook is available from
9.00am to 5.00pm (WST) Monday to Friday by
contacting the Climate and Consultancy section in
the Bureau's Perth Office: (08) 9263 2222.
THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 22nd JUNE 2006
Corresponding temperature outlook
April 2006 rainfall in historical perspective
February to April 2006 rainfall in historical perspective
The Bureau's seasonal outlooks are 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 Australian rainfall/temperatures
and sea surface temperature records for the tropical Pacific and Indian Oceans.
They are not, however, categorical predictions about future rainfall,
and they are not about rainfall within individual months
of the three-month outlook period.
The temperature outlooks are for the average maximum and minimum temperatures
for the entire three-month outlook period.
Information about whether individual days or weeks may be
unusually hot or cold, is unavailable.
This outlook is a summary.
More detail is available from the contact people or from SILO
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.
These outlooks should be used as a 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 should outweigh the disadvantages.
For more information on the use of probabilities,
farmers could contact their local departments of agriculture or primary industry.
Model Consistency and Outlook Confidence:
Strong consistency means that tests of the model on historical
data show a high correlation between the most likely
outlook category (above/below median)
and the verifying observation (above/below median). In this
situation relatively high confidence can be placed in the
Low consistency means the historical relationship, and
therefore outlook confidence, is weak.
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.
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 Niño or La Niña
events in the tropical Pacific Ocean.
It is best considered in conjunction with sea-surface temperatures,
which form the basis of the outlooks.
A moderate to strongly negative SOI (persistently below 10) is
usually characteristic of El Niño, which is often associated with below
average rainfall over eastern Australia, and a weaker than normal monsoon
in the north. A moderate to strongly positive SOI (persistently above +10)
is usually characteristic of La Niña, 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.
The Australian impacts of 23 El Niño events since 1900 are summarized
on the Bureau's web site