 |
 |
 |
|
IntroductionThe Bureau
of Meteorology provides a comprehensive weather service for all of Australia.
Very high priority is given to providing warnings of events that may
endanger life and property. The highest priority of all is given to
tropical cyclones which pose a major threat to coastal areas in Queensland,
Northern Territory and Western Australia.
The Warning
System is based on three Tropical Cyclone Warning Centres (TCWC) located
at the Bureau's Regional Offices in Brisbane, Darwin and Perth. Each
of these centres is staffed by very experienced specialist scientists
and technical support staff. Why is a Cyclone dangerous?Cyclones produce
extreme winds which may exceed 200 km/h. These winds can cause extensive
property damage and cause airborne debris to become potentially lethal
missiles. It is important to remember that the passage of the cyclone
centre or "eye" will produce a temporary lull in the wind but that this
will soon be replaced by extreme winds from another direction.
Cyclones also
produce flood rains which can cause further damage and death by drowning.
The phenomenal seas accompanying cyclones are dangerous both for vessels out at sea and those moored in harbours; serious erosion of the adjacent foreshore can also occur. Another marine
phenomenon which can cause inundation of low lying coastal areas is
the storm surge. This is a raised dome of water about 60 to 80 km across
and typically about 2 to 5m higher than the normal tide level. If the
surge occurs at the same time as a high tide then the area inundated
may be quite extensive.
The Australian Cyclone Warning SystemThe general
name given to cyclone Watch and Warning messages is an Advice or Tropical
Cyclone Advice. The TCWC issues these messages and liaises with the
police, State and Territory Emergency Services and the media.
Tropical cyclones
develop from tropical depressions. They are formally designated tropical
cyclones and named when winds of at least gale force have developed.
The name given to a system at this time is used throughout its life.
Immediately it is recognised that a cyclone might endanger life or property
of coastal or island communities within 48 hours, the Bureau's public
warning system is activated. A tropical cyclone or developing depression
that does not threaten any coastal or island communities will be mentioned
only in the Bureau's weather notes and in advice to shipping and aviation.
A cyclone
watch is issued if a cyclone or potential cyclone exists and there
are strong indications that winds above gale force will affect coastal
or island communities within 24 to 48 hours of issue. The message
contains a brief estimate of the cyclone's location, intensity, severity
category and movement and identifies the coastal area that could be
affected. Watch messages are renewed every six hours. A cyclone
warning is issued as soon as gales or stronger winds are expected
to affect coastal or island communities within 24 hours. It identifies
the communities being threatened and contains the cyclone's name, its
location, intensity (including maximum wind gusts and its severity category)
and its movement. Forecasts of heavy rainfall, flooding and abnormally
high tide are included when necessary. Communities under threat are
also advised to take precautions necessary to safeguard their lives
and property. Cyclone warnings are issued every three hours. When a cyclone is under radar surveillance close to the coast and poses a severe threat hourly advices are issued. A tropical cyclone
advice is prefixed "FLASH" when it is the first warning to a
community not previously alerted by a cyclone watch. It is also issued
when major changes are made to the previous warnings due, for example,
to unexpected movement towards the coast or rapid intensification. Where do I get Cyclone information?The main
sources of cyclone information are radio and television stations.
During a cyclone threat, some stations retain local staff on duty 24 hours a day to broadcast cyclone information. Because of the
high probability of the mains power supply being disrupted, it is important
to have a battery operated radio to listen for cyclone advices. The Bureau, in collaboration with Telstra, also records the latest advices on telephone warning services in each region. The numbers to call are listed below:
NOTE: For contact numbers outside capital cities, please refer to the Disaster/Emergency Information pages near the front of your telephone directory, or ask the operator. Cyclone Severity CategoriesAn estimate
of cyclone severity is now included in all tropical cyclone advices.
The catagories below provide information concerning effects due to
wind which are typical of cyclones in the various categories. Remember
that the system is not designed to give an exact statement of conditions
at individual locations but will give a general idea of the expected
worst conditions.
Categories of cyclone severity range from " 1 " for weak cyclones to "5" for the most severe cyclones. Accordingly, the risk of property and crop damage, shore erosion and danger to life increases from low for a category 1 to very high for a category 5 cyclone.
Using this severity scale, communities will be able to assess the degree of cyclone threat and take appropriate action. It must be emphasised that the category refers to the severity in the zone of maximum winds and therefore the effects felt at individual locations may not be exactly as described in the accompanying table.
CategoriesCategory
1
Strongest gust less than 125 km/h Typical effects (indicative only) - Negligable house damage. Damage to some crops, trees and caravans. Craft may drag moorings. Category
2 Category
3 (eg. Winifred) Category
4 (eg. Tracy) Category
5 (eg. Orson) Cyclone MovementResearch has
shown that cyclones in the Australian region exhibit more erratic paths
than cyclones in other parts of the world. A tropical cyclone can last
for a few days or up to two or three weeks. Movement in any direction
is possible including sharp turns and even loops.
Even if previous cyclones have not moved over your area this is no guarantee that future cyclones will also miss you. How to use the cyclone plotting mapPlotting
maps are published in telephone books and are available from local emergency
services and Bureau of Meteorology offices in the tropics.
Using the sequential positions of the cyclone centres as indicated in the Advices, the cyclone track can be plotted on a cyclone plotting map. Although
plotting successive positions is a good way to appreciate the movement
of the cyclone's centre, it must be realised that the dangerous winds
surrounding the cyclone can extend up to 100 km from its centre and,
on occasions, much further. Generally the place names mentioned in the Tropical Cyclone Advices are restricted to those indicated on the plotting map. However when a cyclone is close to the coast it is sometimes necessary to use other place names. The position
of the cyclone in an Advice is given both in terms of latitude and longitude
and as a bearing (in 16 compass points) and distance from a town or
coastal landmark. The latitude and longitude should always be used for
plotting cyclone positions. The bearing distance location is provided
for those who do not have a plotting map. If you try to plot both there
will often be a small discrepancy caused by the limitation of using
16 compass points. The Tropical Cyclone Advice will indicate the zones of dangerous winds which may be expected. Keep in mind that these dangerous winds may be experienced on the coast several hours ahead of the time of arrival of the cyclone centre.
Cyclone Warning MessagesAs well as
indicating expected maximum wind gusts near the centre of the cyclone,
warning messages will usually indicate the strength of the maximum wind
gusts expected over particular areas in any of the following terms:
The warning may mention above normal tides. The effect in terms of the risk of flooding at the coast will be described as follows:
If evacuation
of waterfront areas becomes necessary additional messages from police
or local government authorities will say who should move and will give
details of evacuation shelters. Example of a cyclone warning messageTOP
PRIORITY--FOR IMMEDIATE BROADCAST
TROPICAL CYCLONE ADVICE NUMBER 22 ISSUED BY THE BUREAU OF METEOROLOGY, BRISBANE AT 5 AM EST MONDAY 11/02/91. A TROPICAL CYCLONE WARNING IS CURRENT FOR COASTAL AND ISLAND COMMUNITIES BETWEEN YEPPOON AND CAPE MORETON. AT 5 AM SEVERE TROPICAL CYCLONE ANN, CATEGORY 3, WAS LOCATED ABOUT 90 KM EAST NORTHEAST OF GLADSTONE AND MOVING IN A SOUTHERLY DIRECTION AT ABOUT 15 KILOMETRES PER HOUR. VERY DESTRUCTIVE WINDS ARE EXPECTED ALONG THE COAST BETWEEN GLADSTONE AND MARYBOROUGH AND ACROSS FRASER ISLAND THIS MORNING. GALES ARE EXPECTED SOUTHWARDS ALONG THE COAST FROM MARYBOROUGH TO CAPE MORETON AND OFFSHORE NORTHWARDS FROM GLADSTONE TO YEPPOON. BEACH FRONT RESIDENTS BETWEEN GLADSTONE AND MARYBOROUGH WARNED THAT DANGEROUS TIDES COULD RISE UP TO 2 METRES ABOVE NORMAL AND CAUSE INUNDATION OF LOW LYING COASTAL LAND THIS MORNING. HEAVY RAIN IS EXPECTED TODAY SOUTH OF BUNDABERG WITH SIGNIFICANT STREAM RISES. DETAILS OF SEVERE TROPICAL CYCLONE ANN, CATEGORY 3, AT 5 AM EST:
RESIDENTS BETWEEN YEPPOON AND CAPE MORETON ARE ADVISED TO TAKE PRECAUTIONS AND LISTEN TO FURTHER ADVICES. THE NEXT ADVICE WILL BE ISSUED AT 6 AM EST.
|
