General Cyclone Information

Tropical cyclones are a seasonally occurring natural hazard that can cause considerable loss of life and material damage. In Australia, Cyclone Tracy caused the loss of 55 lives in Darwin on Christmas Day 1974. The most recent loss of life occurred at Onslow during Cyclone Bobby in 1995.

The strongest cyclone ever to cross the Australian coast, Severe Tropical Cyclone John, crossed north of Karratha near Whim Creek in December 1999. However, Severe Tropical Cyclone Vance, in March 1999, caused more damage, narrowly missing Karratha and Dampier but causing millions of dollars worth of damage to Onslow and Exmouth. Fortunately no lives were lost.

Definition & Occurrence

A tropical cyclone is a circular rotating storm of tropical origin in which the mean wind speed exceeds 63 km/h (gale force). Wind speeds in excess of 100km/h are common by the time a cyclone crosses the coast and higher wind speeds frequently occur. Tropical cyclones are very rare outside the cyclone 'season' from the beginning of November to the end of April. Once cyclones cross the coast, they tend to decay within 24 to 48 hours and the strong central winds die away. Dangerous flooding can occur as heavy rain falls inland.

Cyclone Threats

Cyclones threaten life and property in three ways:

• Fluctuating wind pressure can weaken and possibly cause the collapse of buildings and other structures.
• Loose objects, such as patio furniture, rubbish bins, dog kennels and building material can become lethal windborne projectiles causing severe structural damage to homes and killing or injuring people in their path.
• Flooding can result from an abnormal rise in the level of the ocean (storm surge) caused by the cyclone or as a result of heavy rainfall in river catchments.

Precautions

As cyclones often adopt an erratic course and can change speed suddenly, it is important to be aware of any changes. Tune your radio to the ABC or local commercial radio stations for regular broadcasts of up-to-date information.

Cyclone Season Preparation

• Clean your residence or work area of unwanted materials and rubbish.
• Trim trees of excess height and growth to withstand high winds.
• Clear drains of rubbish or obstructions.
• Have on hand a stock of emergency supplies, eg. first aid kit, candle or torches, batteries for your portable radio, canned food, drinking water containers, etc.
• Decide where best to shelter your pets.
• Determine location and establish cyclone tie-down areas for caravans, boats, trailers, etc.
• Have an emergency plan.

Cyclone Watch Messages

Cyclone Watch Messages are released every 6 hours when there is a possibility that a cyclone may produce gale force winds on the coast within the next 24 to 48 hours.

Cyclone Warnings Messages

Cyclone Warnings are released every three hours when there is a possibility of gales occurring in coastal areas within the next 24 hours. The warnings contain details of the position and movement of the cyclone as well as the coastal areas under threat.

Cyclone Warning messages are issued at 7am, 10am, lpm, 4pm, 7pm and 10pm. Hourly messages are issued when a cyclone is threatening population centres.

Broadcast Schedules

During a cyclone, the Watch or Warning messages are broadcast according to the following schedule: ABC Regional Network - 15min past the hour of issue North West Radio - 30min past the hour of issue Details of cyclone movements are also available by telephoning 1300 659 210 or faxing 1800 630 100. The number to call for road conditions in the Pilbara is 138 138.

Storm Surge

A cyclone may cause a tidal or storm surge which may present a significant hazard to the community of Karratha. Depending on the severity of a surge, some or even a majority of residents may need to be evacuated. The SES monitors the situation continuously, and jointly with the Police will initiate evacuation of residents by area as necessary. Low category cyclones are not likely to cause a tidal surge threat, however, higher categories of 3 to 5 may.