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Severe Thunderstorms and Warning Systems

What is a severe thunderstorm?

Literally thousands of thunderstorms occur each year in NSW and the ACT. Although all thunderstorms produce lightning, that is a danger itself, not all of them are "severe" or likely to produce damage. Severe Thunderstorms are defined as those that produce any of the following:

When is a severe thunderstorm warning issued?

Severe thunderstorms can be quite localised and develop quite quickly. The exact location of severe thunderstorms can be hard to predict. The warnings are usually issued without much lead-time before the event. To try and issue warnings with a greater lead-time would lead to a flood of false alarms, thus rendering the service ineffective.

What format are the warnings in?

Broadbased warnings are issued for the whole state of NSW and the ACT in both text and graphical formats. The Bureau's network of weather watch radars are the prime source of information that can be used to maintain an effective severe thunderstorm warning service. The radar networks are focused on the main population centres, therefore in these locations, the Bureau provides a more detailed severe thunderstorm warning service to cater for the densely populated cities and surrounding areas. Detailed warnings, both text and graphical, are issued for the Newcastle/Sydney/Wollongong area as well as for Canberra. Examples of the 3 different warnings, both text and graphics, can be found at: http://wdev.bom.gov.au/catalogue/warnings/GSTW/graphicalproductsnswact.shtml

Why is the warning system only for severe thunderstorms and not all thunderstorms?

Literally thousands of thunderstorms occur each year in NSW and the ACT. Although all thunderstorms produce lightning, that is a danger itself, not all of them are likely to produce damage. Warnings are issued by the Bureau to alert communities of the threat of these more dangerous thunderstorms. The Severe Thunderstorm Warning Service is one of the Bureau of Meteorology's highest priority programs and operates to minimise the loss of life, property damage and community disruption generated by severe storms.

What information is available when non-severe thunderstorms are around?

The more general forecast products will include thunderstorms in the same way they include showers, rain or other expected weather elements.

 

About Thunderstorms - characteristics, formation and detection

What is a thunderstorm?

Thunderstorms have a three-dimensional structure, but it is best to think of them as a constantly evolving process, rather than just an object. The diagram below represents a vertical cross-section through a thunderstorm moving towards the left of the screen. Each storm, or cluster of storms, is a self-contained system (1.2) with organised regions of upward moving air, known as updraughts, and downward moving air, downdraughts. The warm, moist air which fuels updraughts, typically rises in successive cumulus towers along a flanking line that leads to the storm's main core. As air rises within the core, it cools and condenses into rain or hail which then falls in the downdraught. Near the ground, the downdraught spreads out in what is known as the outflow region. Often, the leading edge of the outflow is marked by an abrupt, cool wind surge, the gust front.

 

thunderstorm
1.2 diagram of a thunderstorm.
 

How do we get thunderstorms and why are some thunderstorms severe

Thunderstorms require 3 main ingredients; a source of moist air, an unstable atmosphere and a mechanism to initiate their development. Moist air is important because when it condenses to form cloud, heat energy is released making the rising air more buoyant and "fueling" further cloud growth. An unstable atmosphere is necessary so that developing cloud is able to rise freely to great heights in the atmosphere. And initiating mechanisms are important as they serve as a focus for storm development. Typical mechanisms that initiate thunderstorms are fronts, troughs and regions of low pressure. Features of topography such as hills and mountains may also enhance storm development.

The severity of any subsequent thunderstorms will depend largely on the buoyancy of the rising air within the storm and the structure of the wind within the atmosphere. Wind direction and speed is rarely constant, and generally tends to increase in speed and turn anti-clockwise (in the southern hemisphere) with increasing altitude. The change in wind direction and speed as you move upwards through the atmosphere is known as "wind shear". Non-severe thunderstorms generally occur within environments possessing only low to moderate instability and minimal wind shear.

If the atmosphere is very unstable with light winds and little shear, storms develop a "pulse-like" character, rising strongly upwards then collapsing over the period of half to one hour. Severe thunderstorms in these conditions may produce large hail and strong bursts of wind but rarely produce widespread damage.

An increase in wind shear produces storms with additional opportunities for regeneration. This allows several storm "cells" at different stages of their lifecycle to be found within the one storm system, increasing their overall lifetime and the area they may affect. These "multicellular" thunderstorms may produce severe hail and wind, with the added possibility of flash flooding and weak tornadoes.

In some storms, the balance between buoyancy and wind shear approaches an optimum, leading to the development of long-lived thunderstorms with strong rotation within their cores. These storms are known as "Supercells" and are responsible for the majority of damage caused by severe thunderstorms. Supercell thunderstorms may produce very large hail, extraordinary wind gusts, powerful tornadoes and heavy rainfall.

Thunderstorms may at times be arranged in lines 100's of kilometres long or in large circular clusters. These large storm systems are known as "squall lines" and "mesoscale convective systems" respectively. When severe weather is associated with these systems, it may occur over large areas.

How do meteorologists identify severe thunderstorms?

Meteorologists rely on frequently updated data from automatic weather stations, weather observers and storm spotters, measurements of upper air conditions from weather balloons, atmospheric profilers and specially instrumented aircraft in the vicinity, as well as computer-generated analysis and forecast charts.

The Bureau’s radar display systems can visualize thunderstorms in 3-dimensions (as opposed to the 2-dimensional display available on the internet), allowing meteorologists to search for and identify severe features that are not apparent at ground level. The detailed cell based warnings depict and describe individual severe thunderstorms and therefore rely heavily on a detailed analysis of radar data.

 

Statistics and climatology of severe thunderstorms in NSW and the ACT.

How many thunderstorms and severe thunderstorms occur per year?

The average number of severe storms reported in NSW and the ACT is around 120 per year, averaged over the last 10 years.

What time of year do most thunderstorms occur in NSW and the ACT?

Although severe thunderstorms can occur at any time, the distribution of events shows a marked pattern throughout the year. The graph below shows the number of severe thunderstorms for each month of the year for the duration of available records (1900 to 2008). There is a marked tendency for severe thunderstorms (indeed all thunderstorms) to occur during the months September through to March. This period is normally referred to as the "Severe Thunderstorm Season" in NSW. The increase in storms during this period is primarily due to the increase in energy provided by the sun during the warmer Spring and Summer months, coupled with Spring and Summer weather patterns that are favourable for storm growth.

 

monthly distribution
 

What time of day do most thunderstorms occur in NSW and the ACT?

Just as there is a marked monthly distribution of storms, there is also a marked daily distribution, peaking between 2pm and 6pm during the afternoon. Again, this is primarily due to the daily heating of the earth's surface by the sun, which is a maximum during the afternoon.

hourly distribution

What are some of the more significant thunderstorms that have occurred in NSW and the ACT through history?


1914 On 25 November 1914, a severe thunderstorm, possibly accompanied by a tornado, struck Sydney’s northern and harbour-side suburbs. The storm tore through a row of shops opposite Lindfield railway station before moving over Middle Head to Watsons Bay where winds lifted a 16’ (5 metre) skiff 50’ (15 metres) into the air.
1919 In Sydney city an exceptionally heavy fall of hail just after midnight on 25 October1919 caused great damage to buildings when the hail blocked gutters and drains allowing heavy rainfall to overflow into ceilings and basements.
1931 On the morning of 6 July 1931, a heavy hailstorm caused £200,000 ($400,000) worth of damage in Sydney city and suburbs. The Manly ferry service was temporarily suspended, following the onset of southerly gales.
1937 One of the more disastrous storms in Sydney’s history occurred on the afternoon of 25 January 1937 when a tornadic thunderstorm tore through suburbs including Marrickville, Mascot, Botany and Matraville. Five people died and many more were injured as buildings collapsed and heavy rain generated flash floods.
1940 On the afternoon of 31 October 1940, a tornado carved a narrow damage path 12 miles (19km) long through thousands of Sydney houses, resulting in two deaths. A well-developed waterspout was seen as the storm moved out to sea. Sydney city recorded a wind gust of 153 km/h as the storm passed overhead.
1941 A tornado swept through Orange on 8 June 1941. It was said to be the fiercest storm ever known in the district up to that time.
1947 On 1 January 1947, a violent thunderstorm pounded Sydney city and the eastern suburbs with hailstones larger than cricket balls (7 cm) causing widespread damage to tiled roofs and cars. Hundreds of people were injured by flying glass
and the damage bill reached hundreds of thousands of pounds.
1957 A tornadic squall at Warriewood and Narrabeen caused more than £40,000 ($80,000) worth of damage on 9 July 1957. The tornado moved inland from the sea for about 3 miles (5 km), un-roofing houses.
1967 On 28 January 1967, thunderstorms left a trail of damage 100 km long through the New South Wales Riverina towns of Finley and Tocumwal and into Victoria. Severe winds, hail as large as tennis balls and possibly several small and transient tornadoes uprooted trees and damaged buildings.
1970 On 1 January 1970, a massive tornado, ‘probably more intense than any other documented in Australian literature’ at that time, carved a damage path 22 km long and up to 1.6 km wide through the Bulahdelah State Forest. Over one million marketable trees were damaged or destroyed by the tornado and cricket ball-sized hail.
1971 On 26 January 1971, torrential rainfall from a thunderstorm over southern Canberra caused severe flash flooding in stormwater channels adjacent to roads in the Woden Valley. Seven people died when their cars were swept into the flood waters. Rainfalls of up to 100 mm in one hour were recorded. Damage was around $9 m. A long-lived complex of thunderstorms brought widespread hail to Sydney on 21 August 1971, with the heaviest falls from Ashfield to Bondi. While the largest hail was generally only up to 2 cm diameter, it accumulated to depths of around 10-15 cm over flat ground and in drifts more than one metre high in some streets.
1975 Flash flooding from thunderstorms resulted in $15 m worth of damage in the Gosford, Sydney and Illawarra regions during 10-11 March 1975. Sydney airport recorded 174.7 mm in the six hours to 3 pm on 10 March, and Mt Kiera in the Illawarra received 591 mm in a 30-hour period.
1976 A series of severe thunderstorms on 10 November 1976 caused considerable damage in western and southern Sydney with tennis-ball size (6 cm) hail and wind damage reported around Lidcombe/Auburn. Damage reached $40 m, with 10 people injured.
1978 Severe thunderstorms struck between Newcastle and Wollongong on 10-11 February 1978, causing extensive damage to buildings and eight injuries. Wind gusts reached 141 km/h at Newcastle and a tornado damaged many homes in the Drummoyne / Hunters Hill area of Sydney. The total damage bill was $15 m. On 1 March 1978, severe thunderstorms on the north coast injured two people, left 15 homeless and caused damage estimated at approximately $5 m.
1984 Late in the evening of 8 November 1984, torrential rainfall from thunderstorms caused flash flooding and $80 m in damage to homes and vehicles in Sydney city and the eastern suburbs. Observatory Hill in Sydney city recorded its highest-ever hourly rainfall of 120.3 mm between 10 and 11 pm.
1986 On 22 January 1986, a severe hailstorm in the Orange district caused extensive damage to homes and vehicles and destroyed much of the apple crop at a cost of $25 m. On 3 October 1986, hail up to 6 cm diameter injured at least 10 people and caused $104 m damage to hundreds of homes in Sydney’s western suburbs, including Hurstville and Rockdale.
1990 On the afternoon of Sunday 18 March 1990 a violent thunderstorm with hailstones up to 9cm diameter battered western Sydney suburbs, with Auburn and Bankstown receiving the most serious damage. Three people were reported injured and the damage bill was over $319 m.
1991 A severe thunderstorm accompanied by destructive winds estimated at 230 km/h, 7cm diameter hail and flash flooding caused over $138 m damage to northern Sydney suburbs on the afternoon of 21 January 1991. The ferocious storm claimed one life and damaged more than 10,000 houses and 50,000 trees.
1992 On 12 February 1992, severe thunderstorms left a trail of damage through Sydney’s western and north-western suburbs, the central coast and as far north as Williamtown in the Hunter district. They brought hail up to 7.5 cm diameter, flash flooding, severe winds and possibly a tornado. Damage was estimated at $118 m.
1994 A severe storm caused $29 m damage in the Sydney metropolitan area on 20 November 1994, with many trees brought down and houses damaged, particularly north of the harbour. Property damage was also reported from inland areas including Narromine and Dubbo where wind gusts reached up to 140 km/h.
1995 A tornado in the Merimbula and Pambula areas on 16 April 1995 injured 34 people and damaged or destroyed more than 100 homes along with vehicles and other buildings. Damage estimates exceeded $25 m.
1996 There were widespread reports of severe thunderstorms across New South Wales on 29 September 1996, with large hail, destructive winds and at least three tornadoes. The worst impact was in Armidale where hail up to 8 cm diameter and 156 km/h wind gusts caused $104 m damage to thousands of homes, public buildings and vehicles. During the evening of 23 November 1996, a band of near-stationary thunderstorms brought torrential rain to the Coffs Harbour Creek catchment with rainfall up to 400 mm recorded in just 4.5 hours. One person drowned as a flash flood combined with a high tide flooded most businesses in the town centre. The total damage bill reached $20 m. On 11 December 1996, Singleton in the Hunter Valley was declared a natural disaster area after a thunderstorm with 7 cm diameter hail damaged roofing on more than 1000 houses and buildings at a cost of $49 m. Severe thunderstorms were reported from many other areas of the State including Springwood, Bundanoon and Kyogle.
1997 Severe thunderstorms were widespread over much of the Central and Northern Tablelands and Sydney metropolitan area with hail up to 5cm diameter, severe winds and flash flooding reported. The damage in Sydney alone reached $40 m.
1998 A severe thunderstorm with destructive winds and golfball-sized hail struck the western New South Wales town of Nyngan about 3:15 pm on 5 January 1998. Three houses were destroyed, 18 wholly unroofed and many other houses, businesses and public buildings were damaged. The damage estimate was $12 m.
1999 The most damaging Australian thunderstorm to date, with a cost of $1.7 billion, savaged Sydney’s eastern suburbs and parts of the city on the evening of 14 April 1999. Hail of at least 9 cm diameter damaged 22,000 homes and 63,000 cars and injured many people. A man died when struck by lightning.
2001 Winds up to 130 km/h, large hailstones and flash flooding caused extensive damage in Dubbo on 6 January 2001. About 400 buildings and 150 motor vehicles were damaged and hundreds of trees uprooted in what locals described as 'the worst storm in living memory'. Dubbo was declared a natural disaster area with a total damage bill exceeding $25 m. On 17 January 2001, hail up to 7cm diameter, high winds and flash flooding devastated Casino in northeast New South Wales, damaging 800 homes and 300 motor vehicles at a cost of $35m. Severe thunderstorms hit many other towns in northern New South Wales, including Grafton where wind gusts were measured at 135 km/h. On 3 December 2001, a thunderstorm at Richmond produced the highest wind gust ever recorded on mainland New South Wales – 174 km/h. Later that afternoon two schoolchildren died when a tree fell on their tent in the northern suburbs.
2005 Broken Hill was declared a natural disaster area after a severe thunderstorm caused extensive damage at 9 pm on 6 November 2005. Around 20 houses were unroofed, another 20 partially unroofed and powerlines brought down. Two people were rescued from a car trapped in floodwaters. The damage bill was $3.8 m. A severe thunderstorm with heavy rain and strong winds, and possibly a tornado, caused widespread damage to more than 50 suburbs in Canberra on 2 December 2005. The storm brought down hundreds of trees and power lines, unroofed buildings and resulted in one death. The total damage bill was $15 m.
2006 On 31 December 2006, a severe late afternoon thunderstorm brought exceptionally heavy rainfall, wind damage and deep drifts of hail to Canberra’s southern suburbs. Lake Tuggeranong was clogged with debris and livestock in rural areas was lost.
2007 On 27 February, the Civic Centre and the Australian National University in Canberra were blanketed in hail at least 20 cm deep. Many vehicles were damaged and more than 1100 homes and buildings were flooded due to blocked drains and gutters. On 10 February, very heavy rain caused flooding and damage to houses in Weston Creek and Kambah. On 8 October 2007, Lismore was declared a natural disaster area after a severe thunderstorm with 7 cm diameter hail and high winds caused $59 m worth of damage and injured more than 17 people. Another severe hailstorm hit South Lismore the following day and on 26 October a tornado damaged the nearby town of Dunoon. On 9 December 2007 a severe thunderstorm devastated western and northwestern Sydney with hail up to 7 cm diameter. The State Emergency Service received over 6000 calls for assistance and Blacktown, Baulkham Hills, Hornsby and Penrith were declared natural disaster areas. The damage bill reached $201 m.

Summaries of severe thunderstorms in NSW by financial year are available through the following links, or from http://www.bom.gov.au/nsw/sevwx/significant.shtml

Are there detailed descriptions or case studies of some significant thunderstorms available?

Details and radar images from severe storms and the damage they caused are available for:

 

RADAR

Why can't I see lightening strikes on the radar images?

Lightning data is measured by a private company, and the Bureau does not have permission to present the data on its radar viewer.

What does a thunderstorm look like on the radar picture on the web?

Information on how to interpret radar images including examples of thunderstorms is available at http://www.bom.gov.au/weather/radar/about/using_radar_images.shtml

How does the radar work?

To learn more about the Australian Weather Watch Radar go to http://www.bom.gov.au/weather/radar/about/index.shtml

 

About Lightning

What is lightning?

Lightning is an electrical feature of ALL thunderstorms. Lightning is therefore not a specific criterion for severe thunderstorms. Across Australia, lightning accounts for between 5 and 10 deaths each year and upwards of 100 injuries.  Meteorologists can’t yet predict exactly where lightning will strike and will not be able to so in the foreseeable future. Thus whenever thunderstorms are about, certain precautions need to be taken to reduce the risk of injury

How is lightning formed?

Lightning occurs to balance an electrical charge separation that is believed to develop due to the presence of ice crystals in a cloud. Generally the icy top of the thunderstorm (or cumulonimbus) cloud becomes positively charged and the base develops a negative charge. Many of the lightning strikes occur within or between clouds. Sometimes, however, the simplest way for the charge to be equalised is via a strike from cloud to ground.

What causes thunder?

Given that the temperature of a lightning bolt can exceed 20,000 degC it is not at all surprising that the surrounding air is heated quickly and "explodes". Thunder is the sound produced by this explosive expansion of the air. Because light travels faster than sound we often see the lightning before the thunder is heard.

How can I stay safe from lightning?

The average distance between successive lightning strikes from the same storm is around 3 to 5 km and at times they can occur up to 16 kilometres from where the rain is falling on the ground. You can judge how far away lightning is if you count the time in seconds between the lightning flash and the resultant thunder and then divide this by three.

If caught outside, you should seek shelter in an enclosed building or a hard-top car. Picnic shelters in parks and on sporting fields are not safe and you should not stand beneath them. If you are caught in the open with no chance of suitable shelter, it is best to crouch on the balls of your feet with your head down between your knees.

 

About Flash Floods

images_hr_flood43x.jpg

The Newell Falls becomes a torrid mass of flood water running down a mountain road after heavy rain near Bellingen, New South Wales, October 2004. Photo: Greg McLagan, Bellingen Shire Courier-Sun

What is a flash Flood?

Flash floods occur when rainfall is too intense to be adequately absorbed by the soil and drainage cannot cope with the deluge. Rainfall resulting in flash floods generally occurs quickly over periods of half an hour to several hours and can reach full peak in a matter of minutes. Flash floods differ from river flooding in that they are usually very localised and occur quickly following intense rainfall.

Why are flash floods dangerous?

Flash floods are extremely dangerous weather events as water in creeks, drains and natural watercourses can rise very rapidly. Fast-moving floodwaters are very powerful and can dislodge trees and boulders, sweep cars off roadways and wash away bridges. They are second only to lightning in thunderstorm-related weather deaths.

How are flash floods caused?

The most frequent cause of flash flooding is intense slow-moving thunderstorms. These storms can deposit extraordinary amounts of water over a small area in a very short time. Rising air within thunderstorms can suspend huge amounts of rain before releasing a deluge onto the ground. Such rain can reach intensities of more than 100 mm per hour. Often topography acts to focus thunderstorm development over a particular location, further accentuating rainfall accumulation.

What are some of the worst Flash Floods in NSW history?

How can I stay safe from possible flash flooding?

During thunderstorms, listen out for the Bureau of Meteorology's latest weather warnings. If caught outside, do not attempt to cross flooded creeks, roads and causeways. Children should not be allowed to play in stormwater drains. If flash flooding is indicated, your safest bet is to move immediately to higher ground.

 

About Severe Wind Gusts

What is a severe wind gust?

A wind gust from a thunderstorm of at least 90 km/h. Peak winds may exceed 160 km/h in the most damaging storms.

What are some of the strongest recorded wind gusts assciated with thunderstorms in NSW?

What causes severe wind gusts?

Wind gusts are generated in thunderstorms when falling rain and hail drag the surrounding air downwards. Evaporation of the raindrops and hail cools the descending air, increasing the air's density, and accelerating the downward rush. The strong downdraft then spreads out once it reaches the ground, producing a cool, gusty wind that can cause damage. If the storm itself is moving quickly, or the atmospheric winds aloft are strong, the wind gusts at ground level may increase further.

When do they occur?

Severe thunderstorm wind gusts can occur at any time of the year, however the most likely times are from August to December. Approximately 40% of all severe thunderstorms are associated with strong wind gusts.

windfact1.jpg (16313 bytes) treedamage.jpg (20116 bytes)

Strong wind gusts wreaked havok in this Caravan Park (left) on the NSW Central Coast in July 1998 and uprooted this tree (right) in Sydney in January 1996.

 

About Hail

What is hail and how does it form?

Temperatures near the tops of thunderstorm clouds are very cold, dropping to around -50 to -60 degrees Celsius even during the summer months. Hailstones form in a thunderstorm when updrafts carry raindrops into these extremely cold areas of the atmosphere where they freeze into ice. They then grow in size by moving with the powerful updrafts of air within the storm and colliding with super-cooled water droplets, which freeze on contact with the ice crystals. A hail stone falls when the thunderstorms updraft can no longer support the weight of the ice. Larger hailstones therefore generally form in clouds with more intense updrafts.

 

hail
hail2

Selection of hailstones that fell in Sydney suburb of Paddington on the evening of 14 April 1999 (left). Hail blankets the ground like snow in the New England city of Armidale on 29 September 1996 (right). Some of the Armidale hailstones reached 8 cm in diameter whilst the Sydney hailstones reached at least 9 cm.

How often does large hail occur and how big can it be?

Large hail (defined as 2cm or more in diameter) is common in New South Wales with an average of about 70 reports each year over the last 10 years. Large hail can occur in any month, but is particularly common from September to March, and reaches maximum frequency in November and December.

The most damaging hailstorm to date in Australian history occurred in Sydney, NSW on the evening of 14 April 1999. This violent storm produced hailstones with measured diameters at 9cm, although larger hail would certainly have fallen in the more severely-damaged areas.

The largest confirmed hailstones New South Wales to October 2008 are listed below. Note that the higher the population in an area the more likely we are to have large hail reported:

District

Largest
confirmed
hailstone (cm)

Year
Northern Rivers
14
1991
Hunter
12
2004
Metropolitan
11
2007
Central West Slopes
10
2004
Illawarra
9
1847
Northern Tablelands
8.5
1993
Upper Western
8
1889
Mid-North coast
7.5
1908
Northwest Slopes
7.5
1912
Riverina
7
1967
Central Tablelands
7
1995, 1986
Northwest Plains
6.3
1969
Southwest Slopes
6.3
2002
Central West Plains
5.5
1906,1908
Southern Tablelands
5.5
1994, 1972
South Coast
5
2000, 1999, 1997, 1958
Lower Western
5
1899

 

About Tornadoes

What is a tornado and how does one form?

A tornado is a rapidly rotating narrow air column extending from the updraught base of a thunderstorm cloud to the ground. Tornadoes are thought to be formed by the interaction between regions of strong updrafts and downdrafts of air within severe thunderstorm clouds. As a thunderstorm becomes stronger and develops an organised inflow, its main updraught may begin to rotate slightly. This is sometimes seen as broad rotation of the cloud base beneath the main updraught or in the circular nature of the wall cloud.

Weaker tornadoes are formed primarily by "tightening-up" of a rotating updraught. They occur as the storm intensifies to a maximum and are found right under the updraught core, sometimes without a significant wall cloud. Weaker tornadoes are most likely during mid-summer storms but may also accompany squall lines and wintertime thunderstorms, mainly in southern parts of Australia. They are still significant events as they may produce narrow strips of severe wind damage.

Stronger tornadoes typically occur with late spring/early summer severe storms and have a more complex cause. It is speculated that at a certain stage in the storms life-cycle, a particularly intense updraught pulse partially blocks the prevailing wind aloft and deflects air down toward the surface. The downward surge interacts with the updraught to produce a tight rotational motion (in much the same way as rolling a pencil between your hands). This "spinning motion" is then tilted into an upright position and enhanced as it moves torwards the ground as a tornado.

tornado_hobart.jpg
Tornado: Sydney to Hobart Yacht Race200???: Credit TWI Australia (Sydney).

 

Do tornadoes occur in Australia?

Tornadoes occur more commonly in Australia, and NSW, than most people would expect. The Bureau of Meteorology's database records 383 tornadoes across NSW from 1795 to December 2007. Most tornadoes in NSW occur in late Spring and Summer but they have been known to occur at all times of the year.

How big are tornadoes?

Tornadoes range in size from a few tens of metres across, up to around one kilometre in diameter. Because of this, damage is normally restricted to a small area, but is very extensive.

How fast are tornadic winds?

One of the highest wind speeds ever actually recorded in a tornado was 241 km/h in Michigan, USA in 1965. However, it will never be known if the measuring instrument received the full force of the wind!

Tornadoes are ranked using the Fujita F-scale which estimates wind speed based on the extent and severity of damage.  

F-Scale rating

F0

F1

F2

F3

F4

F5

Wind speed (km/h)

62-117

118-178

179-250

251-333

334-419

420-511

More recent recorded examples of tornadoes in NSW

 

Safety in Thunderstorms

What should you do in a severe thunderstorm?

The State Emergency Service advises that people should:

For emergency help in floods and storms, ring the SES (NSW and ACT) on 132 500.
See http://www.bom.gov.au/info/thunder/ for more about safety in storms.

 

Storm Spotter Network

What is the storm spotters network?

An important component of the Australian Bureau of Meteorology's Severe Thunderstorm Warning Service is a national network of volunteer Storm Spotters who provide "on-the-spot" information on damaging storms. Spotters are community-minded people drawn from all walks of life. They report either on a freecall telephone number direct to our forecasting offices and/or by lodging report forms or cards. Spotters supplement the Bureau's existing network of weather stations and cooperative observers. The information they provide has a tremendous impact on the Bureau's services, allowing us to verify warnings, calibrate weather radars during events and improve our understanding of the frequency and distribution of these storms across Australia. There are currently over 2000 Storm Spotters active in Australia.

See http://www.bom.gov.au/storm_spotters/index.shtml for more on the national storm spotters network and how to join.

See http://www.bom.gov.au/storm_spotters/handbook/handbook.shtml for the storm spotters guide to identifying thunderstorms and their severity.

Current storm spotters can find the latest NSW/ACT storm spotters newsletter at http://www.bom.gov.au/general/reg/storm_spotters/spotter_newsletter.shtml

Last update 22-Jul-2009

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