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To be inserted: FAQ controls that allow definitions to be turned on or off as per TC page
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:
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.
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
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.
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1.2 diagram of a thunderstorm. |
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.
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.
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.
| 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. |
Lightning data is measured by a private company, and the Bureau does not have permission to present the data on its radar viewer.
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.
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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 |
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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. |
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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. |
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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 |
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 |
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.
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Tornado: Sydney to Hobart Yacht Race200???: Credit TWI Australia (Sydney). |
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.
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.
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 |
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.
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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