Tornado at Broken Hill on Sunday 6 November 2005

A severe thunderstorm struck the western NSW city of Broken Hill and surrounding areas at around 9 pm local time on Sunday 6 November 2005. The storm was accompanied by destructive winds thought to be associated with a tornado. Press reports indicated around 20 houses were unroofed (see photograph below right), another 20 were partially unroofed and power-lines were brought down. Two people were rescued from a car trapped in floodwaters but there were no reports of casualties. The photograph on the left, taken in the worst affected area in southern parts of the town, highlights the danger to people caught without shelter in a severe storm. The piece of roofing timber became airborne and speared through the side of an adjacent house.

broken_hill_image001.jpg broken_hill_image002.jpg

The photographs were taken by a Severe Weather Meteorologist from the Sydney Office of the Bureau of Meteorology, who visited the town to conduct a damage assessment.

The storm was an example of a very devastating type of thunderstorm known as a supercell. While supercells are relatively rare they account for a large amount of the damage from thunderstorms. They can produce violent tornadoes, large hail, damaging wind squalls and very heavy rain and can last for many hours. The frequency of supercells across southern Australia is highest in the period from late October through until December. At this time of the year the right mix of conditions for these storms to form are most common: sufficient moisture and instability in the atmosphere, a lifting mechanism to initiate the storms and wind shear in the atmosphere (a significant increase in wind speed with height above the earth's surface). These conditions were all present in the atmosphere in the period leading up to the Broken Hill storm. The thunderstorms first formed along a cold front that was located over eastern parts of South Australia during the late afternoon. The front was associated with a developing low pressure system near Adelaide (see Mean Seal Level Pressure analysis at 10.30 pm local time below). The front marked the boundary between warm, moist and unstable air from the north and cooler drier air from the southwest.

broken_hill_image003.jpg

The satellite image below taken at 9.00 pm local time shows the storm area associated with the front covering the far south west of New South Wales. The red shading shows cloud top temperatures colder than -60C indicating clouds extending upwards to more than 15 kilometres above the earth's surface.

broken_hill_image004.gif

An image from the Mildura radar (below left) and a merged radar image from the Mildura and Adelaide radars at 8.50pm local time (below right), show the storm cell responsible for the damage over the town. The small red circle shows the location of Broken Hill. The storm cell is approximately 250 kilometres from the Mildura radar, at the very limit of the range at which weather radar provides useful information to forecasters.

broken_hill_image005.gif

A time series of 10-minute radar images in the hour prior to the storm impact showed the storm cell change direction towards the left of the steering winds, a characteristic of supercell thunderstorm tracks.

The Bureau's Automatic Weather Station (AWS) at the Broken Hill Airport is located approximately 4.5 kilometres to the south southeast of the centre of town and about 1 kilometre to the south of where the most intense damage occurred. The AWS observations showed the wind direction changed from the east to the south southwest as the storm struck and the wind speed increased rapidly. The highest wind gust recorded at the AWS was 105 km/h and 33 mm of rain fell in about an hour. Wind gusts of this strength are estimated to occur about once every 5-10 years in this area.

The damage survey conducted by a the Bureau Severe Weather officer revealed that the region of severe damage was confined to an area approximately 200-300 metres wide and 1 kilometre in length along a path from   north northwest to south southeast through southern parts of the town. The localised nature of the more extreme damage suggests that the winds were stronger there than measured at the AWS, more likely of the order of 150 - 200 km/h. The type and extent of the damage is typical of that caused by a tornado. Consistent with this view, amateur video footage of the approaching storm showed a funnel cloud descending from the main cloud base.