The Bureau of Meteorology has developed an enhanced severe thunderstorm warning service. This service has been implemented in Queensland, New South Wales, Victoria, South Australia and the Northern Territory and will be implemented in other states in the future. The aim is to help people more easily picture the areas under threat from dangerous thunderstorms. The new product contains a graphic that will compliment the Bureau's traditional text-based warnings for Darwin and the rural area.
A severe thunderstorm is a thunderstorm that produces one or more of the following:
This definition does not include lightning. By definition, every thunderstorm has lightning associated with it and at present, no method exists for forecasting the severity or frequency of lightning strikes.
Only about 10 percent of thunderstorms are severe, but these account for approximately 90 percent of the damage produced by all thunderstorms.
Individual thunderstorms are small-scale and short-lived phenomena - a thunderstorm is typically only about 10 km across and lives for only 30 minutes or so. Occasionally, and most commonly across the Top End, larger-scale, longer-lived thunderstorms known as squall lines can develop - these squall lines can be 30 km or more in length and can live for many hours. At any one time there can be many thunderstorms affecting an area, only some of which may be severe.
For more information about Severe Thunderstorms refer to the Severe Thunderstorms Brochure.
There are two types of Severe Thunderstorm Warnings provided by the Bureau of Meteorology for the NT. The first provides a detailed service for people living in Darwin and the rural area and includes a new graphic to accompany the warning to show in map format the location of severe thunderstorms and their forecast tracks. The second covers the regional towns of Katherine (including Tindal RAAF Base), Nhulunbuy, Tennant Creek, Alice Springs and Yulara.
Each Severe Thunderstorm Warning describes the area under threat and the particular hazards likely to be associated with the thunderstorms. These warnings are distributed widely to the media and emergency services and are available to the public via the Web and various telephone and fax-based services.
Additionally, for areas within the NT, outside the more populated townships mentioned above, a Severe Weather Warning may be issued for areas under threat from severe weather. More information about Severe Weather Warnings can be found here.
The Darwin and rural area Severe Thunderstorm Warning is issued to the public, emergency services and other organisations when severe thunderstorms are within the warning area, or they are detected outside the warning area and forecast to move into the Darwin and rural area, or there is high confidence that severe thunderstorms will develop within the warning area, during the next 60 minutes.
Occasionally, severe thunderstorms and their associated severe weather can be especially short-lived. Individual thunderstorm locations and forecast tracks would then be of little use because the thunderstorms would likely dissipate before the Warning reached the public. Severe Thunderstorm Warnings are not usually provided for this type of 'pulse' severe thunderstorm in the Darwin and rural area.
Most Severe Thunderstorm Warnings for the Darwin and rural area will be for severe wind gusts associated with tropical squall lines and multi-cellular thunderstorm clusters. Warnings will not usually be issued for very heavy rainfall leading to flash flooding unless it is combined with other severe weather phenomena. This is due to the high frequency and difficulty of forecasting heavy convective rainfall in the tropics.
Severe Thunderstorm Warnings describe the location and forecast movement of individual severe thunderstorms and therefore rely heavily on a detailed analysis of radar data. 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.
Meteorologists also rely on frequently updated data from automatic weather stations, measurements of upper air conditions from weather balloons, atmospheric profilers and specially instrumented aircraft using Darwin Airport, as well as computer-generated analysis or forecast charts.
Reports from registered Storm Spotters are also useful information to meteorologists. More information on becoming a Storm Spotter can be found here.
A sample warning is shown below.
Australian Government Bureau of Meteorology
TOP PRIORITY FOR IMMEDIATE BROADCAST
For people in the
Litchfield Shire areas.
Issued at 2:51 pm Tuesday, 14 September 2010.
The Bureau of Meteorology warns that, at 2:50 pm, potential severe thunderstorms were detected on weather radar near Corroboree Billabong and Annaburroo.
These thunderstorms are moving towards the northwest.
They are forecast to affect Acacia Hills, Middle Point and Lambells Lagoon by 3:20 pm and Palmerston, Humpty Doo and Berry Springs by 3:50 pm.
Damaging winds are likely.
The Northern Territory Emergency Service advises that people should:
The next warning is due to be issued by 3:55 pm.
Warnings are also available through TV and Radio broadcasts, the Bureau's website at www.bom.gov.au or call 1300 659 214. The Bureau and Northern Territory Emergency Service would appreciate warnings being broadcast regularly.
The location of each severe thunderstorm is indicated by a red ellipse. The ellipses show the positions of the thunderstorms at the "valid time" stamped on the image in the top right corner. This will generally be a few minutes before the Warning was issued. The thunderstorm positions shown are derived from radar data. The graphic shows a simplified representation of the situation, which may be very complex. In particular, only potentially severe thunderstorms are depicted.
An arrow indicates the forecast direction of movement of each thunderstorm. This is the direction towards which the thunderstorm is moving. Arcs are used to show the forecast positions of the front edge of the thunderstorm at 10 minute intervals.
The number of ten-minute forecast positions shown will depend on the current behaviour of the thunderstorms. Usually six arcs will be shown, giving forecast positions for the front edge of the thunderstorm out to 60 minutes from the valid time.
The orange cross-hatched "Immediate Threat" area shows the part of the Darwin and rural area which is forecast to be at risk from severe thunderstorms within the next 60 minutes. This area will often be larger than the area under the forecast tracks of the severe thunderstorms shown on the graphic. This is because forecasters need to consider the possibility that thunderstorms might deviate from the forecast tracks, and must allow for the development of new severe thunderstorms or the intensification of thunderstorms not yet meeting severe thunderstorm criteria.
The issue time is the local time at which the Warning was transmitted by the Bureau of Meteorology. The valid time is the time of validity of the initial severe thunderstorm locations shown by red ellipses on the chart.
Darwin and rural area warnings are valid for 60 minutes. The warnings will be updated every 30 to 60 minutes, or as required, as a weather situation evolves. The next warning will be issued by the 'next warning' time given in the text.
A warning cancellation will be issued immediately after the severe thunderstorm threat has ceased.
The warning area for Darwin and rural area Severe Thunderstorm Warnings covers the Darwin, Palmerston and Litchfield Shires, as well as Cox Peninsula, Dundee Beach and the Bynoe Harbour area. The waters of Darwin and Bynoe Harbours are not explicitly included in Severe Thunderstorm Warnings, which are for land areas only. However, severe weather phenomena associated with thunderstorms may occur over the harbours and adjacent coastal waters when a Severe Thunderstorm Warning is current.
The grey "Outside Warning Area" represents the part of the NT for which there is no Severe Thunderstorm Warning service provided.
Only thunderstorms that are identified as potentially severe are depicted and described in a Warning, within the limitations of the Darwin and rural area Severe Thunderstorm Warning service as described above. Other thunderstorms occurring in the warning area and the surrounding region will not be indicated on the map.
For the towns of Katherine, Nhulunbuy, Tennant Creek, Alice Springs and Yulara, a text-based warning is issued when a severe thunderstorm is predicted to affect the town area within the next hour. These warnings are issued as an alert to the public, emergency services and other organisations.
Severe Thunderstorm Warnings for NT towns are not usually issued for short-lived ‘pulse’ severe thunderstorms. Warnings will also not usually be issued for very heavy rainfall leading to flash flooding in Katherine and Nhulunbuy unless it is combined with other severe weather phenomena. This is due to the high frequency and difficulty of forecasting heavy convective rainfall in the tropics.
A sample warning can be found below.
Australian Government Bureau of Meteorology
TOP PRIORITY FOR IMMEDIATE BROADCAST
for Damaging Winds, Large Hailstones and Very Heavy Rainfall
For people in Alice Springs
Issued at 1:15 pm on Saturday 10 April 2010
Thunderstorms with the potential for damaging winds, large hailstones and very heavy rainfall have been observed on radar and are expected to affect Alice Springs between 2:30 pm and 4:30 pm CST.
The Northern Territory Emergency Service advises that people should secure loose outside objects, move cars under cover and seek shelter. Driving conditions may be hazardous - avoid flooded roads and watercourses.
This warning is also available on telephone 1300 659 214 or on the internet at www.bom.gov.au.
Warnings are also available through TV and Radio broadcasts, the Bureau's website at www.bom.gov.au or call 1300 659 214. The Bureau and State Emergency Service would appreciate warnings being broadcast regularly.
The warning area is defined as within a 15 km radius of each town's Post Office, with the exception of Katherine, where Tindal RAAF Base is also included within its warning area, 16 km southeast of the town.
Issued at 1:15 pm.
The issue time is the local time (CST) at which the Bureau of Meteorology transmitted the Warning.
Warnings for regional towns in the NT are valid for up to 2 hours, but can be updated earlier as the weather situation evolves. A warning cancellation will be issued immediately after the severe thunderstorm threat has ceased.