About Graphical Severe Thunderstorm Warnings in Queensland
The Bureau of Meteorology has developed an enhanced severe thunderstorm warning service. This service has been implemented in Victoria, New South Wales. South Australia and Queensland, and will be implemented in other states in the future. The aim is to better define the regions under threat when dangerous thunderstorms are occurring. The new products contain graphics that will complement the Bureau's traditional text-based warning products.
Changes to the format of Severe Thunderstorm Warnings issued for Southeast Queensland
The Bureau will be implementing changes to the format of Severe Thunderstorm Warnings issued for Southeast Queensland. The changes are intended to be implemented on September 17 2008. These changes will bring Severe Thunderstorm Warnings issued in Queensland into line with those in other states.
Summary of changes:
1. The main change will be the use of Local Government Areas (LGAs) as the spatial subdivision mentioned in the text and shown in the associated graphic in SE Queensland Severe Thunderstorm Warnings. This will replace the BoM Forecast Districts.
2. The warning area will be enlarged slightly to span the area east of Dalby from Rainbow Beach to Stanthorpe.
3. If the product IDQ20041 (Severe Thunderstorm Warning - Queensland, is current, the graphic associated with the SEQ warning product will show the Queensland threat area, and the area immediately at threat in SEQ in a striped orange shading. Severe thunderstorm cells and their directions of movement will be shown in red as previously.
Severe ThunderstormsA severe thunderstorm is a thunderstorm that produces one or more of the following:
For more information about Severe Thunderstorms refer to the Severe Thunderstorms Brochure
Only about 10 percent of thunderstorms are severe, but these account for approximately 90 percent of the damage produced by all thunderstorms.
Overview of the Severe Thunderstorm Warning Service in Queensland
There are two types of warning service provided by the Bureau of Meteorology. The first covers all parts of Queensland. The second provides additional detail for people living in the south eastern corner of Queensland, from Gympie to the Gold Coast and west to Dalby. This region includes the greater Brisbane area, the Gold Coast, the Sunshine Coast, as well as Toowoomba and surrounds.
1) Severe Thunderstorm Warnings for QueenslandBasis of Warnings for Queensland
These warnings are issued as an alert to the public, emergency services and other organisations when severe thunderstorms are likely to develop, or extend into, a specified area over the next few hours. The warnings are issued for all parts of Queensland. A sample of the format can be found below.
Important things to note
The area covered by the Warning and under threat from severe thunderstorms is indicated by yellow shading. The area may consist of all or part of one or several weather forecast districts.
The issue time is the local time at which the Bureau transmitted the Warning.
Warnings for Queensland are valid for up to 6 hours but are updated as a weather situation evolves, usually every 3 hours, but more frequently if the situation warrants. The expected issue time of the next Warning is given in the text.
Districts shown on map
The divisions on the chart indicate the boundaries of the Bureau's weather
forecast districts. Some districts are labelled with abbreviations,
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. At any one time there can be many thunderstorms
affecting a district, only some of which may be severe. It is important
to realise that not all locations highlighted in a Warning will experience
severe thunderstorms. The Warning only indicates that some of the thunderstorms
in the area are expected to be severe.
2) Severe Thunderstorm Warning - Southeast Queensland
Basis of Warnings
These Warnings depict and describe 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 Brisbane Airport, and computer-generated analysis or forecast
Important things to note
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. 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 severe thunderstorms (or thunderstorms imminently expected to become severe) 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. Sometimes, for more long-lived thunderstorms, forecast positions will be extended out to 90 minutes. On other occasions, individual severe thunderstorms may be expected to last only for a short period and tracks will be shown only for the next 30 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. In these situations, the broad area under threat from severe thunderstorms will be shown but individual cells will not be depicted.
The shaded orange area indicates the area our meteorologists consider under immediate threat from severe thunderstorms during the warning period. It will often be larger than the area under the forecast tracks of the severe thunderstorms shown on the graphic. Our forecasters need to consider the possibility that thunderstorms might deviate from the forecast tracks, and allow for the development of new severe thunderstorms or the intensification of thunderstorms not yet meeting severe thunderstorm criteria.
Issue time and Valid time
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.
Warnings are valid for up to 90 minutes but will be updated every 30 to 60 minutes as a weather situation evolves.
The boundaries shown on the image are the Local Government Area boundaries (previously the Bureau's weather forecast district boundaries were used).
Only thunderstorms that are identified as severe, or expected to become severe (according to the definition above), are depicted and described in a Warning. Other thunderstorms may be occurring in the region but these will not be indicated on the map. If there are other thunderstorms in the region, then they are not currently showing the radar characteristics usually associated with severe thunderstorms.