About Graphical Severe Thunderstorm Warnings in New South Wales
The Bureau of Meteorology has implemented an enhanced severe thunderstorm warning service in NSW and the ACT. This uses graphics to better define the areas under threat when these dangerous thunderstorms are occurring. The graphics complement the Bureau's traditional text-based warning service.
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 NSW
The Bureau of Meteorology issues Severe Thunderstorm Warnings whenever severe thunderstorms are occurring in an area or are expected to develop or move into the area during the ensuing few hours. The warnings describe 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. An image is available with the warning on the Web to show in map format the area at risk. This service is provided for all parts of NSW.
When severe thunderstorms are actually occurring, or are about to move into, the heavily populated region around Sydney, Newcastle and Wollongong, the Bureau of Meteorology will issue a more detailed Severe Thunderstorm Warning for people in the affected region. High quality, full-time weather radar coverage in this area allows these warnings to describe the current location of individual thunderstorms, and the places likely to be affected within the next 30 to 60 minutes. Again, an image is available with the warning showing in map format the location of the severe thunderstorms and their forecast tracks. The more detailed warnings are available for Sydney, Newcastle, Wollongong and surrounds.
Severe Thunderstorm Warnings for NSW and the ACTBasis of Warnings
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. They warnings are issued, as necessary, for all parts of NSW and the ACT. A sample warning 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 NSW and the ACT 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. Each district is labelled with an abbreviation, as follows:
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.
Severe Thunderstorm Warning - Sydney/Newcastle/Wollongong region
Basis of Warnings
The Severe Thunderstorm Warning - Sydney/Newcastle/Wollongong region is a more detailed alert issued to the public, emergency services and other organisations when severe thunderstorms are actually detected in Sydney, Newcastle, Wollongong or nearby areas.
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 Sydney Airport, as well as computer-generated analysis or forecast charts.
A sample warning is shown below.
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 yellow “Warning Area” indicates the area our meteorologists consider under 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. 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.Area covered
These more detailed warnings are available for Sydney, Newcastle, Wollongong and surrounds. Specifically, the area covered is
Major localities in the area include
The regions shown on the map are groupings of local government (council) areas:
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 occuring 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.