Graphical Severe Thunderstorm Warnings in South Australia
In late 2004, the Bureau of Meteorology upgraded the severe thunderstorm warning service in New South Wales, to provide map-based warnings via the internet. These map-based warnings are now available in four states; NSW, Queensland, Victoria and South Australia. The new products contain maps that complement the Bureau's traditional text-based warnings, and help people to more easily picture the areas under threat from severe weather.
A Severe Thunderstorm is a thunderstorm that produces one or more of the following:
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 SA
The Bureau of Meteorology issues Severe Thunderstorm Warnings whenever severe thunderstorms are occurring in an area or are expected to develop or move into an area during the next several hours. The warnings describe the area under threat and the particular hazards likely to be associated with the thunderstorms. Warnings are distributed widely to the media and emergency services, and are available to the public via the internet and various telephone and fax-based services. A coloured image is available with the warning on the internet to show the area at risk in map format. This service is provided for all parts of South Australia.
When severe thunderstorms are actually occurring, or are about to move into, the populated region around Adelaide, the Bureau issues a more detailed Adelaide Region Severe Thunderstorm Warning. High quality, full-time weather radar coverage of the Adelaide Region 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.
1. Severe Thunderstorm Warnings for South AustraliaThese warnings are issued as an alert to the public, emergency services and other organisations when severe thunderstorms are forecast to develop, or move into, a specified area over the next several hours. The warnings are issued, as necessary, for all parts of South Australia.
Here is an example, which shows what these warnings look like.
Important things to note:
The area covered by the Warning is shaded yellow and represents the area where there is a potential threat from severe thunderstorms. The area may consist of all or part of one or several weather forecast districts.
The issue time is the time that the Bureau of Meteorology transmitted the Warning. It is displayed near the top of the warning text and also in the top right hand corner of the map, in local South Australian time; that is Central Standard Time (CST) or Central Daylight Savings Time (CDT).
Validity Period and Update Frequency
State-based warnings in South Australia are valid for up to 6 hours. The warnings are updated at least every 3 hours, but more frequently if the situation warrants. The expected issue time of the next Warning is given in the text.
The divisions on the chart indicate the boundaries of the Bureau’s weather forecast districts. Districts are 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. So it is important to realise that not all locations highlighted in a Warning will be directly affected by a severe thunderstorm. Also, a number of thunderstorms may occur on a particular day, but it is unlikely that all of these will be severe.
The Adelaide Region Severe Thunderstorm Warning is a more detailed alert issued to the public, emergency services and other organisations when severe thunderstorms are detected in the Adelaide Region, or they are detected outside and forecast to move into the Adelaide Region, or there is a high confidence that severe thunderstorms will develop in the Adelaide Region during the next 60 minutes.
These Warnings describe the location and forecsat movement of individual severe thunderstorms and are based on the forecaster's analysis of weather radar data. The Bureau’s radar display systems enable forecasters to study the structure of thunderstorms in three dimensions and thereby identify storm features that are not apparent on the two dimensional view available on the internet.
Forecasters combine the frequently updated weather radar data with data from
automatic weather stations, satellite imagery, measurements of upper air conditions and
computer-generated forecast charts.
Here is an example of an Adelaide Region Severe Thunderstorm Warning.
Important thngs to note:
The yellow “Warning Area” indicates the area under threat from severe thunderstorms during the next several hours. This yellow area comes from the state-based Severe Thunderstorm Warning.
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 picture shows a simplified representation of the situation, as seen on radar. In particular, only severe thunderstorms (or thunderstorms imminently expected to become severe) are depicted.
A red 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 successive 10 minute intervals.
The number of ten-minute forecast positions shown will depend on the current behaviour of the thunderstorms. Usually six arcs are shown, giving forecast positions for the front edge of the thunderstorm out to 60 minutes from the valid time.
Sometimes, for especially long-lived thunderstorms, forecast tracks may be extended out to 90 minutes. On other occasions, the severe thunderstorms may not be expected to last for 60 minutes and tracks are only shown 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 storms are likely to dissipate before the Warning reaches the public. In these situations, the broad area under threat from severe thunderstorms is described but individual thunderstorm cells are not depicted.
Issue time and Valid time
The issue time is the time that the Warning was transmitted. The valid time is the time when the severe thunderstorms were located as shown by the red ellipses on the image. Both times are in local South Australian time.
Validity Period and Update Frequency
Adelaide Region warnings are valid for 60 minutes. The warnings are updated every 30 to 60 minutes. The expected issue time of the next Warning is given in the text.
These more detailed warnings are available for Adelaide and the surrounding Mount Lofty Ranges district. The zones shown on the Adelaide Region Severe Thunderstorm Warning map are based on Local Government Areas (LGA's). The numerous inner suburban LGA's are grouped to create a more suitable sized "Metropolitan" area.
The "Metropolitan" area comprises the cities of Salisbury, Tea Tree Gully, Port Adelaide Enfield, Charles Sturt, Prospect, Walkerville, Norwood Payneham & St Peters, Campbelltown, West Torrens, Adelaide, Unley, Burnside, Holdfast Bay, Mitcham and Marion.
The remaining council areas within the Adelaide Region are: Playford; Gawler; Barossa; Adelaide Hills; Onkaparinga; Mount Barker; Yankalilla; Victor Harbor and the western parts of Alexandrina and Mid Murray.
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 Adelaide Region but these will not be indicated on the map. If there are other thunderstorms, then they are not currently showing the radar characteristics usually associated with severe thunderstorms.