About Graphical Severe Thunderstorm Warnings in Victoria
The Bureau of Meteorology has developed an enhanced severe thunderstorm warning service. This service has been implemented in Victoria, New South Wales 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.
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.
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.
There are two types of warning service provided by the Bureau of Meteorology. The first covers all parts of the state or territory and the second provides additional detail for people living in the major population centres.
1) Severe Thunderstorm Warnings for the Whole StateThese 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 the state:
Important things to note
The area covered by the Warning and under threat from severe thunderstorms is indicated by a yellow colour. 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 in Victoria are only valid for up to 3 hours. 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. Each district is labelled with an abbreviation, as follows:
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 storm 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 major airports, and
computer-generated analysis or forecast charts.
Description of the Warnings Products
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” shown on the map indicates the general area considered under threat from severe thunderstorm activity over the next several hours. Because of this longer time-frame, it will usually depict a more extensive area than that currently affected by severe thunderstorms. This yellow "Warning Area" is derived from the broad-scale Severe Thunderstorm Warning - Victoria that will normally be in effect to describe the severe thunderstorm threat over the entire Victorian region.
The orange cross-hatched "Immediate Threat Area" shows the
part of the Melbourne Metropolitan region forecast to be at risk from
severe thunderstorms, usually within the next 60 minutes. It too 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
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.
Severe Thunderstorm Warning - Melbourne Area is valid for 60 minutes.
The Local Warning Areas used in the Severe Thunderstorm Warning - Melbourne Area product are groupings of Local Government Areas, which is done to provide suitably sized and a reasonable number of warning areas. The Local Warning Areas and the Local Government Areas they cover are:
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.