About Graphical Severe Thunderstorm Warnings in Victoria

Severe Thunderstorms

A Severe Thunderstorm is a thunderstorm that produces one or more of the following: 

• A tornado 
• Hail of diameter 2cm or greater 
• Wind gusts of 90 km/h or greater 
• Very heavy rain leading to flash flooding. 

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 State

EXAMPLE ONLY IDV65756 Australian Government Bureau of Meteorology Victoria TOP PRIORITY FOR IMMEDIATE BROADCAST SEVERE THUNDERSTORM WARNING for LARGE HAILSTONES, DAMAGING WIND and FLASH FLOODING For people in parts of the Northern Country, Wimmera, North Central, Western and Central Forecast Districts. Issued at 9:45 am Thursday, 8 November 2007. Severe thunderstorms are likely to produce damaging winds and large hailstones in the warning area over the next several hours. Locations which may be affected include Ballarat, Maryborough, Kyneton, Geelong, Melbourne and Bacchus Marsh. The State Emergency Service advises that people should: * Keep clear of fallen power lines. * secure any loose objects in the vicinity of your home. * keep away from creeks and drains. * do not drive vehicles through flooded areas. * stay indoors if possible. * Avoid using the phone during the storm. * if you are outside, avoid sheltering under trees * listen to the radio for storm updates * switch off your computer and electrical appliances The next warning is due to be issued by 12:45 pm. If severe thunderstorms develop in the Melbourne area, a more detailed Severe Thunderstorm Warning will be issued to people in this area. Warnings are also available through TV and Radio broadcasts, the Bureau's website at www.bom.gov.au or call 1300 659 217. The Bureau and State Emergency Service would appreciate warnings being broadcast regularly.

Important things to note

Legend

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.

Issue time

The issue time is the local time at which the Bureau transmitted the Warning.

Update Frequency

Warnings in Victoria are only valid for up to 3 hours. The expected issue time of the next Warning is given in the text.

Districts

The divisions on the chart indicate the boundaries of the Bureau’s weather forecast districts. Each district is labelled with an abbreviation, as follows:  

• CEN - Central
• EG - East Gippsland
• NCE - North Central
• NCY - Northern Country
• NE - North East
• MAL - Mallee
• WES - Western District
• WSG - West and South Gippsland
• WIM - Wimmera
• ALP – Alpine

2) Severe Thunderstorm Warning - Melbourne Area

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.
 

EXAMPLE ONLY IDV65752 Australian Government Bureau of Meteorology Victoria TOP PRIORITY FOR IMMEDIATE BROADCAST SEVERE THUNDERSTORM WARNING - MELBOURNE AREA for LARGE HAILSTONES For people in parts of the Western, Geelong and Bellarine Peninsula and Port Phillip Local Warning Areas. Issued at 9:52 am Thursday, 8 November 2007. The Bureau of Meteorology warns that, at 9:50 am, severe thunderstorms were detected on weather radar near Lara, the area west of Lara and the area northwest of Ballan. These thunderstorms are moving towards the east to southeast. They are forecast to affect the You Yangs, the area east of Lara and the area west of Werribee by 10:20 am and Bacchus Marsh, the area south of Bacchus Marsh and the area south of Werribee by 10:50 am. Large hailstones are likely. The State Emergency Service advises that people should: * Keep clear of fallen power lines. * secure any loose objects in the vicinity of your home. * stay indoors if possible. * Avoid using the phone during the storm. * if you are outside, avoid sheltering under trees * listen to the radio for storm updates * switch off your computer and electrical appliances The next warning is due to be issued by 10:55 am. A more general severe thunderstorm warning is also current for parts of the Northern Country, Wimmera, North Central, Western and Central districts. Warnings are also available through TV and Radio broadcasts, the Bureau's website at www.bom.gov.au or call 1300 659 217. The Bureau and State Emergency Service would appreciate warnings being broadcast regularly.

Description of the Warnings Products

Map

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 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.

Update Frequency

Severe Thunderstorm Warning - Melbourne Area is valid for 60 minutes.

Boundaries

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:

• Inner: Moonee Valley, Maribrynong, Melbourne, Port Phillip, Stonnington, Boroondara, Yarra, Moreland, Darebin and Banyule.
• Geelong and Bellarine Peninsula: Greater Geelong and parts of Surf Coast.
• Northern: Hume, Whittlesea and Nillumbik.
• Western: Wyndham, Melton, Brimbank, Hobson Bay and parts of Moorabool and Macedon Ranges.
• Eastern: Manningham, Whitehorse, Maroondah, Knox, Monash.
• Outer East: Cardinia and part of Yarra ranges.
• South East: Bayside, Glen Eira, Kingston, Greater Dandenong, Casey, Frankston.
• Mornington Peninsula: Mornington Peninsula.
• Port Phillip: All waters and islands within Port Phillip.
• Western Port: All waters and islands within Western Port.

Non-severe thunderstorms

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.