While we experience many thunderstorms, more intense thunderstorms are referred to as severe thunderstorms. Severe thunderstorms can cause significant localised damage due to damaging wind gusts, large hail, tornadoes and flash flooding
The Bureau of Meteorology issues Severe Thunderstorm Warnings to alert communities of the threat of these more dangerous thunderstorms.
How is a severe thunderstorm defined?
A severe thunderstorm is one that produces any of the following:
- Large hail (2cm in diameter or larger)
- Damaging wind gusts (generally wind gusts exceeding 90 km/h)
- Heavy rainfall which may cause flash flooding
Most thunderstorms do not reach the level of intensity needed to produce these dangerous phenomena so the Bureau of Meteorology does not warn for all thunderstorms.
What weather conditions are Severe Thunderstorm Warnings issued for?
A Severe Thunderstorm Warning is issued when:
- A severe thunderstorm is reported
- There is strong evidence of a severe thunderstorm, and it is expected to persist
- Existing thunderstorms are likely to develop into a severe thunderstorm
Severe thunderstorms can be quite localised and can develop quickly. The exact location of severe thunderstorms can be hard to predict. The warnings are usually issued without much lead-time before the event.
Which warnings are issued when thunderstorms occur during other severe weather events?
Some severe weather events can be quite complex with thunderstorms occurring during a more widespread severe weather event (e.g. a band of heavy rain or vigorous cold front). In these circumstances Bureau's forecasters determine which warning(s) to issue to best inform the public about the hazards. This is done with consideration of warnings already in place and may involve consultation with local emergency services agencies. Users are advised to always consider all Bureau warnings issued in a severe weather event for their area.
What information is included in the Severe Thunderstorm Warning?
Severe Thunderstorm Warnings can contain the following information:
- Standard Emergency Warning Signal (SEWS) - sounded only for the most serious events
- Phenomena expected in the warning area, e.g. large hail, damaging winds, flash flooding
- Threat area
- Warning issue time
- Description of the threat
- Action statements
- Advice of next issue time
Types of Severe Thunderstorm Warnings
There are 2 types of Severe Thunderstorm Warnings:
- Detailed - for the densely populated cities and surrounding areas, particularly in the east. They are issued when individual severe thunderstorms are within range of the Bureau's weather-watch radars. They provide more specific information on individual severe thunderstorms. The warning includes a map depicting any existing thunderstorms and the forecast direction of movement for up to 60 minutes
- Broad-based - issued for the entire state or territory affected, these will usually be less detailed. They are issued as an alert to the public, emergency services and other organisations that severe thunderstorms are likely to develop, or extend into, a specified area over the next few hours. They are based on broad areas such as the Bureau's weather forecast districts. They include a map with the areas covered by the warning shaded in yellow.
Severe Thunderstorm Warning for Damaging Wind and Large Hailstones
An example of a broad-based Severe Thunderstorm Warning issued in South Australia.
Severe Thunderstorm Warning - Melbourne Area - for Large Hailstones
An example of a detailed Severe Thunderstorm Warning issued in Melbourne