Severe Thunderstorm Warning Services

While we experience many thunderstorms, more intense thunderstorms are referred to as severe thunderstorms. Severe thunderstorms can produce damaging wind gusts, large hail, tornadoes and heavy rain which may cause flash flooding and these phenomena can all cause significant damage.

Introduction

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 or destructive wind gusts (generally wind gusts exceeding 90 km/h)
  • Heavy rainfall which may cause flash flooding
  • Tornadoes

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?

Severe thunderstorms are monitored using satellite and radar images in conjunction with the Bureau's observation network. 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

Standard public forecasts will include information when there is a reasonable risk of severe storms. This information will allow people to prepare for the potential severe weather. Severe thunderstorms can be quite localised and can develop quickly. The exact location of severe thunderstorms can be hard to predict. As it is difficult to forecast the precise location and movement of severe storms before they have started to develop, detailed warnings will generally be provided once they have been observed or detected. The detailed 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 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
  • Immediate threat area (graphical detailed warning)
  • Storm location and movement (graphical detailed warning)
  • Warning issue time
  • Description of the threat
  • Action statements
  • Advice of next issue time

Types of Severe Thunderstorm Warnings

The Bureau provides 2 types of Severe Thunderstorm Warnings:

  • Detailed - issued for all capital cities and surrounding areas. They are issued when individual severe thunderstorms are within range of the capital city radars. They provide more specific information on individual severe thunderstorm locations. The warning includes a map depicting any existing thunderstorms (red ellipse) and the forecast direction of movement (red arrows) for up to 60 minutes and an Immediate Threat area (orange-hashed).
  • Broad-based statewide - issued for the entire Australian states or territories affected highlighting broad areas where severe storms may occur within the next three hours. 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 (usually in the next 3 hour period). 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.