Call that an extreme event? Describing the extremeness of an event before, during and after it has occurred

Janice Green and Catherine Jolly - Bureau of Meteorology , 13 December, 2019

Being able to define how extreme an event will be; is currently being experienced; or was, is vital information to assist in preparing for extreme events.

There have been some semi-recent changes in the probability terminology that is used to define how extreme an event is. This is being done to ensure that everyone is clear about what the terminology means and are consistently used in messaging/communications to a range of stakeholders.

This new terminology emphasises that there is an equal probability of an event of a specified magnitude occurring or being exceeded in any given year, and that the probability of an event is independent of what has happened in recent years. The use of Average Recurrence Interval (ARI) as in ‘1 in 100 year event’ is now discouraged as it leads to confusion by suggesting an elapsed time between each event, whereas in reality a “100-year flood” can happen in successive years.

The paper demonstrates how the Bureau of Meteorology has used this information for recent large rainfall and flood events.