Flood warning terminology

Background

After the May 2009 floods some members of the community expressed confusion about the terminology used for expected river heights. Below are specific terms used by the Bureau of Meteorology (BOM) when a flood warning is issued:

Exceed

This is the term most likely to be used by the BOM in the developing stages of a flood. It is a forecast of a river height that is predicted to be exceeded, given the current rainfall that has been received in the catchment.

The use of this term does not consider any additional rainfall received after the issue of the warning. Logically any additional rainfall will result in an increase in the ultimate flood height.

Reach

This term is used when the BOM has considered the impact of up to 12 hours of future rainfall. This additional rainfall is assumed from rainfall prediction modelling. This provides the community with additional warning, although the actual rainfall intensity, spread and timing may differ from that predicted by the model.

If the rainfall is less than predicted, then the flood height may be lower or later. Conversely if the rainfall is greater than predicted the flood height may be higher.

Peak

This term is not used until the BOM are confident that the rainfall event is abating and the maximum flood height can be determined with certainty. This is generally late in the flood event after the rainfall event is either finished or is very close to being finished.

With the May 2009 flood event in Kempsey the BOM did not use the term "Peak" for Kempsey in their flood warnings until 3:23 am on 23 May 2009 at a flood peak of 7.0m, with all previous warnings using either the term "Exceed" or "Reach".