The Bureau of Meteorology defines a heatwave as three or more days in a row when both daytime and night-time temperatures are unusually high—in relation to the local long-term climate and the recent past. There is no single temperature threshold for a heatwave in Australia. For each part of the country, the Bureau compares the forecast maximum and minimum temperatures for each three-day period in the coming week (e.g. Monday–Wednesday, Tuesday–Thursday) to the 'normal' temperatures expected for that location at that time of year, and to observed temperatures over the last 30 days.
Heatwaves are classified into three types, based on intensity. Comparing forecast temperatures to the last 30 days of temperatures allows the Bureau to gauge the size of the temperature change, physical and behavioural preparedness for such heat, and how challenging it could be for people and infrastructure:
The heatwave product includes two heatwave assessment (last two three-day periods) and five heatwave forecasts (three-day periods) starting on each of the next five days.
Caution. The maps provided by the Heatwave Forecast will show a reduced severity level or remove the indication of heatwave before the heatwave actually ends. This occurs because the maps are calculated across today, tomorrow and the next day. If the temperature is lower on the last day(s) then the map will indicate a lower risk despite unusually hot conditions being present for the first day or two. The Heatwave Assessment will show how heatwaves are finishing due to the combination of recent days with the current forecast days.