Bureau of Meteorology Australian Digital Forecast Database Grid. Forecasts are issued by the Bureau of Meteorology on a routine basis for each state and merged into one forecast grid for Australia for each forecast element. This forecast element is the 10% chance of more than the rain amount, in millimetres per 3 hours, starting at the hour shown. There is a 10% chance during the three hourly period of getting the stated amount of rainfall, or more. This is given as a single value and should be considered separately from the the Rainfall Amount range given for the whole day.