What does the forecast look like?

With many projects currently undertaking high risk works in accordance with rainfall forecast based risk assessments it’s important to properly understand how the Bureau of Meteorology’s forecast is to be interpreted. The rainfall forecast is commonly misinterpreted by combining the percentage (%) chance of rainfall with the depth (mm) of rainfall possible.

Using the example below, the forecast does not mean there’s a 90% chance of receiving 1-10mm of rain for the day. Rather, it should be read as a 90% chance of receiving any rainfall (taken as >0.2mm). The range shown (in this case 1-10mm) is a bit more complicated. Without expressly saying, it means there’s a 75% chance of receiving more than the 1st number (1mm), and a 25% chance of receiving more than the second number (10mm). 

Another bit of information to note is the size of the range, that is the range from the first number to the second number. Normally, a big range means a localised storm, whilst a small range means steady, widespread rainfall. 

So, when setting rainfall forecast triggers to achieve certain outcomes on site, for example ceasing works during a culvert installation and attempting to stabilise the exposed area it is worth considering how much rainfall poses a risk of sediment release, and what chance you are comfortable undertaken the works up to. For simplicity, you could ignore the percentage chance of any rainfall (0.2mm) and focus on the chance associated with the range presented in the forecast. Depending on historical accuracy of the forecasts, local knowledge of rainfall producing ruoff or actual modelling a default may be: 75% chance of more than 10mm, or 25% chance of 30mm.

Alternatively, you could review the Bureau’s MetEye tool or weather app which would allow a more precise review of the localised weather conditions and a specific rainfall forecast trigger (eg. 50% of more than 15mm). 

Just a reminder that in many cases forecasts can be hard to recover, so in the event of unforeseen rainfall, where the forecast did not trigger remedial works, but a release occurred nonetheless, it may be very important to present the forecast that preceded that event. We recommended diligently saving daily rainfall forecasts during high risk works.