Weather Conditions

Know your BOM forecast

The Bureau issues official seven-day weather forecasts for towns and cities across Australia. Forecasts are issued twice per day—in the early hours of the morning and again in the late afternoon. They may be updated at any time if new information comes to hand that requires a significant change to be made.

We're often asked why we only forecast out to seven days. It’s because that is about the maximum length of time that we can rely on the accuracy of computer weather models. As good as today's weather models are—and they're being improved all the time—they're not perfect. At the end of the day, weather models solve mathematical equations and algorithms that incorporate approximations and assumptions (there's much that scientists are still learning about the atmosphere) for a set number of points around the globe at set times. At this stage it’s impossible to simulate the evolution of weather around the Earth exactly for an infinite number of points at an infinite number of times.

These limitations mean that any calculations the weather model makes that aren’t spot-on at the early stages of the seven-day forecast have an increasing impact as the model calculates further into the future. Therefore shorter-term weather forecasts (say for the next two to three days) are generally more accurate than forecasts towards the end of the seven-day period.

It's also important to remember when looking up the forecast for your town or city that many features of the weather, like rain and showers, can often be on a very local scale—and differ even from suburb to suburb. So the next time you see a forecast for a 'shower or two' but you see mostly blue sky at your place, give your friend on the other side of town a call … they might be sheltering from a downpour!