What rain forecasts tell you
Rain forecasts help you understand when rain is expected, how much may fall, and how likely it is to occur.
Rain amounts are measured and forecast in millimetres (mm). Roughly, one millimetre of rain is equivalent to one litre of water falling on one square metre of ground.
Rain forecasts give you:
- a possible range of rainfall amounts, then
- the chance of receiving at least those amounts.
This is different to a maximum or minimum temperature forecast, which is represented by one single number.
The forecast may also include:
- when rain is expected
- whether widespread or isolated
- the potential for heavy rain.
How meteorologists forecast rain
To forecast when and where rain is likely to fall, bureau meteorologists combine:
- observations from weather stations, radar, satellites and weather balloons
- weather models, both Australian and international
- their expertise and local knowledge to assess results from the models and communicate the forecast.
Learn more about how meteorologists forecast weather.
Why is it hard to forecast rain?
Temperature and wind are often similar across large areas, but rainfall can vary significantly over short distances. A shower may bring heavy rain to one suburb or farm while nearby areas receive little or none.
Meteorologists use complex weather models to identify where rain is likely. While forecasts can often be made with high confidence, predicting exactly when and where rain will fall is more challenging. It's especially hard during thunderstorms.
The type of rain, local geography and changing atmospheric conditions can cause weather systems to develop or move differently than expected. Even small changes can have a major impact on where and how much rain falls.
Watch 'Why is rainfall hard to forecast?' on YouTube.
Why don't rain observations and forecasts match?
Forecasts tell you what is most likely to happen. They give rainfall as a possible range, for 6 km by 6 km areas across Australia.
Observations record what has already occurred. They are shown as a single amount – the exact amount of rain that fell at a particular weather station.
Rain can be patchy and very localised. It is not yet possible to predict exactly how much rain will fall at each location more than a day in advance. But the rainfall range is a good guide.
Will we get the forecast right every time?
The short answer is no.
Weather forecasting has improved dramatically over time. It continues to advance through science, technology, observations and computing power.
But the atmosphere is a complex system. Small differences in current conditions can lead to very different outcomes days later. This means there will always be limits to forecasts. It's why forecasting isn't just about producing numbers.
Forecasts are more than numbers
Forecasting rain is also about helping people understand the range of possible outcomes.
One of the most important roles of a meteorologist is communicating complex weather information in a clear and useful story.
We work closely with emergency services all year round. We support them to prepare and respond to hazards and severe weather events that impact the community. Find out how we support emergency management.
We also tell the story behind the weather on:
- regular radio, TV and online interviews
- our social media channels.
Why do rain forecasts change?
A forecast is our best prediction based on the information available at the time. As time passes, new observations from satellites, weather stations, radars and upper-atmosphere monitoring systems are fed into computer models.
These updated observations can change how weather systems are expected to develop and move. This is why we update forecasts regularly. It does not mean the previous forecast was wrong. Learn more about why forecasts change.
Rain ranges explained
Because rain is highly variable, we use probability to present rain forecasts. While our forecasts show a range, more or less rain is always possible.
When you see a range, such as 15–30 mm, there is a:
- high (75%) chance of at least the first amount
- low (25%) chance of at least the second amount.
This does not apply when the forecast amount is 0 mm. There is always a 100% chance of at least 0 mm in any weather situation.
Watch our short video about rainfall percentages on YouTube.
Calculating rain range
To forecast rain, we run many different weather scenarios through our supercomputer. This helps meteorologist establish the likely rainfall amounts and how confident they can be in the forecast. The rain range reflects the level of certainty.
Narrow rain range – more confidence
When weather models produce similar results, meteorologists have greater confidence in the expected rainfall. This leads to a narrower range, such as 10–15 mm.
Narrower ranges are common when rain is expected to be:
- widespread
- fairly consistent across an area.
For example, rain from a cold front. In these conditions, rainfall amounts are often similar from one location to another.
Wide rain range – less certain
When weather models show different possible outcomes, there is more less confidence in where and how much rain will fall. This results in a wider range, such 1–15 mm or 80–150 mm.
Wide ranges are more common when showers or thunderstorms are forecast. In these conditions, rainfall can vary greatly over short distances. One location may receive little rain while a nearby location gets much more.
Large weather systems such as tropical cyclones or east coast lows can also produce wide ranges. Small changes in the storm's path or the location of the heaviest rain can have a big impact on rainfall totals.
Chance of no rain
When rain is very unlikely, the forecast shows 'No rain'. In hourly forecasts, no rain is shown as 0 mm.
You may still see a percentage with a 'no rain' forecast, such as 95% chance of no rain. This shows how confident forecasters are that no measurable rain will occur.
Research shows many people focus on the word 'rain' and overlook the percentage. Showing the chance of no rain makes it clearer that dry conditions are most likely.
Why do different apps show different rain forecasts?
The bureau is Australia's national weather, water, climate, ocean and space weather information agency. Many other weather providers rely on the data from our observation network and forecast models to provide weather information. They may combine this with different computer models, forecasting techniques and update schedules.
Forecasts may also be generated for slightly different locations, periods or areas. They may provide a forecast for a different percentage chance of rain.
All of this means different weather apps could display different rainfall amounts, even when forecasting for the same town.
The forecasts displayed on our website and app are prepared by our meteorologists who use their understanding of Australia's unique weather patterns and local conditions to interpret the data and advanced weather modelling. Sometimes, you may also notice differences between our website and app.