What is a weather model?
Weather models are computer programs that simulate the atmosphere. They help meteorologists forecast weather.
These models use millions of observations collected from around the world. For example, from weather stations, satellites, radar, aircraft, ships and weather balloons. This large amount of data describes current conditions such as temperature, humidity, wind and air pressure.
Weather models calculate how these conditions are likely to change over time.
How weather models work
Forecasting begins with a snapshot of the atmosphere.
Observations from Australia and around the world are combined. This helps create the best possible picture of current weather conditions.
With this information, the model performs billions of calculations. It simulates how the atmosphere may evolve.
The model repeatedly calculates changes in the atmosphere at thousands of points on a grid. This grid covers Australia and the globe.
The results inform forecasts from the next few hours to many days ahead.
Video: Weather models grid
What weather models does the bureau use?
We use:
- the Australian Community Climate and Earth-System Simulator (ACCESS) weather prediction system
- international weather models.
ACCESS models
ACCESS is Australia's operational weather forecasting model. It is a collaboration between the Bureau of Meteorology, CSIRO, Australian universities and international partners.
We use different versions of ACCESS for different forecasting purposes:
- ACCESS-G – a global version to understand broader weather patterns over longer periods
- ACCESS-C – a higher-resolution version for areas with bigger populations (large cities) and more weather risks.
Higher-resolution models provide more local details but cover smaller areas and shorter forecast periods.
ACCESS-G (global)
ACCESS-G is a global model that simulates weather across the entire planet.
It helps meteorologists understand the movement of large-scale weather systems such as:
- high and low pressure systems
- cold fronts
- tropical cyclones
- broad rainfall patterns.
Global models provide forecasts up to about 10 days ahead.
ACCESS-C (cities)
ACCESS-C is a higher-resolution model focused on Australia. It's used for large cities, and heavily populated areas at risk of greater weather impacts.
ACCESS-C provides more detail about local weather, including:
- rainfall
- thunderstorms
- sea breezes
- strong winds
- temperature variations.
Higher-resolution models can better represent local terrain and weather processes. This helps improve forecasts at regional and local level.
Information from ACCESS-C is available for emergency services and other specialists. Find out about registered access to real-time data services.
ACCESS-S (seasonal)
We use ACCESS-S and some international models when producing long-range forecasts.
International weather models
Meteorologists don't rely on a single weather model. They compare forecasts from several leading international centres. Comparing multiple models helps meteorologists:
- know how confident they can be in a forecast
- identify a range of possible outcomes.
Models have different:
- resolutions (level of detail)
- approaches
- data assimilation systems
- model physics.
This means forecasts can differ between models – especially for rain, thunderstorms and weather systems several days ahead.
A model is run many times with different starting conditions. We can also combine multiple weather models. This helps show a range of potential weather scenarios.
To work out which model delivers the most accurate forecast for a given situation, meteorologists use a process called verification. Find out about measuring forecast accuracy.
For weather forecasting up to 7 days, we use the international models listed below. We use more models for long-range forecasting. Learn about producing long-range forecasts.
| Weather model | Organisation | Country |
|---|---|---|
| ECMWF (IFS) | European Centre for Medium-Range Weather Forecasts | Europe |
| GFS | National Centers for Environmental Prediction (NOAA) | United States |
| JMA GSM | Japan Meteorological Agency | Japan |
| UKMO Unified Model | UK Met Office | United Kingdom |
Why weather models are not perfect
Weather models are powerful forecasting tools. Even so, they cannot capture all the complexity of Earth's atmosphere.
The atmosphere is constantly changing. Even small differences in current conditions can mean different weather several days later.
This is especially true for weather that develops on a small scale, such as:
- showers
- thunderstorms
- fog
- severe local wind events.
Model forecasts become less certain as they extend further into the future. Find out why forecasts change.
And to turn information from models into reliable forecasts, we need meteorologists' scientific expertise and experience.