Weather forecasts are calculated using numerical weather prediction (NWP) models running in simulation software on supercomputers. The inputs to these models are recent and current weather data from airplanes, sensing stations and satellites. Forecast models use computational fluid dynamics to model atmospheric fluid flow. Almost all models are ensembles - combining many simulations and sub model runs. The global models are run at varying times into the future.
The main NWP models are:
| Organization | Model Name | Nickname | Time Span |
| UK Met Office | Unified Model | Unified | 6 days |
| European Centre for Medium-Range Weather Forecasts (ECMWF) | Integrated Forecasting System (IFS) | ECMWF | 10 days |
| Environment Canada | Global Environmental Multiscale Model | Canadian | 10 days |
| Environmental Modeling Center | Global Forecast System | NOAA | 10 days |
The visual output produced by a model solution is known as a prognostic chart, or prog. Astronomy websites present the results of these models. Some websites combine numerous models to increase accuracy.
Clear Outside uses MeteoSource to deliver the combined data.
Meteoblue uses their Learning MultiModel (mLM) to post-process the output from NWP models using current weather measurements. The mLM gets current weather measurement data and, among a set of models, selects the one with the most accurate simulation, to then run the forecast.
Windy.com allows you to select from eight different models.
The websites then present this data in many ways. Some are tailored for astronomy.
For more information: Numerical Weather Prediction