The process of using mathematical equations to estimate the atmosphere's state at some point and time in the future. This is possible because the atmosphere is a fluid. Its motions and forces can therefore be expressed using equations of thermodynamics and fluid dynamics.
The set of equations used to represent the atmosphere's governing processes include:
- the equations of motion (conservation of momentum),
- the thermodynamic equation (conservation of energy),
- the continuity equation (conservation of mass),
- and the equation of state.
Since each are differential equations (meaning, they describe a rate of change over a specified time), when current weather observations are used as input, the result is an estimate of the future state of an atmospheric condition--that is, a forecast.
NWP calculations are done by powerful supercomputers housed at the NOAA National Centers for Environmental Prediction (NCEP). The computer programs responsible for generating NWP forecasts are known as forecast models.