Numerical Weather Prediction is the science of predicting the weather using computational techniques. Manipulating the huge datasets and performing the complex calculations necessary to do this on a resolution small enough to make it accurate requires some of the most powerful supercomputers in the world.

Models

There are many different methods of determining the weather in this way; each of these methods is called a Model. Some of the better known ones are:

• GFS Global Forecast System - developed by the NOAA - output is freely available
• ETA (now called NAM) an international model originating in Yugoslavia in the 1970s
• UKMET developed by the UK Met Office - output is restricted

others include RUC, AFWA, GEM, NOGAPS, COAMPS. The University Corporation for Atmospheric Reasearch has a page describing the characteristics of these models

Ensemble

Given the chaotic nature of the weather, very small changes in starting conditions can produce a radically different forecast. For this reason, where time allows, many models are run and the results compared to give an Ensemble forecast. If the output of all the models is similar, one can be confident in the predictions. If the results diverge, forecasts produced are likely to be prone to errors.

GFS

The Global Forecast System is the main model run by the NOAA. Its output can be downloaded for free from the National Centers for Environmental Prediction website. This model is run 4 times a day and produces hourly forecasts up 16 days in advance, but with decreasing spatial and temporal resolution over time (though it is widely accepted that beyond 7 days the forecast is little better than guesswork). Unlike conventional forecasts the 6 hour gap between runs does not allow actual measurements to be collected in time to be used as the starting conditions, nor is the distribution of weather stations globally of high enough density to give worthwhile data. For this reason the input of the GFS is satellite derived data.

The model is run in two parts: the first part has a higher resolution and goes out to 180 hours in the future, the second part runs from 180 to 284 hours. The resolution of the model varies in each part of the model: Horizontally, it divides the surface of the earth into 55 or 80 kilometre grid squares; Vertically, it divides the atmosphere into 64 or 42 layers and Temporally, it produces a forecast for every 3rd hour for the first 180 hours, after that they are produced for every 12th hour .

Incidentally, this is the only global NWP output available for free over the internet (as determined by the US legislature), and as such is the basis for all web based forecast services, eg Weather Underground, AccuWeather, The Weather Channel, MetCheck.

See also

• ECMWF