Notes taken from : http://www.wxdude.com/basics.html
A daily weather forecast involves the work of thousands of observers and meteorologists all over the world, and the work of thousands of machines. Modern computers make forecasts more accurate than ever, and weather satellites orbiting the earth take photographs of clouds from space. Forecasters use the observations from ground and space, along with formulas and rules based on experience of what has happened in the past, and then make their forecast.
1. Persistence Forecasting: This assumes that what the weather is doing now is what it will continue to do.
A thermometer measures temperature.
A barometer measures air pressure.
A rain gauge measures precipitation.
An anemometer measures wind speed.
A radiosonde attached to a weather balloon measures weather high in the atmosphere.
A satellite orbiting Earth takes pictures of clouds from space to help us see where and how fast clouds are moving.
A radar shoots a radio signal into a cloud to shows where precipitation is falling and how much. It can also spot severe storms and how fast they are moving.
Eyes and ears are probably the most accurate tools. Meteorologists all over the world observe clouds and precipitation, and relay that information and their measurements to other meteorologists.
2. Synoptic Forecasting: This method uses basic rules that the atmosphere follows.
3. Statistical Forecasting: Meteorologists ask themselves, what does it usually do this time of year? Records give forecasters an idea of what the weather is "supposed to be like" at a certain time of year.
4. Computer forecasting: Forecasters take their observations and plug the numbers into complicated equations.
The Weather Network
Online Guide to Weather Forecasting