"The presence of land masses on the earth's otherwise watery surface modifies the distribution of temperature, because continents heat and cool at a dramatically different rate than do the oceans. The topography of the land also influences the jet stream's location. Mountain ranges and plains on large continents, for example, significantly affect the distribution of atmospheric temperature. And since the jet stream is a thermally driven phenomenon, the more complicated the three-dimensional temperature structure of the earth's atmosphere, the more 'wandering' will take place in the course of the jet stream.
"Knowing where the jet stream will be located is essential for forecasting the movement and evolution of weather systems. The first step in the forecast process is observation. Many thousands of atmospheric observations are made each day by civilian and military aircraft, land and maritime weather reporting stations and ships and weather balloons. These observations help to define the present state and location of atmospheric circulations and weather systems, including the jet stream.
"These observational data are then plugged into numerical weather-prediction models. These models assimilate this snapshot of the present state of the atmosphere and mathematically diagnose how the jet stream and other circulations and weather systems will change with time. The output from the models is then made available to meteorologists, who apply their knowledge, experience and expertise in making the final forecast."