Upward-moving air cools, and if this cooling is great enough, the air will cool to the point where it cannot hold any more water vapor. This water vapor then condenses, and falls as rain or snow. Weather data is gathered by weather instruments at over 10,000 airports worldwide, from weather balloons launched twice daily from about 1,000 locations around the world, and from weather satellites orbiting the earth. The first task in making a forecast is to look at these current measurements of the atmosphere and understand what the weather is doing now. It also helps to watch some time-lapse weather animations to see what has been happening over the past day or so, and make some estimates of how these trends will carry over into the future.
Tornadoes always form inside severe thunderstorms.
Usually, a large area of clouds inside of the thunderstorm will start to spin about 30 minutes before a tornado appears. So, once we see this spin start to develop, a tornado warning can be issued. However, not all spinning thunderstorms make tornadoes, so there are a lot of false warnings issued. More information about tornadoes is available from the National Severe Storm Laboratory . Please explain lake effect.