Why are weather forecasts valid only for a short period of time?
The atmosphere is so dynamic and ever changing. These sometimes rapid changes in any one of many factors can cause weather patterns to shift quickly. Weather changes in response to wind,water,temperature,precipitation, smog, and several other factors. Everything from the jet stream in the troposphere to the polar fronts plays a role in determining the weather pattern.
Forecasts are what they are because of these rapid environmental changes that dictate the weather.
Smog and pollution also play a role in weather patterns. Just ask someone who has lived in a city with heavy smog.
Meteorologists have to stay busy to keep up with all the daily patterns. They do updates many times a day. They have computer software that helps them. Not to mention forest fires and volcanic eruptions, these to bring about changes in the atmosphere that helps shape the forecast.
I'll assume you mean weather forecasts. First, they're not always valid at all haha. The ones for my valley are often wrong. But you have to look at weather as part of an all-inclusive climate and atmosphere. All of the systems that combine to generate weather - temperature, humidity, wind patterns, cloud formation, jetstream, etc. - are constantly shifting and interacting. Even volcanic eruptions have an effect.
So as these shifts occur, weather forecasters have to adjust their predicitons based on what the shift was. Most forecasts are valid for only 12 - 36 hours before new calculations are required.
Weather can change. When meteorologists tell us on the evening news what the weather will be like tomorrow, it is only a prediction. Many things have an influence on the weather such as density, which is the temperature and moisture that is in the air. Polar (cold) air and tropical (warm) air combine and when they do it is called the jet stream. The jet stream can become very unstable and when this happens we get unfavorable weather. This depends on pressure changes and air mass that is over the region. Most weather that we experience occurs in the troposphere.