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.