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.