Weather predictions can change from day to day but the way weather forecasters predict the weather is through satellites and imaging and a lot of equipment.
Meteorologists use several computer programs and specialized tools to measure atmospheric conditions that occurred in the past and conditions that are happening in the present, and they then apply this information to create educated guesses about the future weather.
That's basically it the weather forecasts are just predictions and although the weather forecasts says it may snow or rain a few days out the weather could change to better weather or worse weather.
Forecasts that are further out than one day are very hard to predict and be accurate although they're pretty close.