Re: Why are weather forecasts based on the movements of fronts?

Fronts are the boundaries between air masses.  Air masses are large regions
that have relatively the same temperature and moisture content.
Whenever you mix air masses with differing temperatures and moisture
interesting things can happen.  One example is your breath on
a cold day.  The cool dry air mixes with the warm moist air
and you see "clouds" come out of your mouth.

At a frontal boundary one air mass is displacing another
and this usually causes lifting of air.  Rising air is
usually what causes precipitation and thunderstorms among other things.
When a cold air mass encounters a stationary warm one, the colder air 
is less dense and acts like a bulldozer, pushing and lifting the
warmer less dense air up and out of the way.  This rapid rising
of air ignites thunderstorms and severe weather.  
When a moving warm air mass meets a stationary cool one, the warmer
less dense air tends to ride up over the cooler more dense air
creating clouds and precipitation.

Air frontal boundaries are to weather what the front lines
are to a war, it is where the action is.