a storm is a source of precipitation & snow is a kind of precipitation & rain is a kind of precipitation ->  a storm is a source of rain / snow  a storm is a source of rain / snow & a thunderstorm is a kind of storm ->  a thunderstorm is a source of rain / snow  a thunderstorm is a source of rain / snow & cold fronts cause thunderstorms as they pass by -> cold fronts will cause rain / snow as they pass by